CA1138997A - Modem diagnostic and control system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A system for diagnosing and controlling operation of a plurality of modems, some located at a central site and others at various remote sites. A processor located at the central site selectively addresses microprocessor test and control units at each modem over a secondary channel. The microprocessor test and control units respond to commands to configure and perform various modem tests, operate autonomously to monitor various alarm con-ditions, and format status reports and alarm maydays for trans-mission back to the central processor. The system is capable of performing a wide variety of testing, monitoring and network control functions for a very large network of modems.

Description

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ll4l 1`10Dl~M l)~ A~;NO.STIC
I; ~NIj C()~`~;l'l'l)J, SY~ 5 .~ BZ~CKGROI~ND_OI;' THE INVI~lTION
The subject invention relates generally to automaticnetwork diagnostic systems and more particularly to a system ~ for automatically testing and controlling, from a central site~
a plurality of centrally and remotely located data modems.
With the increasing complexity of distributed data pro-cessing systems, particu~arly those utilizing telephone data jlcommunication between central and remote site data processing Ijapparatus a~d their associated data modems, the need for testing land control of the data modems has increased. The complexity of present and proposed systems requires the ability to communicate rapidly wi~h modems at diverse and numerou~ sites~ Modem mal- ¦

~ unctions become increasingly critical in that one malfunctio~ing ! modem may interrupt transmission ~y man~ othe~ in the network.
¦ Since even very small amounts of down time can mean big dollar losses in distributed processing ~ystems; the need arise~ to automatically control modems in a distributed system to minimize ~I time los~es. To provide efficient and effective operation~ it -- would be highly desirable to pro~ide the modem with the capability

2~ to raise alarm 6ignals to a central controller and perform the network reco~figuration and contxol functions necessary to respond to various trouble conditions~ Both ~or 6peed and reli-ability, it is des~rable to have as many o these func~ions a~
po8sibl~ perfor~ed automatically by appara~us of the ~ystem.

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., SUMMARY OF_THE INVENTION
It is, therefore, an object of the invention to pro-vide an improved method and apparatus for monitoring, testing, and controlling modems at diverse physical locationsO
In a preferred embodiment of the invention a test and control unit is provided at each modem, responsive to commands from a central system controller. The test and control unit monitors various parameters of the modem. The test and control unit may respond to test commands from the system controller to perform various tests of these parameters and send various status reports back to the system controller. The system controller may automatically scan all primary and secondary drops of the network for modem status without interrupting modem or network operation. The test and control unit may also detect parameters indicating abnormal operation or mal-functions, and send "mayday"~alarm signal~ to the central system controller.
The preferred embodiment of the invention; the test and control unit is also capable of responding to commands to control the operation of its associated modem. Control over the network configuration is also achieved. The system of the invention is applicable to multi-tiered networks.

11 31~ 7 In accordance with one aspect of the invention there i8 provid~d in a data communication system including a plurality of data modems each at a remote site, the diagnostic apparatus at each said remote site compris~ng:
means for providing a first plurality of signals from which malfunction~ in said data communication system may be detected;
means supplied with said first plurality of signals for repeatedly testing",said signals and responsive to said 0 tests for detecting said malfunctions; and means responsive to detection of a said malfunction for generating a second plurality of signals, each indica-tive of the occurrence of a said malfunction.

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Further in accordance with the invention there is provided ~or use in a data communication system including a plurality of data modems, the diagnostic apparatus comprising:
means for providing a plurality of first signals from .~hich faults in said data communication system may be detected, said faults including an indication of a failure in a said data modem and an indication of a streaming condition;
means contained within a said data modem and supplied with said first signals for automatically detecting from said first signals the occurrence of one or more of said faults and for generating a message including information indicative of the type of faul-t and the location of the fault in the data communication system; and means for transmitting said message to a central site in said data communication system.
Also further in accordance with the invention there is provided for use in a data communication system including a plurality of remote sites and at least one remote site modem located at each remote site, a method of performing remote site diagnostics comprisi.ng the steps of:
providing a plurality of first signals Erom which malfunctions in said data communication system at a remote site may he detected;
monitoring said signals at said remote site to auto-matically detect said malfunctions~ said monitoring and detect-ing being performed at least in part under control of a program stored at said remote site; and generating a message signal containing information indicating that a malfunction has occurred in the data communi-cation system~

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Further in accordance with the invention there is provided ln a data modem, adapted for use as part of a data communication system including a central controll.er, the method of performing data communication system diagnostics comprising the steps of:
providing a plurality of signals from which a plurality of alarm conditions may be detected;
monitoring said plurality of signals under stored program control, autonomously of said central controll.er;
detecting alarm conditions from the monitored signals under stored program control, autonomously of said central controll.er;
storing digital indications of the presence or absence of said alarm conditions; and responding to the detection of an alarm condition by forming a message indicative of the occurrence of a said alarm condition for transfer to selected apparatus in said data communication system.
Further in accordance Wit]l the invention tllere is provided a data communication system including:
a plurality of data modems each a~ a remote site, a data terminal communicating with each remote site modem, at least one central-site modem, a central site processor communi-cating with said central site modem, said central-site modem and remote-site modems being adapted for communication with one another over a transmission medium; and diagnostic apparatus at each said remote site comprising:
means for providing a first plurality of signals from which malfunctions in said data communication system may be detected;

- 3b -means supplied with said first plurality of signals for repeatedly testing said signals and responsive to said tests for detecring said malfunctions; and means responsive to detection of a said malfunction for generating a second plurality of signals, each indicative of the occurrence of a said malfunction.

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BRIEF DESCRIPTION OF TIIE l)RAWINGS
~ The preferred emhodiment and be~t mode contemplated for I practicing the just summarized invention will now be de~cribed in conjunction wit~ the drawings of which:
FIGURE 1 is a generalized ~lock diagram of a portion of a 6ystem configured according to t~e preferred em~odLment of the in~ention.
FIGURE 2 i~ a block diagram of a two-tier modem ~y~tem incorporating the preferred embodLment of the invention.
FIGURE 3 illu~trates ths datQ format for transmitting a command according to the preferred e~odLment of the inYentiOn.
FIGURE 4 illu~trates the for~at of a conci~e ~tatus word.
FIGURE 5 is a ~lock diagram of a dial ~ack-up technique of the preferred embodiment.
FIGURE 6 is a c;rcuit diagram of a test and control unit accordïng to t~e preferred em~od~ment.
FIGURE 7 ls a circu;t d~agram of receiYe-tran~mit cir-cuitry of the preferred em~odLment.
FIGURE 8 i~ a c;rcuit diagram of thQ pO~9~ fail detect circuitxy of the preferred emfiodiment.
FIGURES ~ ~ 21 ar~ flo~ diagrams illu~trating the ... .
operation of the test and control unit.
F~GURE 22 illu~trate~ the receiYe function of t~e test . . j and control unit.

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DET~IL~D D~SC~IPTION OF TIIE
PREFERR~D EMOBDIM~NT OF THE INV~NTION
Fig. 1 illustrate~ in functional block form a portion of a system accordlng to the preferred embodiment of the invention providing remote test capabilities for modems. Such modems may -be, for example, in four wire controlled carrier multipoint or continuous carrier point-to-point data communications networks.
Testing is accomplished according to commands ~ent to system modems such as a central modem 11 and a remote modem 13 by a system con-troller 15, which may be a programmed minicomputer such as the DEC
~DP-ll.* Testing and control of the remote modem 13 i8 accomplished :
under control of a remote test and control unit 17. The test and control unit 17 receives commands transmitted acxoss a secondary channel by the central modem 11~ The central modem 11 contains a .. , , . I
.test and control unit similar to 17.
The test and control unit 17 includes a secondary channel transmitter and receiver 19, a control unit 21, and monitoring circuitry 23. As discussed later, the test and control unit 21 is preferably configured aroun~ a microprocessor, which~ by way of example, may be a Fairchild F~ CPU and PSU. The test and control ;unit 17 decodes addresses and commands from the syst~m controller ;15~ performs a ~pecific test i~ addressed~ frames and formats the test results, and transmits these;results back to the system con-troller 15. Some tests arq aqcomplished without interfering with normal network operation while othe~ teqts temporarily interrupt 1.
.portions of the network.
l In addition to responding to certain te~ts initiated by ithe cvntroller 15r the test and control unit senses certain , r PDP-ll is a trademark ,., ~ . . ..
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~13~;37 anomalous co~ditions in the mod~m 13 and transmits suitable alarm messages back to the system controller 15. Because it has the ability to address a particular test and control unit 17, the preferred embodiment is also capable of performing certain network control functions such as dial back-up.
In the preferred embodiment, the secondary channel 25 operates asynchronously at a relatively low speed data rate such as 75 bits per second. The modulation technique utilized by the ;secondary transmitter and receivex 19 is frequency shift keying (FSK). The tones utilized to encode the data are preferably at 392 Hz and 447 Hz, where a space equals 392 Hz and a mark equals 447 Hz. The secondary channel is transmitted SdB below the primary channel of the modem. Tones other than 392 Hz and 447 Hz may also be used.
A typical system configuration according to the preferred embodiment is illustrated in Fig. 2. As shown, the system central controller 15 includes a number of ports 1, 2, 3,...N. ~ number of modems, for example, from 1 to 254, can be associated with each of these ports l...N. Each central site port l,...N communicates with a central site modem 11, which in turn communicates with a number of remot~ site modems 13 across a four wire multi-drop line 27. These modems l3 are typically synchronous operating but may also be asynchronous types. The 1 to 254 modems can be either centrally or remotely located.
One of the modems 13 is illustrated communicating with a remote controller 29 and a digital mixer 31, which communicate with . .
a number of additional modems 33, 35. Typically, the ~nodems 33, 'j35 will be asy~chronous and are, separated by the remote controller ¦l29 from the synchronous modem 13. Central second tier modems 33 communicate across a four wire multidrop line 34 with remote second ''`` I' .:- Ij , . .

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tier modems 35. The digital mixer functions as an OR-gate 31 to provide a path around-the controller ~or the secondary telemetry channel~ In a typical set-up, the modems 13 would be synchronous, 2400 bps data sets and each modem 33, 35 would be an asynchrDnous 1200 bps data set. However, the synchronous 2400 bps data sets may also be used after the controller 29. The remote controller 29 is~
a standard controller such as might be used in a bank to buffer eommunieation between a number of terminals using modems 33, 35 and j the modem 13.
jj To faeilitate eommunieation, each modem 13, 33, 11, 35 ', , has a unique address. The test and eontrol unit 17 of each central site modem 11 reeeives its test and network eommands from the , , central controller 15 in a digital format. If the eentral modem , .
11 has been addressed, its appropriate response will be transmitted back to the central controller 15 by an asynehronous data stream, - Eaeh eentral site modem 11 performs a regenerative function to pass on eontrol commands to remote sites. The digital data re-eeived from the system controller 15 by the eentral modem 11 supplies a modulating signal to the ~requency shift key tFSK) seeondary ehannel transmitter 19 in the central site modem 11.
This transmitter 19 plaees the system eontroller eommands in , proper analog forma~ for transmission across a telephone line to the remote sites 13. The eentral site 11 also receives FSK
signals from the remote sites'l3 in analog ~orm across the tele-, phone line. The secondary receiver 19 in the central site modem11 demodulates the analog signal to pass an asynehronous bit stream to the system eontroller 15 at the FSK ehannel data rate~
i e.g, 75 bps. The,e~ntral site test and eontrol unit 17 does n~t ~' I

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monitor this received bit stream for its own address because it only receives commands from the system controller 15.
The test and control unit 17 at a remote site modem 13 receives its commands in analog format from the central site modem 11. The address command decode logic of the test and control unit 17 receives its commands from the FSK demodulator in the receiver 19. When a particular remote site 13 is addressed, it frames and formats its response to the command presented by the system controller in an asynchronous data stream. This data stream is applied to the ~SX modulator in the transmitter 19 where it is converted to an analog format for transmission back to the central site modem 11.
, The test and control unit 17 provides the regenerative capability for signals supplied across the digital interface created by the mixer 31. The test and control unit 17 in the modem 13, which is connected to the mixer 31, demodulates commands from the system controller 15 which are in analog format and converts these commands to a digital data stream, for example, at 75 bps. If the data stream is addressed to an asynchronous modem 33, the digital mixer 31 connects the data stream to the secondary channel input leads of all the associated asynchronous modems 33~ If one of the asynchronous modems 33 has been ad-dressed and must transmit a response back to the system controller 15, the regenerative function necessary for transmission will be .. . .
;accomplished in the synchronous higher speed modem 13. To ac--' complish this function, the mixer 31 OR~s the secondary channel ' received data from all of the asynchronous modems and supplies , that data to the secondary transmit data input of the synchronous I
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mOdem 13 in digital format. This digital data stream, for example, at 75 bps, is then connected to the FSK modulator in the test and control unit 17 of the modem 13 for transfer back to the system central controller 15.
In operation according to the preferred embodiment, a central site synchronous modem 11 will be in a continuous broad-cast mode on its secondary channel. When the system central con-troller 15 is idle, a 390 Hz tone corresponding to a marking con-dition is transmitted. In response to this marking condition, the secondary DCD (data carrier detect) line of each remote modem 13 will be activated (on). Also in response to the marking condition, the secondary channel transmitters of each modem 13 are set to be in a controlled carrier mode. These secondary channel remote transmitters at each synchronous modem 13, 19 are enabled upon any of the following conditions: (1) The site has been addressed by the central controller 15 and must respond with status or test re-sults (2) it is necessary for the remote site to send an alarm message back to the central controller of its own accord, or (3) secondary DCD from an associated asynchronous modem 33 turns on, 2Q indicating the necessity of transmitting informatiorl from the _ asynchronous portion of the network back to the central site.
Normally, all asynchronous central modems 33 are in a broadcast mode on their secondary channel, and all remote asynchro-nous modems 35 are in a controlled carrier mode on their secondary - 25 channel. In implementing this function, secondary DCD from the asynchronous modems will be OR-gated by the gate 31 to the synchro-nous remote modem 13 to serve as secondary RTS (request to send).
No connection like this in the opposite _ 9 _ ., ~3~

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direction is needed because the secondary channel of each ce~tral asynchronous modem 33 is in a continuous carrier mode.
According to the preferred embodiment, khree general modes of operation are provided. These include test modes wherein the test and control unit 17 performs status reportlng and testing functions in response to commands from the system controller 15; monitoring functions, wherein the test and control unit monitors for certain anomalous functions and supplies alarm or mayday messages back to the system controller; and performance of network control functions. Each of these opera-tions will be discussed in further detail below.
All test modes are remotely initiated and terminated by commands from the system controller 15. Of course~various .
sequences of commands to per~orm desired combinations of tests ! ;
may be provided by the system controller 15. These test modes range through various degrees o~ status checks to actual test runs.
As illustrated in Fig. 3, the format for the received test commands is a six word ~;equence, including a sync word (ASCII delete character;, synch word, address word 1, address word 2, command word, and block check character. Each word is eleven bi~s in length and include~ a start bit (logic 0), seven information bits, an even parity ~it and ~wo s~op bits (logic 1).
The seven bit delete character DEL is composed of all ones and serves to allow the receiver to synchronize to the asyn-chronous data.
If bit To is a logic zero, the message is a command from the system controller 15. If it is a logic one, the message is an acknowledgement from the test and control unit 17. The block i ..

--1 0-- . , check characJ;er COil tains a longitudinal even parity value ~enerated by computin~ ~he e~clusive OR ~or each bit posi~ion of the t~o address characters and the con~land char~cter. For example, the first bit of the block check character is the value obtained by computing even parity for Ao~ and C3. ~eplies from the test and control unit 17 have the same format as commands from the;
central controller 15 with the exception 'chat the first DEL~TE
character is replaced by a M~RX characte- (all 11 bits are logie l's). Replies are transmi'cted on a one shot hasis. As discussed , bclow, the command word is replaced by either one or three in-formation words. Maydays and concise status require one informa-tion word. ~xtended status and error counts require three informa-tion words. The test and control unit 17 will respond to a com-mand from the system controller 15 if the following conditions are met: (1) one DELETE character is detected; (2) an address decode is obtained; (3) parity is correct for each word, and (4) the block check character is correct.
For most of khe con~nancls~ the test and eontrol unit 17 provides an acknowledgemen-k bac}c to the system controller 15.
This acknowledgement consists of an echo of the original command with one eontrol bit '~ chan~ed in Address Word lo Conm~nds which are not echoecl are Squelch Primary Transmitter~ Simulate Power Fail, AnalocJ Loop, and all commands which require a response from the test and control unitu In addition to responding to its own address~ a particular test and,con-trol unit 17 has the ability to respond I,' to a group address (represented by all zeros)~ This address !
, addresses each modem on a given portO Ahy command can be used~
, ~3~$7 Return to nor~lal (Rlr~) clears an existin~ tcst or alar;n mode and s~ops the ~ransmission o~ all alarrn m~ssac3es. It further resets all storag~ regis-Lers in the test and control uni'c 17 and causes the associa~ed modem and iLS te~t channel to revert ~o normal operation. ~TN will also clear an existing RSI state~
However if the alarm condiJcion persists, the control unit will again transmit ano-ther mayday.
The return 'LO normal storage and inhibit (RSI) instruc-tion clears an existing 'cest mode~ stops the transmission of all alarm messages and causes ~che modem to revert to normal operation.
~lowever, the existancel and type of the particular mayday is saved.
RSI inhibits the transmission of an alarm even if the condition still persists, until it is reset b~ an RTN command. RSI may be used by the central control 15 to clear th2 net~Jork of alarm messages before the central controller institutes diagnostics to isolate a problem. RSI is also used in 'che situation where mayday messages are transmitted slmultaneously ~rom two or more locations~
; Simultaneous maydays result in the central contxoller receiving a noncoherent data stream with continuous paric~ and framiny errors. The controller will then trallsmit an RSI with a group address to all modems on thac line. ~11 mayday messages will be inhibited but their existence will be stored. The controller can then selectively address each modem on that line with a dump stored mayday command (DS~) to recover all of the alarm conditions which had previously occurred simultaneously.
The various test modes which may be entered and performed in conjunction with -the above corNmand format will now be discussed.
These include two t~pes of status checks, concise and expanded.
The test functions available include sel~ test, end to end test, analog loop with test pattern, digital loop with t~st pattern, ~nalog loop and dic;-ital loop~

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Tl~c ~irs-~ o:f the st~atus mo~ r moclcs is conci~e status mo;li.or. In this mode, '~ e ceiltxal con~rollcl. 15 scans a yroup or all of th~ modems in ~he systeM, sequentially addressing each modem with a transmit concise status (TCS) command. In response, each modem sequentially transmits back to the system controller 15 a concise status worcl. I'he concise status word has ~he format illustrated in Fig. 4. The concise status word contains the ~ollowing information:
(A) DCD - ON~OFF (remote) or RTS - ON/O~F (central) (B) DSR - ON/OFF
(C) M'E Power - ON/OFF~ The voltage level of the RTS
! lead from the data terminal equipment is continu-ously monitored. Specificâtion RS232C requires ~, I
that the voltage of any DTE interface lead be bet~een +3 and ~r25 vO~ ts or between -3 and -25 volts~ The status monitor yields an o~f condition for this parameker i~ the monitoring circuit senses a voltage level ~etween -3 volts and ~3 volts as an open circui.t~
(D) Modem in either Locally or remotel~ initated ,analog or digital loop - YES/NO
tE) Modem is either a Central Si.e or Remote Site (~or multipoint) RTS - ON/OFF (remote)~ DCD - ON/OFF (Central) ~point to point~ I
(F) Logic 1 indicates that modem is connected to dial lines and logic 0 indicates that modem is c~nnected to dedic2ted lines.

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f ., , , ~3~ 7 ~G) Si~nai Quali!-y - ON/OI~ ,n ON condition corres-pon~s ,o either ~he case where there exits a low probability for error on the primary channel ~GOOD QU.~LiTY) or DCD is off when .he signal quality lead is checkedO An OFF con~ition corres-ponds to the case where DCD is ON and signal ~uality is unacc~pta~le.
In the preferred em~odiment, the co~troiler does not evaiuate the ~; status reply response. A second -type of status monitor available is the expanded statusu In the e~panded mode9the system controller 15 transmits a Dump Modem Sta~us (DMS) command~ Receipt of this col~land by the test and control unit 17 will result in the trans-~mission of a three word status message~ The first status word is the same as for the concise status mode. The format for the second and t~ird words is as IOllOWS:
Status Word 2 .
Bit 0 - DCD (central or RTS (remote) - current state ~multipoint~
Logic 0 (point to point) Bit 1 - ~eceive Clock - transitiolls are occurring at the data rate of the modem Bit 2 - Transmit Data - current state Bit 3 - Receive Data - current state Bit 4 - CTS -- current state Bit 5 - Transmit Cloc~ - transitions are occurrin~ at leas~
at the data rate o-& the modem Bit 6 - Digital Loop - ~he modem is in ei.her a local or remote digltal loop modeO
`Bit 7 - Parit~

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St;~us ~orcl 3 Bit O - DCD Transi~ions ~centr~l multipoint3 - at least one one ~ransition has occurred since the last DMS command.
RTS Transitions (remote multipoint) - Logic O for point to pOillt Bit 1 ~ Not Used Bit 2 - Transmit Data Transition Bit 3 - Receive Data Transition , Bit 4 - CTS Transition Bit 5 - Modem Type Bit 6 - Modem Type 1~
Bit 7 - Parity A flrst of the possible modes perform~ble is the self test mode. ~his test can be performed on either a central or remote modem. ~hen placed in ia self test mode~ the transmit out-put of th~ modem is connected back to its receive input. Internal ;
RTS of the modem is turned on~ A pseud~ndm test pattern generator is connected to the modulator input and a pattern detector and error counter is conllected ~o the demodulator output.
Errors are accumulated for transmission back to the system con-troller 15 on the secondary channelO The secondary channel analog transn,itter is connected to the telephone line~ This connection is not made ~or the primary channel, which contains the test pattern~ Normal primary channel data traffic is affected only in that the transmission from the modem under test is inhibited. To perform ~his testO ~he system controller transmits in sequence a self test ~n~ble (STE) command~ an enable !

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' ' crror countex (E~C) command, a dump error CQUnt (DEC) command and a return ~o normal (~TN) co~nand.
In response to the ST~ com~nand, the test and control ;unit 17, causes ~he modem transmi-~ outpu~ ~,o be connected to its receive input, turns internal RTS of the modem on, and enables the test pattern genera~or and detector. The ~EC command resets the error counter and ini-tiates the accumulation of errorsO
The delay between the STE and EEC command providessufficient time for the scrambler and descram~ler to synchronize.
After the ST~ and EEC commands have been performed, I ~he DEC command initiates reply back to the central system con-troller 15. The s~stem cont~oller performs the timing function to determine the length of the self test run. An approximate error rate (errors in 10- or errors in 105 can then be computed by the system controller 15.
The format of the response of the test and control unit to a D~C col~nand is as follows: ;

Error Count ~ord 1 Bits 0 - 6 - Character error count - binary coded number which represents the number of parity or framing errors in all message~ received from the system icontroller since the last DEC command.
Bit 7 - Parity 1.

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Error Count Word 2 Blts O ~ 3 - L~w ~rder 4 bits of the primary channel test error count jj Bi~s ~ - 6 - Logic O's ~it 7 - Parity Error Count r~ord 3 , Bits O - 3 - EIi~h order 4 ~its of the primary channel j test error count Bits_~ - 6 - Logic O's Bit 7 - Parity ' A total of 8 bits in a binary coded format are utilized for the 1, primary channel test error count. Hence~ it is possible to count up to 255 test errors~ All error counts are reset to zero after j being reported. Character errors are tabulated for all messages received by a particular test and control unit whether or not that modem has been addressed. An incorrect block check character is counted as one character error. Proper framing for a received character is determined in the following manner.
(1) A M~ to SPAC~ transition is recognized as the be~Jinnin~ o~ a start bit.
(2) The center of the start bit is checked to see that it is still a SPACE. If it is notJ one character error is counted.

(3) The 9th bit after the start ~it is checked to determine il if it is a proper stop bit (MA~K)~ I~ this is not the case a frarning error is reco~nized and the character ', Il ~

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error counts~r is incremen~d by one.
The characts~r error count ~or a particular modcm in the s~stern provides an indica~ion of the guality of the secondary channel da~a received a~- that locat~on.
After the error information is transmitted back to the system controller 15, the controller 15 generates an RTN com~and.
Upon receipt of this RTN, the test is terminated and the modem reverts to normal operation.
A second type of test performable by the preferred embodiment is an end-to-end test between a central modem and a I remote modem. Such a test is a full duplex test with an error count obtained for the recei~er of each modem. Normal primary channel data traffic is inhibited during this test for all the modems branching off of a particular central site. As in the case of self test, the internal scrambler/descrambler of the modem is used to generate and detect the test pattern. To implement this test, the central s~stem controller 15 transmits in sequence the followinc3 commands: t~st pattern cnable (TPE), EEC, DEC, and RTN.
The s~stem control:ler 15 will sequentially send the TPE command first to the cen~ral modem and then to the remote modem. The TPE instruction enab~es the pseudo random pattern generator in the modem transmitter and the pseudo random pattern detector in the modem receiver~ Internal RTS of the modem is forced on. A delay is prGvided before -the next EEC command to allow the scramblers and descramblers in ~he two modems to synchronize. '~
! The other commands E~C, DEC~ RTN ~ cause the test and control unit to perform as discussed above. The EEC command , i~ I
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is scnt first to the central mo~em and then to the remote modem, as is the DEC con~andO
- Another form of test enabled by the subject preferred e~r~odiment is an analog loop with test pattern~ which is always conducted between a central and remote modem. This test is operator-controlled and may take advantage of the fact that the central system controller lS can have the network structure stored ! l in its data base. For example, the operator of the system controller 15 presses iD sequence first an analog loop key and then a test ~ey. Then, it is only necessary for the operator to key in the address of the remote modem. Since the system con-troller has the network structure stored in its data base, the address of the central slte will be implicit in the command. The system controller 15 can then address the necessary analog loop test commands to the proper siteu To perform the analog loop test, an analog loop command is transmitted by the system controller 15 to the remote modem and a test pattern enable command is transmitted to the central modem. The test and cont~ol unit is desicJncd such that even though placing the modem in an analo~ loop mode causes loss of carrier, a subsequent receive line fault mayday will not result.
The se~uence of commands utilized is discussed in the ~ollowing paragra~h.
In performing the analog loop test pattern mode, an analog loop (ACL) command is sen-t to a remote site under test.
The analog loop command causes the receive input of the modem under test to be connec~ed bac~ to its transmit output through a stage of gain. Next TPE is sent to the central site, enabling its scrambler and descran~leru An EEC command is also sent to the central site to initiate the accumulation of errors. Next DEC

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is sent to the central site where it elicits a reply, including the crror count at the end o~ the test in an 8-bit binary format.
RTN is then sent to all modems on the central line, utilizing the group address previously discussed. The sequence of commands emanating from ~he central controller during this test may be summarized as follows: !
(RMT ADD 3) (ACL), (CE~ ADD~ ~TPE), (CEN ADD) (TPE) ack. back to CSC 15 (CEN ADD) (EE:C), (CEN ADD) (EEC), ack. back to lS
(CEN ADD) (DEC), (C~N Al)D) (I:RI~OR COU21T), (GROUP ADD) (RTN) response to CSC 15 The repertoire of commands for the test and control unit includes a Stop Error Counter (SEC). Receipt of this command causes the accumulation o~ test errors to cease and the total to be stored.
jThis command is useful for modems which loop both the primary and secondary channels in an analog loop mde. This would result in the controller receiving echoes of its own channels. SEC
can be used to hold an error count while tlle modem is removed from an analog loop mode~
A digital loop with test pattern mode is also available.
~This test follows the same format as the previously described `
analo~ loop test~ Central site modems are not placed in a j digital loop mode. The test is conducted between a central site and a remote site. The scran~ler~descrambler and error count~r are enabled for the central modem~ r~he remote modem is placed in a digital loop mode. In this mode, received data becomes transmit ~' data, received clock becomes an external tr~nsmit clock, DCD beaomes ~rs. I
l!The DTE loop serves to isolate the DTE from the modem. D5R will be i ! of ~ at the interface to indicace to the con~oller tha~ a tesc is in pr~gress.j `, 20 . ' :

~ igital loop conu~ nd ~DCL) is fi~t sent to ~e r~m~e si-~ which is und~r ~st. TP~ is th~n sen-t to ~le c~tral site, followed by EEC, foll~Jcd by aftex the central system controller 15 has timed out the appropri-ate len~th of the error count. Finally, RTN is ~ransmitted by means of the group address~
A final test mode available is the ana]o~ loop, or digital loop mode wherein the scrambler and descrambler and error counter of the central modem are not enabled. Tlle central site modem operates in its normal mode. The test is performed accordin~ to the following sequence of commands: ACL or DCL to remote site under test, and RTN by group address. This test mode enables connecting external test sets to the central modem site.
As before noted, the test and control unit 17 has the ability to monitor its associated modem for certain anomalous conditions and transmit alarm messages back to the central controller 15. The format for the transmitted messages is the same as for the concise status message~ Each possible alarm condition is assigned a bit position in the mayday word~ Presence of the mayday is indicated by a one in the correspondirg bit position.
All other bits are logic O's. The alarm bit assignment is as follows:
Bit O - Customer Alarm Bit 1 - Streaming Bit 2 - Receive Line Fault Bit 3 - Modem Power Failure Bit 4 - Dedicated Line Not Restored Bit 5 -Bit 6 - O
Bit 7 - Parity A mayday m~ssago is continuously transmitted by the test ~L3~

and control unit 17 until it receives an RTN or RSI command from ,he central system controller 15. The RTN command stops transmission ol the mayday but if the cause of the alarm persists, further alarms will be transmitted. RSI will inhibit ~he trans-missiorl of an alarm, even if the cause still persists until reset by an RTN command. A single exception t~ this rule is the receive line fault mayday, which alarm message is controlled to exist for a fixed period, e.g. 12 seconds at 75 bps. As noted earlier, the RSI command will inhibit the transmission of alarm messages by the test and control unit 17, but the condition which !~
caused the alarm will be stored. With the network then free of alarms, diagnostic procedures can be undertaken either with the aid of the system controller 15 (usin~ the Dump Stored Mayday command) or by other procedures, to deten~ne the cause of the problem.
Thexe exists the possibility that multiple mayday messages may be transmitted simultaneously or that a mayday from one modem may be transmitted while another modem is responding back to the system controller 15 wil:h test or status information.
In either case, the result will be that the system controller 15 will detect f~aming errors on its receive data line. After a certain number of framing errors are counted, the system con~trol-ler will cease issuing test commands. If the framing errors per-sist, the system controller 15 will transmit an RSI command with a group address. This RSI inhibits all mayday messages ~rom the group, and causes their storage at the sites which have ex-perienced an anomalous conditions. The system controller 15 may then poll each site on the central line with a DMS command to dump the stored mayday status. Receipt of this command at a remote site , 1~3~`J~P~

which has stored a mayday messa~e results in retransmission of the mayday messa~e b~lck to the central system controller 15.
In this manner no mayday message will be lost. The following para~raphs describe mayday messages provided according to the preferred e~bodimellt of the invention.
If the request to send si~nal from the data terminal equipment DTE associated with a particular modem is held in an "ON" condition for an excessively long time, preventing other modems on a multidrop line from transmitting, the test and control unit 15 will send a streaming alarm (STR) back ~o the system controller 15~ An "excessive" period of time may be identitied according to a strap selection. Remote site test and control units 17 sense RTS being on for a prolonged period of time while central site test a~ control units sense DCD indicating the presence of carrier from a remote modem.
The central modem is always strapped for a longer stream time than its associated remote modem. In this way if the stream-ing condition is due to RTS being ON for a long period of time, the remote modem will always maydav first, and for a period of time there will be no multiple mayday signals. If a s~reaming mayday is only received Erom a central site, then it is known that the condition has been caused by a modem failure and not interfac~e RTS i being on for a long period of time. The test and control unit 17 transmits this alarm until it receives a RTN or RSI command from the system controller 15. RSI always s~uelches a mayday. RTN
will not, if the mayday condition is still present upon RTN receiptD
A customer alarm message (CAM) may also be provided in response to an extra input signal from the customer. In response to an ON condition, a mayday message is transmitted back to the system contr~ller. ~gain, the alarm is squel~hed by receipt of either RTN orRSI.

~.

:

e3r7 The failure to dctect a carrier on the primary data channcl of a remote modem or RTS being off at a central modem for a prolonged period of time will cause the test and con~rol unit 17 to transmit a receive line fault (RLF) mayday message bac~ to the central s~stem controller 15. This period of time may, for example, be 3.4 seconds. The RLF alarm is transmitted for a period, for example 8-13 seconds, and then squelched automatically by the test and control unit 17. It cannot ~e terminated by a command from the system controller 15 because the receive line to the modem has failed. ~fter the mayday times out, the alarm condition is stored and a DSM command again produces the mayday.
Only the RTN command can clear the stored mayday. The central . .
modem operatea in a continuous carrier mode and has its RTS on continuously. Its associated remote modem has DCD on continuously.
A remote modem transmits a RLF mayday if primary channel DCD is off for 3.4 sec. A central modem transmits a RLF mayday if RTS
is off for 3.4 sec. If a central modem failure occurs so that RTS is off, both the central and remote sites ~ill transmit rnay-days simultaneously and framing crrors will occur at the system controller lS. The DSM command can then be used to recover both mayday messages. If a telephone line failure occurs then only the remote modem will respond. lf a fault occurs on the 4 wire trunk from the central modem to the bridge from which lines to individual remote modems branch out, all modems associated with the central site will experience a receive line fault. This condition re-sults in the transmission of simultaneous multiple alarm messages back to the system controller l5. The exis~ence of simultaneous messages on line may prevent the system controller 15 from decoding a unique alarm. Only after the receive lineshave been restored can -2~-~L~3~

the DSM co~mand be used to determine which modems have p~eviously experienced a failure.
The test and control unit 15 utilizes an auxiliary power source to generate and transmit a tone whenever a modem power supply failure occursO This alarm is termed modem power failure (MPF). The central site modem of the port branch in which the power supply failure occurs detects the MPF tone and transmits an alarm message with its own address to the central eontroller 15. The eentral controller may then conduct a scan of the modems associated with the central modem whieh transmitted !;
the MPF mayday. The results of this sean are then analyzed by the eentral eontroller to determine whieh modem in the network has the failed power supply.
Since the only line of eommunieation between the eentral site modem unit 11 and the system controller 15 are transmit and receive secondary data channels, no power fail alarm signal is provided for these modems. A digital power fail alarm signal is generated for second tier central site modems 33. Thls digital alarm is neeessary beeause the interface between ,he two m~dems 13, 33 is of a digital nature. Hence, if a second tier c~ltral modem 33 e~-pericnces a power supply failure, it will present a digital alarm signal aeross the DTE interface to its associated modem 13.
This modem 13, which is actually a remote site modem of another eentral line will detect the digital power failure alarm eondition and will send an analog power fail signal with its address to the system controller 15. The digital alarm signal lead is bi-; directional. By strap selection it will be an output if thesynchronous modem is a eentral site moclem 11 and an input if the . . , . . , ~
-2~- ~

`
~31~ '7 , ' synchronous modem is a remote site modem 13.
Finally, a dedicated line not restored tDNR) message is supplied in a situation where a dial back-up connection has been made but the modem has been temporarily switched back to the dedicated line to determine if the dedicated line has been restored. If the modem has not received a transmit concise status (TCS) command from the system controller 15 within 10 seconds after a switch has been made to the dedicated line, a switch back to the dial line will be automatically initiated and the DNR alarm will be transmitted over the dial line. The purF~se of the DNR alarm is to indicate to the system controller 15 that the modem has been switched back to the dial line. The system,controller 15 could then transmit either an RTN or RSI
command over the dial line to squelch the alarm message as discussed above.
An additional feature of the test and control unit 17 is~, its ability to cause its associated modem to respond to certain network control commands which are generated by the central controller 15,. Thcse commands have the same format as the test commands prev:iously discussed. Commands provided according to the preferred embodiment are discussed in the succeeding paragraphs.
The squelch primary transmitter (SPT) command has been mentioned ' previously. In response to this command, the test and control unit causes the primary channel transmitter of the addressed modem' to be squelched and sets DSR at the DTE interface to an "OFF"
Ii state by forcing internal RTS of the modem to an off conditicn.
The SPT command is used when streaming is detected.

After the central controller supplies an RSI command, the next step is for the central controller lS to transmit an SPT command~
.

.'' ' ', , ' , -2~--Receipt of this co~nancl stops the st~e~ng condition and also causes DSR to drop. With DSR off, the D~E may turn its RTS
signal off. If this should occur, the cause of the streaming condition will have been removed. The system controller 15 may then send a dump modem status DMS command to check if RTS is now in an "OFF" condition. If dropping DSR does not cause RTS to be turned off, operator intervention at the remote site will be required to remedy the difficulty. The site with the streaming terminal will be temporarily inoperative. However, with the SPT command still in effect, the other sites on the central line can now communicate with the central modem.
il The SPT co~mand also provides a diagnostic tool in the situation when ~wo or more sites respond to the same primary chan-nel address, for example when a DTE is programmed or an incorrect address. The SPT command can be used to selectively squelch certain remote sites, using the secon~ar~ channel addressing SC~le~ . The operator at the central site may then determine which DTE is responding incorrectly.
An additional network comlnand which may be provicled is one to simulate power ~ailure, termed SP~. Upon receipt of this command, the tes~ and control unit 17 enables the power failure mayday circuits and causes either the power fail tone (remote site) or digital power failure pulse (central site) to be transmitted.
This command SPF can then be used as a test function to insure that the power failure circuits are operating properly.
The SPF command may also be used as an aid to the central controller 15 in checking the actual network configuration. In its data base, the system controller 15 may have the entire networ~;

I . j !

ll -27-` ' , !
:',`'. .

. 3 bJ ~ ~ 7 .. . .
configuration stored. Each remote modem is associated with a par-ticular central line, as already discussed. The accuracy of the configuration information stored by the system controller 15 may be checked by causing the xemote modem to transmit a po~er failur~
mayday and then monitoring which central modem 11 responds to the central controller 15. In this manner, one may detect if a particular remote modem is operating through the central modem which the system controller 15 thinks it isO
As illustrated in Fig. 5, the preferred er~odiment provides an advantageous, automatic dial back-up arrangement.
Figure S represents the first tier of the network shown in F~g. 2 ! Dial back-up is implemented by utilizing a well-knWn multiline adapter 71, a number of data access arrangments (DAA) 73, and their associated telephones and a dial back-up unit 77. The adapter 71 provides an AC bridge to connect the transmit and re-iceive line pairs of the central site modem to the associated re-; mote site modems. ~or the dedicated lines, the telephone company supplies this function by means of an AC bridge 75, usually located in the telephone company central switching office. The AC bridge 75 comrnunicdtes with a dial back-up unit 77 at each remote site, which unit 77 simply switches the remote modems between the dial and dedicated lines. Hence, each of the calls placed from the central site will be automatically answered at the , unattended remote site.
! When a failure occurs, it is necessary to place two phone calls to the remote site, one to each DAA 73. The null~er of remote sites which must be dialed depends upon the location of the telephone line fault. If a fault occurs between the central site rnodem and the telephone company AC bridge 75, all of the remote ~sites must be dialed. If a fault occurs on one of the lines from .. , i, ~ , .

.

.3.3 .

the bridse 75 to a remote site, then only that location must be called. For this case, the dedicated lines to the telephone company bridge 75 must also be connec~ed to the multiline adapter 71.
Once a remote site modem has been dialed, the system controller 15 is utilized to send a Switch to Dial Back-Up command to that modem. In response, the test and control unit 17 changes the state of a Dedicated/Dial control signal to the dial mode indication and hence, will be in "sync" with the state of the dial back-up unit 77 (Fig. 5). A check for possible restoration of the dedicated connection is then made by sending a switch to dedicated line (SDL) command to the remote site's test and control unit 17 over the dial lines. U~on receipt of this command SDL the test and control unit 17 will transrnit a control signal to the dial back-up unit 77 which will cause the dial back-up unit 77 to switch the modem to the dedicated lines. The test and control unit 17 contains a timer circuit which is enabled when the switch ~rom the dial lines back to the dedicated lines occurs. If the test and contrc,l unit 17 docs not detect a transmit concise status (TCS) colt~and on the dedicated channel within a ixed interval t for exarnple 3.4 seconds, the tes~ and control unit 71 will send a control signal to the dial back-up unit 77. This control signal causes the dial bac}c-up unit 77 to switch back to the dial lines. Over the dial lines, the test and control unit 17 will send a dedicated-line-not-restored tDNR) mayday bac~.
to the central controller.
I the dedicated line has been restored, an end-to-end test is conducted between the central and remote modems to determine if the line is of satisfactory quallty. If the error ` 29 , rate is satisfactory, the system controller 15 will transmit a disconnect dial back-up (DDB) command over the dedicated line to the remote site. Vpon receipt of 'his command, the test and control unit 17 sends a signal to the dial back-up unit 77 which causes it to hang up the dial lines. If the error rate, determined in the end-to-end test, is not satisfactory, the system controller 15 will transmit a switch to dial back-up (SDB) command over the dedicated line to the remote site. Upon receipt of this command, an appropriate control signal is transmitted from the test and control unit 17 to the dial back-up unit 77 to switch the trans-mit and receive lines of the modem to the dial lines.
If the test and control unit 17 is in the dial mode ¦ and it detects a receive line fault, it will transmit the required mayday and will generate a disconnect dial pulse. This is the I same pulse that is generated in response to a DDB command.
Receipt of this pulse by the dial back-up unit 77 will cause it to hang up the dial lines and switch the modem to the dedicated lines. If the dedicated line has not been restored, the operator at the site of the system controller 15 can again place the requixed calls to establish the dial back-up connection. If it were not for this protocol, it would not be possible to reestablish the dial connection because subsequent calls would encounter~a busy signal (dial back-up still holding the dial lines).
According to the preferred embodiment, it is possible to ;use the secondary channel as a data channel. For this purpose, the repertoire of commands lncludes an Inhibit Test and Control (ITC) command. Upon receipt of this command from the system controller 15, the test and control unit 17 will not monitor its . i, 1 _30_ r ;

~38~i7 received sccondary channel data for possible test and control con~na2~ds. l~ence, it will not inadvertently go into a test mode by decoding a command in a random data stream. If an alarm con-di~ion occurs while the moclem is in an ITC mode, the test and control unit 17 will clear this mode and will transmit the appro-priate mayday. A Return to Normal (RTN) command resets to normal operation when it is desired to remove it from an ITC mode. Use as a data channel is preferably subject to the following limita-tions in the preferred embodime~t:
1. No secondary CTS.
2. 4 Wire operation.
3. Only secondary RTS control. No reverse channel operation ¦ under control of primary RTS.

4. If the modem is a central site of a multidrop network, it must operate in a continuous carrier mode on the secondary channel.
¦ 5. If the modem is a remote site of a multidrop network, its secondary channel is operated in a controlled carrier mode but secondary DCD will not be present at the DTE interface. `
6. The data transmitted by the secondary channel cannot I include either the RTN or RSI commands. For a cent~ral ¦ site modem, if the received data is in a SPACING condi-tion for greater than 300 msec, it will be clamped at the DTE interface to a ~RK until a SPACE to MARK
transition occurs.
When cperated in a data mode, the secondary channel accepts 0-150 bps asynchronous data.

', ~3E~7 Figure ~ illustrates a particular st~ucture for a ~est and control unit accor~ing tv t~lc preferred embodimen~ of the invention. The test and control unit includes four multiplexcrs, 55, 57, 59, 61, a microprocessor central processing unit ~CPU) 63, and program storage unit (PSU) 65. The multiplexers 55, 57,59, 61 serve to double the number of possible inputs to the micro-processor C~U 63. ~ach multiplexer has eight inputs A , B , and four outputs, Y . Each of ~he multiplexers are controlled by - a select line 64 on which a control signal is outputted from , the PSU 65. ~hen the select lines 6g are activated (are a logic 1), the B inputs to the multiplexers are gated to the multiplexer outputs Yn, while if no select signal (the select lines are a logic 0) appears, the A inputs are gated to the outputs Y . Thus, the microprocessor 63, 65 selects the set of inputs it will need for a particular operation under program control. Thirty-two possible inputs to the microprocessor exist. The various input I signals may be level converted as necessary.
The signal on the Al input represents either primary RTS of a remote site, pri~ary DCD oE a central site,or a logic high ;l or low. If the modem is operating as a remote unit in a point-to-point con~iguration, the input is ~TS of the remote modem. If the modem is operating as a central unit in a point-to-point configuration, the input is DCD of the central unit. If the modem is operating as a remote modem in a multipoint network, the input is a high logic level, whereas if the modem is operating as a central unit in a multipoint network, the input Al is a low loyic level. The Bl input is a fixed low logic level, representing no input. The Yl output is Al/0 Thus, in a point-to-point con-figuration, RTS/DCD is saved for status purposes. Otherwise, . I , ~
ii , the Al input indicates whether the modem is a reMotc or central multipoint unit~
If the modem is opcrating as a rcmote unit, the A2 sisnal is DCD; and, if the modem is operating as a central unit, the signal is ~TS. For a remote unit, DCD should always be on, as should RTS ~or a central unit. An off condition at A2 thus indicates a receive line fault. The B2 input is cne bit of a speed select code, either a logic O or logic 1. The output Y2 is then a receive line fault signal or one bit of the speed select logic code. The speed select code is used to program the particular data rate at which the secondary channel of the system i5 to operate.
; The A3 input signal is primary DSR and the B3 input signal is the second bit of the speed select code, either a logic O or logic 1. The output Y3 is then either DSR or a second speed select bit. The inputs B2 and B3 thus supply a two-digit speed code upon proper selection by the select line 64 to the multiplex er 57.
The A4 input is a signal quality indication. The signal quality indication may be developed from primary DCD and the ; signal quality level produced by the associated modem. The modem signal quality indication is inverted and serves as an input to an AND gate. The other input to the AND gate is primary DCD, and the output of the AND gate is the A4 input. An off condition ;at the output of the AND gate indicates that DCD is on and the signal quality is poor. The B4 input is binary logic level, which serves as a one bit of a stream time code STL. The Y4 output is alternatively a signal quality indication or the STL bit.

' I ~
,,. I
~ ~ -33-., , 3~7 The ~5 input is primary ~TS from the data terminal equipment DTE. Primary RTS is prererably supplicd to the circuits of ~ig. 4 by a window comparator which m~nitors the voltage of the RTS lead circuit. An "OFF" condition is supplied to the A5 input if this volta~e is between + 3 volts or an open circuit. Thisindicates da~a terminal equipment power failure.
A strap may be provided to connect the primary RTS to a bias voltage supply in case a DTE not providing RTS is being used. The B5 input is the other bit of the stream time code STH. The Y5 output is then either an indication of whether the data terminal DTE has power, or is a second stream time code .
bit STH, depending on the state of the select iine 64.
The A6 input signal is a dial mode status bit. This bit indicates that the modem is operating either on a dedicated or a dial line. The B6 input signal is thefirst bit of a m~d~m type code. The Y2 output signal is then either,a.dial mode indication or a modem type indication.
The A7 input signal in a rem~te modem is a digital power fail pulse from an associated second tier central modem indication.
For central site modems the A7 input is demodulated received data.
This also serves as a power fail indication. If a remote modem, connected to a particular central modem, experiences a power fail-ure, then it will transmit a tone corresponding to a SPACING con-dition on the secondary channel. Detec~ion of the SPACING con-dition for a particular period of time causes the central modem to transmit a modem power fail mayday. The B7 input is the second bit of the modem type code. The Y7 output provides a power fail indication or a second modem type bit. The B6 and B7 ''inputs form a modem type code.

, ~.3~7 .

!
The A8 input is a customer alarm signal. This signal is provided by the modem user and may, for example, be a burglar alarm. The B8 input is a bi~ stream represen~ing the number of test errors occurring during a modem test. The test error signal may be provided by gating a test level with the receiver clock and supplying the result to the B8 input. The Y8 output is either a customer alarm or an error signal.
The Ag input is either DCD (OFF in test) in central multipoint modems or RTS in remote multipoint modems. In point-to-point modems, the A9 input is grounded. The A9 input serves to detect a streaming condition. If DCD or RTS for the central and remote modems, respectively, is continuously on for an in-ordinate period of time, a streaming condition is indicated. In point-to-point operation, no streaming condition is necessary ;
because no other modems would be interfered with. Therefore, in point-to-point, the streaming input is effectively disabled by the connection to ground. The B9 input is a first bit ~Do o~ the eight bit test and control unit address. The output Y9 is thus either a streaming indication or the first address bit.
The Alo input supplies a signal which indicates that the receiver clock is operating properly. This signal is developed by feeding the receiver clock to a retriggerable monostable multi-vibrator. The pulse width of the monostable is set such that if the receiver clock i5 at the proper frequency, a continuous pulse level is produced at the output of the monostable. The input signal to the input terminal Blo is the second bit of the test and control unit address ADl. The output signal Ylo is the recei~e clocking indication or the second address bit. The All input is primary channel transmit data of the modem.
, I
-3~5-, I

3~l3~'7 The input to the tcrminal Bll is the third bit of the ~ microprocessor address ~D2. The output Yll is either an indica-I tion of the s~ate of modem transmit data or the third address `~ bit A~2.
s The input signal to terminal A12 is the receivc data signal while the B12 input is the fourth bit in the test and control unit address AD3. The output Y12 is either the receive data state . or the fourth address bit AD3.
: ', The input A13 to the multiplexer 61 is the modem Clear to Send signal CTS, whose current state is monitored, and the I , input to terminal B13 is the fifth address bit AD4~ The output ' Y13 is then either the Clear to Send signal CTS or the fifth 'address bit AD4.
The input to A14 is a transmitter clocking signal. This ' signal is again pxoduced from the transmitter clock utilizing ,a retriggerable monostable circuit, as previously described for : the receive c:Lock. The B14 input is the sixth test and control ' unit address bit AD5. The.output Yl~ is an indication of the ; transmit cloclc operation or ~he sixth address bit AD5 The input A15 is an indication of whether or not the ' modem is in the digital loop test mode. The digital loop signal , is tapped from the digital loop control output of the micropro- l j cessor PSU 65. The input B15 is the,seventh test and control unit ' address bit ~D6. Therefore, the output Y15 is either the digital loop mode in~ication or the sixth address bit AD6.
' The final multiplexer input A16 provides an indication ~of whether the modem is :in the analog loop tes~ mode. This signal ' , is again tapped from the analog loop control signal at the output of the microprocessor PSU 65. The B16 input is the last test and -36~

control unit address bit ~D6. Therefore, the output Y15 is either the digital loop mode indication or the sixth address bit AD6 .
The final multiplexer input A16 provides an indication - of whether the modem is in the analog loop test mode. This siynal is again tapped from the analog loop control signal at the output of the microprocessor PSU 65. The B16 input is the last test and control unit address bit AD7. The output Y16 provides an indica-tion to the microprocessor of whether or not the modem is in the ;, analog loop test mode or alternatively provides the eighth and the final address bit AD7. The address bits ADl~ Ad2...Ad7, are selectively connectable to O and 1 logic levels to set the address of the test and control unit in any particular modem site.
Formatted data including commands are received by the ; micxoprocessor PSU 65 at a receive data input 64. The formatted ~data is then transformed by the microprocessor, as later discussed.
The microprocessor PSU 6S supplies a number of control signals to its' associated modem, as well as transmit and receive I signals. As discussed above, the analog loop and digital loop control signals cause the modem to perform either the analog loop or digital loop self tests. ~ dedicated/dial control signal con-trols whether the modem is connected to the dedicated or dial-up transmission line. This control signal provides automatic switching between the dial and dedicated lines. The break line loop control signal is activated in the self-test mode to disable the connection of the telephone line loop which normally occurs in the analog test mode. At the same time, the inverse of the break-time loop control signal RCC disables the primary 'I
channels test pattern signal from appearing on the telephon lines. ', jl . I
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~;~3~

The secondary channel may then be used to transfer the self-test results back to the modem~ The SPT control signal causes the primary transmitter to be squelched at proper times in response to a SPT command from the controller. The TPE control signal enables the test pattern genera~or and detector in the associated modem for particular test operations. The secondary channel trans-mit enable signal controls the activity of the secondary channel transmitter. The disconnect dial control signals is a 13 micro-second pulse, which disconnects the modem from dial lines.
Messages which are formatted for response back to the system controller are outputted in the proper format at the message out terminal. At reMote sites, data outputted fxom the message out channel is applied to the modulator for transmission on the secondary FSK channel. At central sites, the message out of the microprocessor PSK is OR ~ated with secondary channel receive data, which has been demodulated from a remote site, or its is the digital output of the central site test and control unit.
Finally, the SPF output control signal is a control signal which causes the apparatus to simulate a power ~ail for purposes of checking out the power failure circuitry.
This power fail circuitry is illustrated in Fig. 7.
As shown, the power fail sensing circuitry includes a power f~ail sensing relay driver 121, a relay 123, a capacitor 125, a power fail oscillator 127, and a low-pass filter 129. The power fail sensing relay driver 121 detects an AC power failure, for example, a blown fluse or a pulled plug. It also senses short circuit or open circuit conditions on any of the modem power supply Il . I
,, I

l! l l!
! ` -38 , . . .

1~3~C~

voltages and opens on the secondary powcr side. When a power failure is detected, relay contact Kl opens and relay contact K2 closes. The opening of Kl insures that the power fail oscillator and low pass filter are powered solely by the capacitor. When the capacitor discharges the oscillator stops and the power fail tone ceases. It is desirable that this tone be only transmitted on a one shot basis. The closing of K2 applies the tone to the output of the modem. The frequency of the tone corresponds to a SPACING condition on the secondary channel. Its duration is approximately 10 seconds. When the tone is emitted by a remote modem, the central site will detect the SP~CING condition for 5ix hundred milliseconds and will transmit a modem power fail mayday with its address. If the modem is operating as a central site modem, its power fail output must be in digital format. In this event, relay contact K2 applies the capaci~or voltage to a pulse generator which provides a digital power fail signal of approxirnately seven seconds duration to an associated remote modem as modems 13 and 33 in Fiy. 2.
Fig. 8 illustrates the preferrcd processing of the secondary channel receive and transmit siqnals. The receive line signal in analog form is first applied to a band-pass filter 91 with a center frequency o~ 420 Hz to separate the secondary channel from the main channel. The output of the band-pass filter 91 is sliced by a comparator 93 and supplied in digital format to the demodulator 95. In actual implementation, the FSK digital de-modulator 95 is preferably formed as part of the associated modem digital LSI circuitry. The output of the demodulator 95 is applied to a post filter 97, which is a low-pass filter centered .
, . .

` t at 130 ~Iz. The output of the low-pass filter is applied to a second comparator 99, the output of which is secon~ary channel - demodulated data. The secondary channel carrier is detected by ; a carrier detect circuit 101 which provides a positive indication when the level of the secondary channel signal exceeds a fixed threshold and which ouptuts a carrier detect signal to an AND
gate 103. The other input to the AND gate 103 is the output of the second comparator 99. The output of the ~ND gate 103 is jlsecondary channel received data which is supplied to the input !1 terminal A7 in Fig. 4 for power fail detection. The output of the demodulator is gated with carrier detect so that secondary receive data is changed to an off condition when the modem is not receiving secondary carrier. The output of the second comparator ~99 and a delayed form of the carrier detect signal are applied 'to a second AND gate 105. AND gate 105 accomplishes the same ilfunction as AND gate 103.
In remote modem, the output of the AND gate 105 is otuputted to the receive data input 64 of the microprocessor and to the secondary channel receive data O~ gate 108. In the 'l'central site mode, the output of the AND gate 105 is applied to a three hundred millisecond space inhibit timer 107 and from there, ~supplies a demodulated secondary channel receive data output l~through the OR gate 108 for the system controller 15. If the ,processor 65, 67 is at a central site, the output of the OR gate ~108 is the message output of the processor. The receive data to the microprocessor is supplied by the secondary transmit data ¦input 110 in the central site modem. The 300 millisecond space ¦linhabit timer limits the propagation of the power fail space ~tone to a single tier of the network.

1. I

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~ t rcmo~e sites, t~ message out of ~he microprocessor is ~pl~lied to a digital modul~tor 114, pre~crably on the modern LSI chip, and then to a band-pass filter 15 for transmission across thc transmission line channel. If the mod~m is a central site, digital data from the s~stem controller 15 is also applied to the modulator 114 through an OR gate 113 for transmission to remote sites.
The operation of the processor 65, 67 is illustrated in the Flow Diagrams 9-21. As shown in Fig. 9, the processor normally runs in an idle loop, monitoring various system parameters.
First, the processor tests the power failure indication and receive line fault indication and resets respective timers if no alarm condition has been detected. The processor then tests and saves transitions in relevant status bits. Next, the streaming indication is checked and a timer reset if no streaming condition is indicated. The next test in Fig. 9 is for data spacing. If data spacing is detected, subsequent reception of a character is indicated. When a start bit is detected, flags are set to indica~e character rece~tion. Once the st:art bit has been detected, a character wi:Ll then be received. Ncxt, if a DNR time-out is not in pro~ress, the DNR timer is reset to 2ero. If a DNR time-out is in progress, the timer is allowed to run and the next step is performed. In this step, the countin~ test errors condition is checked. If errors are being counted, and an error has occurred, a counter is incremented. The idle loop is then re-peated.
Every 3.348 mil:liseconds, an interrupt from idle occurs, as illustrated in Fig. 10. The time 3~348 milliseconds is set to enable sampling in the middle of each receive bit at the highest baud rate. Ak 75 BPS, four clocks per bit time are provided.

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The first test in the interrupt chain is to ascertain if a character is being received according to the flags set during the idle loop. If so,-the processor jumps to the routine indicated in Fig. 1~. If not, the alarm conditions in a temporary alarm storage register are cleared and a number of alarm conditions then monitored. The power failure time-out is monitored; and if a time-out has occurred, a power fail bit is set in the tem-porary alarm storage register. If not, the power fail timer is advanced and a test for receive line fault time-out is made. If a receive line fault time-out has occurred, the receive line fault bit in the alarm register is set and a check for dial mode is made. If the modem is in the dia~ mode, the processor instructs that the dial line be dropped. If a receive line fault time-out has not occurred, the receive line fault timer is advanced and the next test is made. In this test, the streaming time-out indication is monitored. If a streaming condition has occurred, th~ streaming bit in the temporary alarm storage register is set. If not, the streaming timer is advanced and the flow pro-ceeds to Fig. 11.
In the flow of Fig. 11, a test for a customer alarm condition is made, and the custoemr alarm bit in the temporary stora~e register is set if necessary. Next, a test for DNR
timing is made. If DNR is timed out, the processor sets the DNR bit in the alarm storage register and produces an output control signal causing a switch-back to the dial line.
Next, a flag is examined to indicate whether a message transmission is re~uired. If so, the flow of Fig. 15 is-per- ;
formed. If not, a test is made to indicate whether any alarms i -42- , ~3~

havo been sct If no alarms havc been set, the idcl condition is reentered. If alarms have becn set, it is necessary to send an alarm mcssage back to the Central System Controllcr 15, if the alarm condition is nct one which has been inhibited by a received RSI command. In such case, the ITC mode is cleared and the alarm ty~e is saved in a transmit buffer~ The program then proceeds to the flow of Fig. 14 for configuring a single data byte message. If the receipt of a charac-ter is detected according to Flows 9 and 10~ the receive process of Fig. 12 is entered.
This process is controlled by a programmable counter in the PSU 67, which resynchronizes upon the transi~ion which indicates .
,beginning of a start bit. The counter operation upon an input message is illustrated in Fig. 22. The first test in ~ig. 12 examines the count of the programmable counter to determine if it is time to detect a bit. If not, the RX routine is exited back to the flow of Fig. 10. If it is time for a bit, the next test is to detect if it is time for a start bit. This detection is made by utilizing a second counter which decrements on each sampling interval and is idle duriny stop bits. The count of 10 indicates a start bit and count of 1 indicates the last bit.
Sampling occurs at the mid-point of each bit time. If it is time for a start bit, a test is made to determine if the start bit is good, and an error is counted if it is not good. If it is not time for a start bit, a test is made to see if it is time for a stop bit. It it is time for a stop bit, the quality of the stop bit is again ascertained. If the stop bit is proper, a parity check is then made on the entire message word, and if the parity is good, the receive process is cancelled, saving l! .
, ' ,1 ' ' ~3~ 3 the new word in a receive stack. If it is not time fo~ a stop ~it, ~he new bit is sampled, detectedt and shifted into a regis-ter storins the current data word. The receive routine is then exited.
If a new word was saved in a stack according to the flo~ of Fig. 12, the flow of Fig. 13 is entered. The received characters are stacked successively with the first received character being shifted successively upward in the stack as ~additional characters are received. Thus, the first step in Fig. 13 is to check the top of the stack (the oldest character position~ to determine if it is a "DEL" character. If so, a I
'complete message possibly has been received. If not, the process enters the idle mode. If DEL has been detected at the top of the stack, the next test is to ascertain if the address is a group address. If not, the address is tested to ascertain if it is the address of the microprocessor under discussion, as set by the address bits ADo...AD7. If the addxess is improper, the processor returns to the idle mode. If the address is correct, the processor checks the Block Check Character (BCC~. If the Block Check Character is proper, the command number is saved for a later Acknowledge operation according to flow 14. The command is then examined to determine if it is a member of the permis~sible co~and set. If it i5 not, the process returns to the idle mode.
If the command is a proper command, a test is made to ascertain if the processor is in the ITC mode. If not, the processor branches to the commanded routine through a table, as indicated.
Fig. 14 illustrates the Acknowledge procedure. The command number saved during the flow of ~ig. 13 is placed in ' . .

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the data byte location of the transmit st~ck. Parity is then calcul~ted for the data byte, and the Block Check Code is generated and saved in the transmit stack. In the case of a single data byte message, a terminat~r is placed in the transmit stac~ to indicate a single data byte message. A pointer to the top of the stack is then set to indicate the next character to be transmitted, ; and the "transmit busy" flag is set. Setting of the transmit busy flag will cause the flow of Fig. 11 to branch to the transmit I
routine of Fig. 15. If a mar~ ~yte is to be sent, the start bit is set to mark. This causes a character to be transmitted ~,which substitutes a data mark bit where the start bit would normally occur. Parity for a character containing all ones is also a one (mark), resulting in a continuous mark-hold condition for the duration of one character time. This delays the second character, a sync byte, until the corresponding receiving circuit - can detect the mark hold and prepare to receive without missing any bits. The start bit is then reset to space to allow normal transmission of subsequent characters. The receive line fault counter is then loaded to time the sending of the RLF alarm and the processor returns to the idle mode.
When the processor enters the transmit mode, ~ig. 15, secondary channel carrier is turned on. A counter is again used ! to time the transmission of the message, which is stored in a transmit stack. Iirst, a test is made to determine if it is time to send a bit. If not, the idle mode is re-entered. The count is then examined to determine if it is time for a start bit. If yes, t~he start bit is sent. If not, a test is made to idetermine if it is time to send a stop bit, and if so, a mark ¦is sent. If it is not time for a start bi-~ or a stop bit, a .

test is made to determinc'if the end of the character being transmitted has been reached. If the end of a character has not been reached, a data bit is sent and the processor returns to the idle mode. If the end of a charac~er has been detected, the bit counter is reset, the transmit stack pointer is advanced, and a test is performcd to ascertain if the bottom of the transmit stac~ has been reached. If the bottom of the transmit stack has been reached, the carrier is dropped, the transmit busy flas is cleared, and the processor returns to the idle loop. If the bo~tom of the transmit stack has not been reached, the flow of Fig. 16 is performed.
In the flow of Fig. 16, if the end of the message has not been reached, the transmit bit counter is re-loaded and the flow of Fig. 15 is re-entered. If the message is at an end, ., a test is made to determine if'the message was an alarm message, ''which is repeatedly sent. If it was not, the flow of Fig. 15 .. . .
is re-entered at point 5. If the message was an alarm message, a test is made to ascertain if it was a receive line fault alarm, which times itself out. If not, the transmit stack pointer is set back to the top of the stack and the flow of Fig. 15 is re-entered at 4. If the alarm was a receive line fault, a test is ,1 '.:
made to determine if the time for transmission of the RLF has~
expired. Normally 10 seconds of transmission. Varies -~ two seconds depending on T7 buad rate. It so, the RLF signal is ~inhibited and the flow of ~ig. 15 is re-entered at point 5.
The flow of Fig. 17 indicates the response of the pro-cessor to commands which are acknowledged by the processor. In response to an RSI command, any alarms are loaded from temporary alarm storage to the mayday inhibit flags. In response to an RTN
command, all mayday inhibits are cleared. The SPT, TPE, BLL, ACL, , .

DCL, and any active alarm bits are cleared as well as ITC mode, (Not a bit) before returning to the Acknowledgement flow. The SDB command sets a bit which causes the modem to switch back to the dial back-up lines. The SEC command clears the error count flag. The DCL command sets the DC loop control bit, and the ACL
command sets the AC loop control bit. The ITC command sets a flag indicating the ITC mode. All of these commands then return to the Acknowledge flow of Fig. 14.
Fig. 18 illustrates the performance of the DEC and DMS
con~ands, which require the processor to configure a 3-data byte messaye. For the DEC command, the error counters are placed in ... . .
the transmit stack. For the DMS command, the status bits are collected and placed in the transmit stack. The flow then returns to the 3-byte message entry point of Fig. 14.
The flow of Fig. 19 illustrates the performance of the E~C, STE, and TPE commands. In response to the EEC command, the test error counter is cleared. The error count flag is set and the current error bit state i~ stored. In response to the STE command, the ~C loop and break line loop bits are set and the test pattern enable bit is set. Acknowledgement is then provided by reverting to the flow of Fig. 14.
Fig. 20 illustrates the SPF and SPT commands. It may be noted that this flow 20 returns to the idle state and no Acknowledgement is provided.
Fig. 21 illustrates performance of the SDL and DDB.
In response to SDL, a conlrol bit is set to cause the switch to dedicated lines and the DNR flag is set to start time-out for ,the DNR mayday. The next step in responding to the SDL command ~, ' I
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and the only step in responding to the DD~ command is to drop the dial line and return to the Acknowledgement routine.
As may be appreciated the many features of the just described preferred embodiment are subject to numerous modifi-cations and adaptations without departing from the scope and spirit of the invention. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described above.

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Claims (19)

DIV III

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a data communication system including a plurality of data modems each at a remote site, the diagnostic apparatus at each said remote site comprising:
means for providing a first plurality of signals from which malfunctions in said data communication system may be detected;
means supplied with said first plurality of signals for repeatedly testing said signals and responsive to said tests for detecting said malfunctions; and means responsive to detection of a said malfunction for generating a second plurality of signals, each indicative of the occurrence of a said malfunction.

2. The diagnostic apparatus of claim 1 further including means for generating a message including the location and type of a detected malfunction and transferring said message out of said diagnostic apparatus.

3. The diagnostic apparatus of claim 2 wherein said message is transferred to other apparatus within said modem.

4. The diagnostic apparatus of claim 2 wherein said message is transmitted through a secondary channel to a central location in said data communication system.

5. For use in a data communication system including a plurality of data modems, the diagnostic apparatus comprising:
means for providing a plurality of first signals from which faults in said data communication system may be detected, said faults including an indication of a failure in a said data modem and an indication of a streaming condition;

means contained within a said data modem and supplied with said first signals for automatically detecting from said first signals the occurrence of one or more of said faults and for generating a message including infor-mation indicative of the type of fault and the location of the fault in the data communication system; and means for transmitting said message to a central site in said data communication system.

6. The apparatus of claim 5 wherein said means supplied with said first signals performs said automatic detection under control of a stored program.

7. The apparatus of claim 6 wherein said first signals are available within said data modem or at the interface of said data modem.

8 For use in a data communication system including a plurality of remote sites and at least one remote site modem loca-ted at each remote site, a method of performing remote site diagnostics comprising the steps of:
providing a plurality of first signals from which malfunctions in said data communication system at a remote site may be detected;
monitoring said signals at said remote site to automatically detect said malfunctions, said monitoring and detecting being performed at least in part under control of a program stored at said remote site; and generating a message signal containing information indicating that a malfunction has occurred in the data communication system.

9. The method of claim 8 wherein said message includes information indicative of the type of malfunction which has occurred.

10. The method of claim 8 wherein said message includes information indicative of the location of the malfunction.

11. The method of claim 10 wherein said message further includes information indicative of the type of malfunction which has occurred.

12. The method of claim 11 further including the step of transmitting said message over a secondary communication channel to a central location in said data communication system.

13. In a data modem, adapted for use as part of a data communication system including a central controller, the method of performing data communication system diagnostics comprising the steps of:
providing a plurality of signals from which a plurality of alarm conditions may be detected;
monitoring said plurality of signals under stored program control, autonomously of said central controller;
detecting alarm conditions from the monitored signals under stored program control, autonomously of said central controller;
storing digital indications of the presence or absence of said alarm conditions; and responding to the detection of an alarm condition by forming a message indicative of the occurrence of a said alarm condition for transfer to selected apparatus in said data communication system.

14. The method of claim 13 wherein said message includes information indicating the type and location of said alarm condi-tion.

15. A data communication system including:
a plurality of data modems each at a remote site, a data terminal communicating with each remote site modem, at least one central-site modem, a central site processor communicating with said central site modem, said central-site modem and remote-site modems being adapted for communication with one another over a transmission medium; and diagnostic apparatus at each said remote site comprising:
means for providing a first plurality of signals from which malfunctions in said data communication system may be detected;
means supplied with said first plurality of signals for repeatedly testing said signals and respon-sive to said tests for detecting said malfunctions, and means responsive to detection of a said malfunc-tion for generating a second plurality of signals, each indicative of the occurrence of a said malfunc-tion.

16. The diagnostic apparatus of claim 15 further in-cluding means for generating a message including the location of a detected malfunction and transferring said message out of said diagnostic apparatus.

17. The diagnostic apparatus of claim 16 wherein said message further includes the type of detected malfunction.

18. The diagnostic apparatus of claim 16 or claim 17 wherein said message is transferred to other apparatus within said modem.

19. The diagnostic apparatus of claim 16 or claim 17 wherein said message is transmitted through said transmission medium to the central site processor in said data communication system.