CA1138999A - Communication system with primary and back-up transmission lines - 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

:1:3L3B999 i 1 1141 MOI~]'M I~J7\.(;NO.';'rIC
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B~(~ICGROUND OF THE INVE:NTION
i ; The subject inVen~iGn relates generally to automatic I ne~work diagnostic systems and more particularly to a system .j 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 ~apparatus and their associated data modems, the need for testing land control o~ the data modems has increased. The complexity of present and proposed systems requires the ability to communicate ~rapidly with modems at diverse and numerous sitesO Modem mal- ¦
. .functions become increasingly critical in that one malfunctioni~g .. ~ I modem may interrupt transmission by many othe~s in the networ~.
¦ Since even very small amounts of down time can mean big dollar ¦ losses in distr.ibuted processing systems, the need ar.ises to ¦automatically co~trol modems in a distributed ~ystem to minimize ~I time losses. To provide efficient and efective operation, it ~ would be highly desirable to provide the modem with the capabil ty2~ to raise alarm ~ignal~ to a central co~troller and perform the network reconfiguratio~ and control function~ necessary to respond to var~ous trouble conditions. Both for speed a~d rel~-ability, it i8 desirable to have a~ many of these ~unction~ as - possible performed automatically by apparatu~ of the sy~tem.

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SUMM~RY OF THE INVENTION
It is, therefore, an ob~ect of the invention to pro-vide an improved method and apparatus for monitoring, testing, and controlling modems at diverse physical locations.
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 - 10 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 operationO The test and control unit may also detect parameters indicating abnormal operation or mal-functions, and send "mayday"-alarm signals 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 modemO Control over the network configuration is also achieved. The systam of the - invention is applicable to multi-tiered networks.

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In accordance w~th one aspect of the invention there is provided in a data modem adapted to be switched between backup and primary transm.ission lines and adapted to receive command signals from a central controller, the apparatus comprising:
: means responsive to a first command from said ~entral controller to switch from said back-up line to said primary line for timing an interval during which a second command is expected to be received from said central controller and for switching said modem back to said back-up line if said second ; command is not received during said interval.
Further in accordance with the invention there i~
provided a communication system comprising:
a data modem adapted to be switched between,back-up and primary transmission lines;
Y a central controller me~ns for communicating commands over either of said transmission lines to said modem, said : commands including a first command to switch said da~a modem from a back-up line to a primary line; and . 20 means at said data modem responsive to said first command, for timing an interval during which a second command is expected to be received from said central controller~ and for switching ~aid modem back to said back-up line if said second command is not received during said interval.

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BRIEF DESCI~IPTION OF TIIE DR}~WINGS
~ The preferred embodiment and beat mode contemplated for i practicing the just summarized invention will now he de~cribed in conjunction with the drawings of w~ich:
FIGURE 1 is a generalized ~lock diagram of a portion of a 6ystem conigured accoxding to the preerred embodLment of the .invention.
. FIGURE 2 i~ a block diagram of a two-tier modem ayatem ~; incorporating the preferre.d embodLment of the invention.
FIGURE 3 illu~trates the dat~ format for transmitting a command according to the preferred em~odiment of the in~ention.
FIGURE 4 illustrates the format of a conci~e ~tatus word.
FIGURE 5 is a block diagram of a dial ~ack-up techni~ue of the preferred embodLment.
FIGURE 6 is a circuit diagram of a test and control unit accordïng to the preferred emhodiment.
FIGURE 7 i$ a circu;t di.agram of receiYe-tran~mit cir-cuitry of the preferre.d em~odLment.
FIGURE 8 i5 a circuit diagram of the po~er fail ~etect .. . .
circuitry of t~e preferred e~fiodLment.
FIGURES 9 - 21 are flo~ diagram~ illustrating th~
operatlon of t~e. te~t and control unit.
FIGURE 22 illu~;trate~ the receive function of t~ te~t and control unit.

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~L3~g9~ , DET~ILED DESCRIPTION OF TIIE
PREFERRED EMOBDIM~NT OF THE INVENTION
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Fig. 1 illustrates in functional block form a portion of a system according to the preferred embodiment of the invention . -providing remote test capabilities for modems. Such mod~ms may -;be, for example, in four wire controlled carrier multipoint orcontinuous 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
PDP-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 across a secondary channel by the central modem 11~ The central modem 11 contain~ a .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. A~ discussed later, the test and control unit 21 is pre~erably configured around a microprocessor, which, by way of example, may be a Fairchild F8 CPU and PSU. The test and control unit 17 decodes addresses and Fommands from the systçm controller ;15~performs a ~pecific test if addressed~ frames and formats the test re~u~ts, and transmits these results back to the system con-,.
troller 15. Some tests arq a~complished without interfering with normal network operation while other tests temporarily interrupt portions of the network~
¦l In addit~on to responding to certain te8t8 initiated by ithe controllex 15, the test and control unit ~ense~ certain ~ PDP-ll is a trademark I . . .
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anomalous conditions in the modem 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 pre~exred 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 da~a rate such as 75 bits per second. The modulation technique utilized by the secondary transmitter and receiver 19 is frequency shi~t 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 5dB below the primary channel of the modem~ Tones other than 392 Hz and 497 Hz may also be used.
A typical system configllration 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. A numher 'of modems, for example, from 1 to 254, can be ;-ssociated with each of these ports l~..N. Each central site por~ I,...N communicates with a central ~Lte modem 11, which in turn colTImunicates with a ~' number of remot~ si,te modems 13 across a f'our wire multi- !
drop line 27. These modems,l3 are typically ~nchronous operating but may also be asynchronous types. The 1 to 2r~4 modems can be either centrally or remotely locatqd.
One of the modems 13 is illustrated coll~unicating with a i remote controller 29 and a diyital mixer 31, which communicate with a number of additional modems 33, 35. Typically, the modems 33, ! 35 will be asyr~c}lronous and are, separated by the remote controller I
29 from the sync~ronous modem 13~ Central secon-,i tier modems 33 communicate acro~;s a four wire multidrop line 34 ~tith remote second i -6- 1 ,, . , , ~ , .

1/ 113~999 tier modems 35. The digital mixer functions as an OR-gate 31 to provide a path ~round-the controller for 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. ~owever, the synchronous 2400 bps data sets may also be used after the controller 29. The remote controller 29 i5, a standard controller such as might be used in a bank to buffer t communication between a number of terminals using modems 33, 35 and the modem 13.
To facilitate communication, each modem 13~ 33, 11, 35 has a unique address. The test and control unit 17 of each central site modem 11 receives its test and network commands from the central controller 15 in a digital format. If the central modem 11 has been addressed, its appropriate response will be transmitted back to the central controller 15 by an asynchronous data stream~
Each central site modem 11 performs a regenerative function to pass on control commands to remote sites. The digital data re-ceived rom the system controller 15 by the central modem 11 supplies a modulating signal to the frequency shift key (FS~) j secondary channel transmitter 19 in the central site modem 11.
This transmitter 19 places the system controller commands in proper analog format for transmission across a telephone line to the remote sites 13. The central site 11 also receives FSK
signals from the remote sites l3 in analog form across the tele-phone line. The secondary receiver 19 in the central site modem 11 demodulate~ the analog signal to pass an asynchronous bit stream to the system controller 15 at the FSK channel data rate, e.g. 75 bps. ~he c~ntral site test and control unit 17 does n~
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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 remot~ site 13 is addressed, it frames and formats ltS response to the command presented by the system contr~ller in an asynchronous data stream. This data stream is applied to the FS~ modulator in the transmitter 19 where it is converted to an analog ~ormat for transmission back to ;
the central site modem 11.
The test and zontrol 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. I~ 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 necessa~y 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 tran~mit data input of the ~ynchronous , 1. 1.
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modem 13 in digital format. This digital data stream, for example, at 75 bps, is then connected to the FSX 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 broadcast mode on its secondary channel. When the system central , controller 15 is idle, a 390 Hz tone corresponding to a marking condition is transmitted. In response to this marking condition, the secondary DCD (data carrier detect) line of each remote modem 13 will be acti~ated ~on). .~150 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 ad-dressed by the central controller 15 and must respond with status or test results ~2) it is necessary for the remote site to send an alarm message back to the central controller of its o~n accord, or (3) secondary DCD from an associated asynchronous modem 33 turns on, indicating the necessity of transmitting information 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 asyn-chronous modems 35 are in a controlIed carrier mode on their secondary channel. ~n implementing this function, secondary DCD from the asynchronous modems will be OR-gated by the gate 31 i~ to the synchronous remote modem 13 to serve as secondary RTS
j (request to send)~ No connection like this in the opposits .
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direction is needed because the ~econdary channel o~ each central asynchronous modem 33 is in a continuous carrier mode.
According to the preferred embodiment, three general modes of operation are provided. These include test modes wherein the test and control unit 17 performs status reporting and testing functions in response to commands from the system ,. . I
controller 15; monitoring functions, wherein the test and control i. i , 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. OE course~various ~ i sequences of commands to perform desired combinations of tests may be provided by the system controller 15. These test modes range through various degrees of status checks to actual test runs.
As illustrated in Fig. 3, the format for the received test commands is a six word se~uence, 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 bits in length and includes a start bit (logic 0), seven information~bits, an even parity bit and two stop bits (logic 1). t 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 l5o If it is a logic one, the message is an acknowledgement from the test and control unit 17. The block !

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chec~ charac~er con~ains a longitudinal even parity value senerated ~y computing ~he exclusive OR for each bi~ position of the two address characters and the command char~ct~r. For e~ample, tlle first bit of the block check character is the value o~tainea by computing even parity for A~, A~, and C0. Replies from the test and control unit 17 have the same ~ormat as commands Erom the central controller lS with the exception that the firs~ DEL~TE
character is replaced by a MARK charac~er (all 11 bits are logic l's). Repliesare transmitted on a one shot basis. As discussed below, the comrnand word is replaced by either one or three in- i formation words. Maydays and concise status require one informa-tion word. Extended status and error counts reguire 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 obta:ined; (3) parity is correct for each word, and (4) ~he bloc~ check charactcr is correct.
For most of the col~nands, the test and control unit 17 provides an acknowledyemcnt back to the system controller 15.
This acknowledgement consists of an echo o~ the original command with one control bit To changed in Address Word 1~ Comn~nds which are not echoed are Squelch Primar~ Trans~nitter~ Simulatc Power Fail, Analog Loop~ and all cor~nands which require a response ~rom ' the test and control unit.
In addition to responding to its own address, a particular test and control unit 17 has ~he a~ y to respond to a group address (represented b~ alï eros)~ This address addresses each modem o~ a given port~ Ahy comrnand can be used.

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, Return to normc~l (r~rrN) clcars an existi~ cesic or alarm mode and s,ops ~ne transll-ission of all al~rm mcss~ges. Xt ~ur~her resets all storacJe re~isters in thc test and control unic 17 and causes the associated modem and its test channel to rev2rt to normal operation. ~TN will also clear an existing RSI s.ate.
However if ~he alarm condition persists, the control unit will again transmit another mayday.
The return to normal s.orage and inhibit (RSI) instruc-tion clears an existing test mode~ s'cops the transmission of all alarm messages and causes the modem to revert to normal operation.
~Iowever, the existancel and type of ~he particular mayday is saved.
RSI inhibits the transmission of an alarm even if the condition still persists, until it is reset by an RTN command. RSI may be used by the central control 15 to clear the network of alarm messages before the central controller institutes diacJnostics to isolate a problem. RSI is also used in the situation where mayday messages are transmitted simultaneously rrom ~wo or more locations.
Simultaneous maydays result in the central controller receivinc~
a noncoherent data stream with contin-lous parity and framiny errors. The controller will then tr~nsmit an RSI with a yroup address to all modems on that line. All mayday messages will be inhibited but their exis'~ence will be stored. The controller can then selectively address each modem on that line with a dump stored mayday command (DSM~ 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 ~he above command format wlll now be discussed.
These include two t~pes ol status chec~s~ concise and expànded.
The test functions available include sel~ test, end to end -test~ !
~analog loop with test pat~ern~ dic~ital loop with tes-t pattern~-~analog loop and disital loop~

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The rirst or ~he sta~cus monicor modes is concise s'ca'cus monitor. In this mode, the central con~roller i5 scans a ~roup or all of the modems in ~he sys~em, sequelltially ad~ressing each modem with a transmi~ concise s~atus ~TCS~ cornrnand. Xn rcsponse, each modem sequenLially transmits back to the system con-troller 15 a concise status word. The concise sta~cus word has the format illustrated in Fig. 4. The concise status word contains the ~ollowing in~ormation: . .
: i (~) DCD - ON/OEF ~remote) or RTS - ON/OFF ~central) : (B~ DSR - ON/OFF
(C) DTE Power - ON/OF~'. The voltaye level of the RTS
! lead from thc data terminal equipment is continu-ously monitored~ iSpecification RS232C requires that the ~oltage of any DTE interlace lead be between ~3 and ~25 volts or bet~Jeen -3 and -25 volts~ The s-tatus monitor yields an off condition for this parameter if the monitoring circuit senses a voltage level between -3 volts and ~3 volts as an open circuil..
(D) ,Modem in either Locally or rernotely initated analog or digital loop - YES/NO
(E) Modem is either a Central Site or Remote Site ~or multipoint~
RTS - ON/OFF (remote)~ DCD - ON/OFF (Central) ~point to point~
(F) Logic 1 indicates that modem is connec-ted to dial lines and logic 0 indlc~'ces that modem is connected to dedi.cated linesO

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(G) Signai Quali~y - ON/OFF. An ON condition corres-ponds LO ei~her the case where ~here exits a low pro~ability for error on th~ primary channcl (GOOD Q~ALITY) or DCD is orf when the signal quality lead is checked. ~n OF~ condi~cion corres ponds to the case where ~CD i5 ON and signal quality is unacceptable.
In t~e preferred embodiment~ the controller does not evalu~te the ;status reply responseO A second t~pe of status monitor available is the expanded status. In the expanded mode~the sysbem controller ;

15 transmits a Dump Modem Status (DMS) command. Receipt of this 1~ i command by the t~st and contro] unit 17 will result in the trans-mission of a three word status messaye. The first status word is the same as for the concise status mode. The format for the second and t~ixd words is as follows:
Status Word 2 Bit 0 - DCD (central or RTS tremote) - current state ~multipoint~
Logic 0 (point to point) Bit 1 - Receive Clock - transitions are occuxriny 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 Clock - transitions are occurring at least at the data rate of the modem Bit 6 - Digital Loop - the modem is in either a local or remote digital loop mode.
Bit 7 - 2arity . 1, i . ' `' ' .
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S~a~us Word 3 Bit O - DCD Transitions (central multipoint) - at least one ; one transition has occurred since the last DMS command.
RTS Transitions (remote multipoint) - Logic O for point to point ` Bit 1 - Not Used .
Bit 2 - Transmit Data Transition Bit 3 - Receive Data Transition ~--Bit 4 - CTS Transition .~ I
Bit 5 - Modem Type ~it 6 - Modem Type , !
Bit 7 - Parity A first of the possible modes performable is the self test mode. This test can be performed on either a central or . i remote modem. ~hen placed in a 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 pseudor2ndQm test pattern generator is connected to the modulator input and a pattern detector and error counter is connected to the demodulator output.
Errors are accumulated for transmission back to the system con-troller 15 on the secondary channel. The secondary channel . .; ' !
analog transmitter is connected to the telephone line. This connection is not made for 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 this test, thè system controller ;~ transmits in sequence a self test enable tSTE) command~ an enable , .~ .
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error countçr (E~C) command, a dump error count (D~C) command and a recurn to normal ~RTN) co~mand.
In response to the STE co~ and, the test and control unit 17, causes ~he modem transmit outpu~ to be connected to its receive input, turns internal RTS of the modem on, and enables the test pattern generator ancl de~ector. The EEC command resets the error counter and initiates the accumulation of errors~
l~he delay between the STE and EEC command providessufficient time for the scrambler and descrambler to synchronize~ .
After the STE and EEC cor~nands have been performed, l'the DEC command initiates reply back to the central system con-troller 15. The system cont~oller performs the timing function to determine the length of the sel. '~est run. An approximate error rate (errors in 106 or errors in 105 can then be computed by the.system controller 15.
The format of the response of ~he test and control unit to a DEC command is as follows:

Error Count Word 1 Bits 0 - 6 - Character error count ~ binary codcd number which represents the number of parity or framin~
errors in all messages received from the system controller si.nce the last D~C command.
Bitl - Parity il .
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Error Count Word 2Bits 0 - 3 - Low order 4 bits o~ the primary channel test error count j Bits 4 - 6 - Lo~ic 0's Bit 7 - Parit~
I~ Error Count Word 3 -Bits 0 -_3 ~ High order 4 bits of the primary channel .! test error count Bits ~1 - 6 - Logic O ' s , Bit 7 - Parity j A total of 8 bits in a binar~ coded format are utilized for the primary channel test error count~ Hence~ it is possible to count . ; up to 255 test errors. ~11 error counts are reset to zero after being reported. Character errors are tabulated for all messages received by a particular test and control unit whether or not that modem has been acldressed~ An incorrect block chcck character I is counted as one character error~ Proper framing for a received : I character is determined in the following manner~
(1) A M~ to SPACE transition is recognized as the ' beginninc~ of a start bit~ ¦

(2) The center of the start bit is checked to see that it is still a SPACE~ If it is.not, one character error is j counted~

(3) The ~th bit after the start bit is checked to determine if it is a proper stop bit. (~ K~ If this is not the case, a framing errox is recognized and the character ;

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error counter is incremented by one.
Thc character elror count for a particular modem in the system provides an indica~ion of the quality of ~he secondary channel data received at that location.
After the error information is transmitted back to the system controller 15, the controller 15 generates an RTN command.
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 remote modem. Such a test is a full duplex test with an error count obtained for the receiver 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 i modem is used to generate and detect the test pattern. To implement this test, the central system controller 15 transmits in seguence the following commands: test pattern enable ~TPE), .. ~ I
EEC, DEC, and RTN.
The system controller 15 will sequentially send the TPE command first to the central modem and then to the remote modem. The TPE instruction enables the pseudo random pattern ~enerator 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 provided before the next EEC command to .
allow the scramblers and descramblers in the two modems to synchronize.
¦ The other commands EEC, DECd ~TN , cause the test and control unit to perform as discussed above. The EEC command ,, . -18 ' :

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is scnc first to the central modem and then to the remote modem, as is the D~C command.
Another ~orm of tes~ enabled by the s~bject preferred embodimcnt is an analog loop with test pattern, which is always conducted between a cenLral and remote modem. This test is operator-controlled and may take advantage of the fact that the central system controller 15 can have the network structure stored in its data base. For example~ the operator of the syste~
controller 15 presses in sequence first an analog loop key and then a test key. T'nen~ it is only necessar~ 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 site will be implici~ in the command. The system controller 15 can then address the necessary analog loop test commands to the proper site~
To perEorm 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 control unit is designed 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 foll~wing paragraph.
In perEorming the analog loop test pattern mode, an analog loop (ACL) command is sent to a remote site under test.
' The analog loop command causes the receive input of the modem under test to be connected back to its transmit output through a stage of gain. Next TPE is sent to the central site, enabling I its scrambler and descran~ler~ ~n EEC command is also sent to I the central si~e to initiate the accumulation of errors. Next ~EC ' Il . , . ~ , .

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is sent to the central site where it elicits a reply, including the crror count at the end of the test in an ~-bit binary format.
RTN is then scnt to all modems on t7ne central line, utilizing the group address previously discussed. The se~uence of commands emanating ~rom the cen~ral controller during this test may be summarized as follows:

(R~5T ADD 3) (ACL), (CE:N ADD) ~TPE), (CEN ADD) (TP~
ack. back to CSC 15 (CEN ADD) (EEC), (CEN ADD) (EEC), ack. back to 15 (CI::N ADD) (DEC), (CEN ADr~ RROr~ COUNT), (GROUP ADD) (RTN) respo~se 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 of test errors to cease and the total to be stored.
'This 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 o~ its own channels. SEC
can be used to hold an error count while the rnodem is removed from an analog loop mode.
A digital loop with test pattern mode is also available.
'~ This test ~ollows the same format as the previously described `
analog loop testu Central site modems arc not placed in a digital loop mode. The test is conducted between a central site and a remote site. The scrambler~descrambler and error counter are enabled for the central modem. I`he remote modem is placed in a digital loop mode. In this mode, received dat~ becomes transmit data, received clock becomes an extiemal tr ~ mit clock~ DCD bccomes R~S.
I The DT~ loop serves to isolate the DTE from the modem. DSR will be i off at the interface to indica~ to the con~roll~r ~hat a tes~ is in prc~r~ss.

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113~9 A cli~ital loop commalld (DCL) is fi~t sent to ~he rcmo~e sit~ which is under ~st. Tl~ is thellsent to ~le ccntral site, followed by E~C, follo~ed by DES
a~ter the central system con-troller 15 has timed out the appropri~
ate length o~ the error count. Finally, RTN is transmitted by means of the group address.
A final test mode available is the analog loop, or digit~1 loop mode wherein the scrambler and descrambler and error counter of the central modem are not enabled. The central site modem operates in its normal mode. '~he test is performcd according 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 be~ore 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 messaye. Each possible alarm condition :: i is assigned a bit position in the mayday word. Presence of the mayday is indicated by a onc in the corresponding bit position.

A11 other bits are logic Ols. The alarm bit assignmeIlt 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 -~ G

Bit 6 - O

Bit 7 - Parity A mayday messagc is continuously transmitted by the test ,~ ' ~ ' ` i i I

~3~9 , and control unit 17 until it receives an RTN or RSI command from tile central system controller 15. The RTN command stops transmission of the mayday but if the cause of the alarm persists, further alarms will be transmitted. RSI will inhibit the trans-mission of an alarm, even if the cause still persists until reset by an RTN co~mand. A single exception to this rule is the receive line faul~ 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 messacJes 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 (using the Dump Stored Mayday command) or by other procedures, to deten~ne the cause of the prcblem.

There 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 respon~ing back to the system controller 15 with test or status information.
In either case, the result will be that the system controller 15 will detect framing 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 from the group, and causes their storage at the sites which have ex-perienced an anomalous conditions. The system con~roller 15 m~y then , po]l each site on the central line with a DMS command to dump the ' stored mayday status. Receipt of this command at a remote site !! .

.

~3~9~9 which has stored a mayday messa~e results in retransmission of ~he mayday message back to the central system controller i5.
In this manner no mayday message will be lost. The following paragraphs describe mayday messases provided according to the preferred embodim~nt of the invention.
If the request to send signal from th~ data terminal equipment DT~ 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 (Sq'R) back to the system controller lS. An "excessive" period of time may be identitied according to a strap selection. Remote site test and contxol units 17 sense RTS being on for a prolonged period of time while central site test and 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 ~he stream-ing condition is due to RTS being ON for a long period of time, the remote modem will always mayday first, and for a period of time there will be no multiple mayday si~nals. If a streaming mayday is only received from a central site, then it is known that the condition has been caused by a modem failure and not interfac~e RTS
being on for a long period of time. The test and control unit 17 transmits this alarm until it receives a RTN or RSX command from the system controller 15. RSI always squelches a mayday. RTN .~
will not, if the mayday condition is s~ill present upon RTN receipt.
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 sys~m con ~ ller. Again, the alarm is squelched by receipt of either RTN orRSI.
i i ` !
-23- ' ~:13~9~9 ; The failure to detect a carrier on the primary data channel of a remo~e modem or RTS being off at a central modem for a prolonsed period of time will cause the test and control unit 17 to transmit a receive line fault ~RLF) mayday mes,sac3e back to the central system controller 15. This period o~ 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 ,i by the test and control unit 17. It cannot be terminated by a command from the system con-troller 15 because the receive line to the modem has failed. ~fter the mayday times out, the alarm condition is stor~d and a DSM con~and again produces the mayday.
Only the RTN command can clear the stored mayday. The central modem operates in a continuous carrier mode and has its RTS on continuously. Its associated remote modem has DCD on continuously.
remote modem transmits a RLF mayday if primary channel DCD is off for 3.4 sec. A central modem transmits a RLF mayday if ~TS
is off for 3.4 sec. If a central modem failure occurs so that RTS is off, both the central and remote sites will transmit may-days simultaneously and framing errors wlll occur at the system controller 15. 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. If 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 15. The existence 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~- ~

~3?~
.

the DSM command be used to d~ermine which modems have previously experienced a failure.
The test and control unit lS utilizes an auxiliary power source to ~enerate and transmit a tone whenever a modem power supply failure occurs. 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 controller 15. The central controller may then conduct a scan of the modems associated with the central modem which transmitted il the MPF mayday. The results of this scan are then analyzed by the central controller to determine which modem in the network has the failed power supply.
Since the only line of communication bet~-een the central 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 i5 generated for second tier central site modems 33. This digital alarm is necessary because the interface between the two m~dems 13, 33 is of a digital nature. Hence, if a second tier central m~dem 33 ex-periences a power supply failure, it will present a digital alarm signal across the DTE interface to its associated modem 13.
This modem 13, which is actually a remote site modem of another central line will detect the digital power failure alarm condition 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 the synchronous modem is a central site modem 11 and an input if the . . .
~, ;

113~

synchronous modem is a remote site modem 13.
Finally, a dedicated line not restored (DNR) message is supplied in a situation where a dlal back-up connection has bcen made but the modem has been temporarily switched back to the dedicated line to determine if the dedicated line has been rcstored. If thc modem has not received a transmit concise status (TCS~ command from the system controller 15 within lO
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 sys~em controller 15 could then transmit either an RTN or RSI
command over the dial line to squelch the alarm message as discussed above.
~n 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 I controller 15. These commallds havc the same format as the test commands previously discussed. Commands provided according to the preferred embodiment are discussed in the succeediny 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' i! ' i to be squelched and sets DSR at the DTE interface to an "OFF"

state by forcing internal RTS of the modem to an off condition.

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 15 to transmit an SPT command.

.

113~

1 Receipt of this cor~nand stops the strec~g condition and also i causes DS~ to drop. With DSR off, the DTE 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 cornrnunicate with the central modem.
il The SPT co~nand also provides a diagnostic tool in the situation when two or more sites respond to the samé primary chan-nel address, for example when a DTE is programmed for an incorrect ; address. The SPT command can be used to selectively squelch certain remote sites~ using the secondary channel addressing scheme. The operator at the central site may then determine which DTE is responding incorrectly.
An additional network command which may be provided is one to simulate power failure, termed SPF. Upon receipt of this comrnand, the test 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 comrnand 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 network I;
.

` -27-:

9~

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 remote mod-~m to transmit a power failure 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 throu~h the central modem which the system controller 15 thinks it is.
As illustrated in Fig. 5, the preferred e~odiment provides an advantageous, automa~ic dial back-up arrangement.
Figure 5 represents the first tier of the network shown in Fig 2 Dial back-Up is implemented by utilizing a well-kn~n multiline adapter 71, a number of data access arrangmen-ts (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-ceive line pairs of the central site modem to the associated re-mote site modems. For 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 communicates 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 number of remote si~es which must be dialed depends upon the location of the telephone line fault. ~f a fault occurs between the central site ~modem and the telephone company AC bridge 75, all of the remote ~sites must be dialed. If à fault occurs on one of the lines from ., -28- , . . , . , ' '.

~13~9g~

tile bridge 75 to a remote site, then only that location must be called. For this case, the dcdicated lines to the telephone company bridge 75 must also be connected to ~he 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 1~ 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 transmit a control signal to the dial back-up unit 77 whic~ will cause the dial back-up unit 77 to switch the modem to the dedicated lines. Tlle test and control unit 17 contains a timer circuit which is enabled when the switch from the dial lines back to the dedicated lines occurs. If the test and control unit 17 does not detect a transmit concise status (TCS) command on the dedicated channel within a fixed interval, for example 3.4 seconds, the test and control unit 71 will send a control signal to the dial back-up unit 77. This control sign~l causes the dial hack-up unit 77 to switch back to the dial lines Over the dial lines, i~e test and control unit 17 will send a dedicated-1ine-not-restored (D~R) mayday back to the central controller.
If 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 satisactory ~ality. If the error !l 1' , ~3~9~5~

- rate is satisfac~ory, the system controller 15 will transmit a disconnect dial back-up (DD~) command ove~r the dedicated line to the remote site. Upon receipt of this 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 ~he 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 con'rol unit 17 is in the dial mode and it de~ects a receive line fault, it will transmit the required mayday and will generate a disconnect dial pulse. This is the same pulse that is generated in response to a DDB command.
Receipt o~ this pulse by the dial back-up Ullit 77 will cause it to hang up the dial lines and switch the modem to the dedicated lines. If the c1edicated line has not been restored, the operator at the site of the system controller 15 can again place the re~uired 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 chall~el as a data channel. For this purpose, the repertoire of conmlands ~ncludes 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 1l !

~ 30 1. ` ~

3~9~

received ~econdary channel data ~or possible test and control commands. I~ence, it will not inadvertently go into a tes~ mode by decoding a con~and in a random data stream. If an alarm con~
dition occurs while the modem is in an ITC mode, the test and control unit 17 will clear this mode and will transmit the appro-priate mayday. A ~eturn to Normal (RTN) command resets to normal operation when it is desired to remove it from an ITC modeO Use as a da~a channel is preferably subject to the following limita-tions in the preferred embodiment:
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 I secondary channel.
¦ 5. If ~he 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 central j si~e 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 MARK unt~il a SPACE to MARK
transition occurs.
When operated in a data mode, the secondary challnel accepts 0-150 bps asynchronous data.

.. ' ' .

~1.3fl~

~ i~ure 6 illustrates a particular structure for a test and control unit accordiny to the prcferrcd embodiment of the invention. The test and control unit includes four multiplexers, 55, 57, 59, 61, a microprocessor central processing unit (CP~) 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. Each multiplexer has eight inputs A , B , and four outputs, Y . Each of the multiplexers are controlled by a select line 64 on which a control signal is outputted from the PSU 65. When the select lines 64 are activated (are a logic 1), the Bn 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 outp~ts 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 signals may be level converted as necessary.
The signal on the Al input represents either primary RTS of a remote site, primary DCD of a central sitelor a logic high or low. If the modem is operating as a remote unit in a point-to-point configuration, the input is RTS of the remote modem. If the modem is operating as a central unit in a point-to-point configuration, the input is DCD o~ 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 logic level. The Bl input is a fixed low logic level, representing no input. I'he Yl output is Al/0. Thus, in a point-to-point con-figuration, RTS/DCD is saved for status purposes. Otherwise, I

' i -32~
.

~L ~ 3~

the Al input indicates whetller the modem is a remote or central multipoint unit.
~ f thc modem is operating as a remote unit, the A2 signal 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 ~TS for a central unit. An off condition at A2 thus indicates a receive line fault. The B2 input is one 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 speS-d select code is used to program the particular ~lata rate at which the secondary channel of the l ! system is to operate.
;1 The A3 input signal is primary DSR and the B3 input signal i5 the second bit of the speed select code, either a logic O or logic l. 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, Iwhich serves as a one bit of a stream time code STL. The Y4 output is alternatively a signal quality indication or the STL bit.

1. , Il .

' ~ -33- 1 `I 113~

The ~5 illpU~ iS primary RTS from the data tcrminal equipment DTE. Primary RTS is prefera~ly supplied to the circuits of ~ig. 4 by a window comparator which monitors the voltage of the RTS lead circuit. An "OFF" condition is supplied to the A5 input if this voltage is between + 3 volts or an open circuit. Thisindicates data 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 line 64, The A6 input signal is a dial mode status bit. This ~bit indicates that the modem is operating either on a dedicated Il or a dial line. The B6 input signal is thefirst bit of a m~dem type code. The Y2 output signal is then either a.dial mode indication j or a modem type indication.
The A7 input signal in a remote modem is a digital power fail pulse from an associated second tier central modcm indication.
~or 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 ~ail-ure, then it will transmit a tone corresponding to a SPACING con-dition on the secondary channel. Detec~on 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. l`he Y7 output provides a power fail indication or a second modem ~ype bit. The B6 and B7 inputs form a modem type code.

.

~ I - 113~9~9 . . , ` , ,.. . . .
The A8 input is a customer alarm sig~al. This signal is provided by the modem user and may, for example, be a burglar alarm. The B8 input is a bit stream representing the number of test errors occurring during a modem test. The test error j signal may be provided by gating a test level with the receiver clock and supplying the resu}t to the B8 input. The Y8 output - i is either a customer alarm or an error signal.
The A9 input is~either DCD (OFF in t~st) in central multipoint modems or RTS in remote multipoint modems. In point- !

A~'~' ~ to-point modems, the Ag input is grounded. The Ag input serves ! to detect a streaming condition. If DCD or RTS for the central ; ~ and remote modems, respeotively, is continuously on for an in-~` - ordinaee 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 inpùt~is effectively disabled by the connection to ground. The B9~input is~a first bit ADo of the eight bit test and control unit address. The output Yg is th~s v,~ ~
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 monostabIe multi-i~ ~ vibrator. The pulse width of the monostable is set such that if the receiver clock is 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 receive cIocking indication or the second address bit. The A 1 input is llprimary channel transmit data of the modem.

.i i, ~ ' i ~ 3~5-,,,ij , . . .
. , ' . ' :
~, . . . .

~3~ 9 The ir~put to t-hc t~rlni~lal ~11 is the thir~ bi~ of thc microprocessor address ~D2. The ou~put Yll is either an indica-I tion of the state of modem transmit da~a or the third address 1~ bit AD2.
The input signal to ~erminal A12 is the receive datasignal 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 A~3 to the multiplexer 61 is the modem Clear to Send signal CTS, whose current state is monitored, and the input to terminal B13 is the fifth address bit AD4. The output : Y13 is then either the Clear to Send signal CTS or the fith address bit AD4.
~: The input to A14 is a transmitter clocking signal. This .: signal is again produced ~rom the transmitter clock utilizing ', a retriggerable monostable circuit, as previously described for the receive clock. The Bl~ input is the sixth test and control unit address bit AD5. The.output Y14 is an indication of the ; transmit clock operation or the sixth address ~it AD5~ ¦
The input A15 is an indica tiOII 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-cessor PSU 65. The input B15 is the seventh test and çontrol unit address bit AD6. Therefore, the output Y15 is either the digital jloop mode indication or the sixth address bit AD6.
The final multiplexer input A16 provi.des an indication of whether the modem is in the analog loop test 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-~13~9~

control unit address bit AD6. 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 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 control unit address bit A~7. 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 i;the final address bit AD7. The address bits ADl, Ad2...Ad7, are selectively connectable to O and 1 logic levels to set the ad~ress of the test and control unit in any particular modem site.
Formatted data including commands are received by the microprocessor PSU 65 at a receive data input 64. The formatted data is then transformed by the microprocessor, as later discussed.
The microprocessor PSU 65 supplies a number of control signals to its' associated modem, as well as transmit and receive 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. A dedicated/dial control signal con-trols whether the modem is connected to the dedicated or dial-up tran~mission 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 channels test pattern signal from appearing on the telephon lines. , .
~.

'; ' 113~

The sccondary channel may then b~ used to tr~nsfer thc 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 generator 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 ~o the system controller are outputted in the proper format at the message out terminal. At remote sites, data outputted from the message out channel is applied to the modulator for transmission on the secondary FSK channel. ~t central sites, the message out of the microprocessor PSK is OR gated 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 fail for purposes o~
checking ou~ the power failure circuitry.
This power fail circuitry is illustrated in Fig. 7.
As shown, the power fail sensing circuitry includes a power fail sensing relay driver 121, a relay 123, a ~apacitor 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 condition, on any of the modem power supply !

. . , ~1.3~
, voltages and opens on the secolld~ry powcr side. When a power failure is detected, relay contact Kl OpellS and rclay 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 SPACING condition for six 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 capacitor voltage to a pulse generator which provides a digital power fail signal of approximately seven seconds duration to an associated remote modem as modems 13 and 33 in Fig. 2.
Fig. 8 illustrates the preEerred processing of the secondary channel receive and transmit signals. The receive line signal in analog form is first applied to a band-pass filter 91 with a center frequency of 420 Hz to separate the secondary channel rom 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 circuitr~. The output of the demodulator 95 is applied to a post filter 97, which is a low-pass filter centered ' ,, I

.

~ 1~3~9~9 ; , at 130 ~z. The output of the low pass filter is applied to a second comparator 99, the output of whlch is secondary channel I ~ 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 ou-tput of the ~ND gate 103 is ¦
I! secondary channel received data which is supplied to the input ¦Iterminal A7 in Fig. 4 for power fail detection. The output of ¦Ithe demodulator is gated with carrier detect so that secondary I receive data is changed to an off condition when the modem is not j~receiving secondary carrier. The output of the second comparator ~99 and a delayed form of the earrier detect si~nal are applied to a second AND gate 105. AND gate 105 accomplishes the same ¦jfunction as AND gate 103.
, In xemote modem, the output of the AND gate 105 is otuputted to the receive data input 6~ of the mieroprocessor land to the secondary channel receive data OR gate 108. In the 'lleentral 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 through the OR gate 108 for the system controller lS~ If the ~proeessor 65, 67 is at a eentral 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 inhabit timer limits the propagation of the power fail spaee ~tone to a single tier of the network.

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At rcmotc sites, the messase out of ~he microprocessor i5 ~pplicd to a digi~al modulator 11~, preferably on thc modem LSI chip, and then to ~ band-pass filter 15 for tran~mission across the transmission line c};annel. If the modem is a central site, digital data from the system 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 paramet~rs`.
First, the processor tests the power failure indication and receive line fault indication and resets respec~ive 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 strearning condition is indicated. The next test in Fig. 9 is ~or data spacing. If data spacing is detected, subsequent reception of a character is indicated. When a start bit is detected, flags are set to indicate character rece~tion. Once the start bit has been detected, a character will then be received. ~ext, if a DNR time-out is not in progress, the DNR timer is reset to zero. 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 milliseconds, an interrupt from idle occurs, as illustrated in Fig. 10. The time 30348 miiliseconds is set to enable sampling in the middle of each receive bit at the highest baud rate. At 75 BPS, four clocks per bit time are provided~
., ` , ' ~

~13~9 The first test in the interrupt chain is to ascertain if a character is being received according to the flass set during ~he idle loop. If so, the processor jumps to the routine indicated in Fig. 12. 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 tha~ 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, the 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 storage 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 required. If so, the flow of ~ig. 15 is per-~formed. If not, a test is made to indicate whether any alarms 1:~3~g 11~1VC becn set. If ~o alarms havc been sct, ~he i~el condition is rccntcred. If alarms have been set, it is nece~s~r~ to send an alarm messa~e back to the Central System Controllcr 15, iE
the alarm corldition is not one which has been inhibited by a received RSI command. In such case, the TTC mode is cleared and the alarm type 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 character 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 transition which indicates .
beginning of a start bit. The counter operation upon an input message is illustrated in Fig. 22. The first test in Fig. 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 decremen-ts on each sampling interval and is idle during stop bits. The count of 10 indicates a start bit and count oE 1 indica~es 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 chec~ is then made on the entire message word, and if the parity is good, the receive process is cancelled, saving I ! .
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the new word ln a receive stack. If it is not time for a stop bit, the new bit is sampled, detected, and shifted into a regis-ter storing the eurrent data word. The reeeive routine is then exited.
If a new word was saved in a s~ack according to the flow of FigO 12, the flow of ~ig. 13 is entered. The received charaeters are staeked suecessively with the first received character being shifted sucessively upward in the stack as additional characters are received. Thus, the first step in Fig. 13 is to eheek the top of the stack (the o]dest eharacter position) to determine if i~: is a "D~L" eharac~er. If sor a 'eomplete message possibly has been reeeived. If not, the proeess enters the idle mode. If D~L has been deteeted at the top of the stack, the next test is to aseertain if the address is a group address. If no~, the address is tested to aseertain if it is the address of the mieroproeessor under diseussion, as set by the address bits ADo...AD7. If the address is improper, the processor returns to t~e idle mode. If the address is eorreet, the proeessor ehecks the Block Cheek Character (BCC). If the Block Cheek Character is proper, the commancl number is saved for a later Aeknowledge operation aceording to f]ow 14. The eommand is then examined to determine if it is a member of the permis~sible eommand set. If it is not, the proeess re,urns lo the idle mode.
If the eommand is a proper eommand, a test is ma~e to aseertain if the procc.ssor is in the ITC mode. If not, the proeessor branehes to the eommanded routine through a table, as indieated.
Fig. 14 illustral:es the Aeknowledge pr~cedure. The eommand nun~er saved during the flow of Pig. 13 is plaeed in , t i I

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i the data byte location of the transmit stack. Parity is then calcula~ed for the data byte, and the Block Check Code is generated and saved in the transmit stack. In the case of a single data by~e message, a terminator is placed in the transmit stack to indicate a single data byte message. A pointer to the top of the stack is then set to indicate the next charactcx to be transmitted, and the "transmit busy" flag is set. Settin~ of the transmit , busy flag will cause the flow of Fig. 11 to branch ~o the transmit I
routine of Pig. 15. If a mark byte 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 eharaeter time. This delays the second character, a syne byte, until the corresponding reeeiving circuit ean deteet 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 eounter is then loaded to time the sending of the RLF alarm and ; the processor returns to the idle mode.
! When the proeessor enters the transmit mode, Fig. 15, secondary channel carrier is turned on. A co~r~ter is again used to time the transmission of the message, which is stored in a transmit stack. ~irst, a test is made to deterrlline if it is time to send a bi~. 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, the start bit is sent. If not, a ~est is made to determirle if it is time to send a stop bit, and if so, a mark ~is sent. I~ it is not time for ~ s~art bit or a stop bit, a i -~5- 1 aq~

test is made to determine if the en(i of the chax~cter bcing transmitted has been reached. If the end of a character has not bcen reachcd, a data hit is sent and the processor returns to the idle mode. If the end OL a character has been detected, the bit counter is reset, the transmit stack pointer is advanced, and a test is performed to ascertain if the bottom of the transmit stack 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 bottom of the transmit stack has not been reached, the flow of Fig. 16 is i performed.
.. . ,; i 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 i~ was a receive line fault alarm, I which times itself out. If not, the transmit stack pointer is set back to t:he top of the stack ancl the flow of Fig. 15 is re-entered at 4. If the alarm was a receive line fault, a test is made to determine if the time for transmission of the RLF has~
expired. Normally lO seconds of transmission. Varies + two I seconds dependiny on T7 buad rate. It so, the RLF signal is inhibited and the flow of Fig. 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 . . 1 response to an ~SI command, any alarms are loaded from temporary ,alarm stoxage to the mayday inhibit flags. ~n 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 Ac~nowledgement flow. The SDB co,~nand sets a bit w~hich 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
co~nand sets the RC 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
commands, which require the processor to configure a 3-data byte message. 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 EEC, STE, and ~PE cor.~ands. In response to the EEC command, the test error counter is cleared. The error count flag is set and the current error bit state is stored. In response to the STE command, the AC loop and break line loop bits are set and the test pattern enable bit i5 set. Acknowledgement is then provided by reverting to the flow of Fig. 14.
Fig. 20 illustrates the SPF and SPT co~nands. 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 control 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 ~DB 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 spiri.t 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 (6)

DIV II

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

1. In a data modem adapted to be switched between back-up and primary transmission lines and adapted to receive command signals from a central controller, the apparatus comprising:
means responsive to a first command from said central controller to switch from said back-up line to said primary line for timing an interval during which a second command is expected to be received from said central con-troller and for switching said modem back to said back-up line if said second command is not received during said interval.

2. The apparatus of claim 1 wherein said means also transmits an alarm signal to said central controller if said second command is not received during said interval.

3. The apparatus of claim 1 wherein said back-up line is a dial back-up line.

4. A communication system comprising:
a data modem adapted to be switched between back-up and primary transmission lines;
a central controller means for communicating commands over either of said transmission lines to said modem, said commands including a first command to switch said data modem from a back-up line to a primary line; and means at said data modem responsive to said first command, for timing an interval during which a second command is expected to be received from said central controller, and for switching said modem back to said back-up line if said second command is not received during said interval.

5. The apparatus of claim 4 wherein said means at said data modem also transmits an alarm signal to said central controller if said second command is not received during said interval.

6. The apparatus of claims 4 or 5 wherein said back-up line is a dial back-up line.