CA1138996A - Test and control unit for use in conjunction with a data modem - 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

~3~6~6 i : l 1141 ~101)],M l)~ NO~TIC' ` - i ~NI)- C~ oi, S--YS'l'E:

'BI~CKGI~OI~ND OF THE INVE:NTION
; The sub~ect invention relates generally to automatic network diagnostic systems and more particularly to a system for automatically testing and controlling/ from a centxal site, a plurality of centrally and remotely located data modems.
With the increasing complexity of distributed data pro-cessing systems, particularly those utilizing telephone data ,Icommunication between central and remote site data processing llapparatus and their associated data modems, the need for testing land control of the da~a modems has incxeased. The complexity f present a~d proposed systems requires the ability to communicate ¦ rapidly with modems at diverse and numerous sites~ ~odem mal- ¦

functions become increasingly critical in that one malfunctioning I modem may lnte;rrupt transmission by many othe~s in the network.
¦ Since even very small amounts of down time can mean big dollar losse~ in distributed processing systems, the need arises to l¦automatically control modems in a distributed ~ystem to minimize ¦I time losses. q'o provide e~ficient and effective operation, ~t - would be highly desirable to provide the modem with the capability

2~ to raise alarm signals to a central controller and perform the network reconfiguration and control function~ necessary to respond to various trouble conditions~ Both ~or speed and reli-ability, it ~8 desirable to have as many of these fu~ction~ a~
possible performed automatically by apparatu~ of the systemO

~13~

S UMMARY OF THE I NVENT I ON
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 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 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 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 modem. Control over the network configuration is also achieved. The system of the invention is applicable to multi-tiered networks.

~3L3~

- In accordance with one aspect of the invention there is provided a test and control unit for use in conjunction with a data modem comprising:
means for detecting a plurality of alarm conditions in the operation of said modem and setting corresponding alarm bits;
means for receiving a formatted message, including a plurality of words each of which contains an information portion, one of which portions is a command, and for storing the received informati.on portions in a stack;
means for checking the contents of the stack to verify correct receipt of a command and for setting control bits to effectuate a con~and if said contents are correct;

1~L31~

m~ans for format-~ing and storing an acknowledyement m(~ssage in response to detection of an alarm condition by said detecting means or in response to a correctly received command; and means for transmitting a said acknowledgement message out of said test and con-trol unit.
Further in accordance with the invention there is provided a test and control unit for use in conjunction with a data modem comprising:
means for detecting a plurality of alarm conditions in the operation of said modem and setting corresponding alarm bits;
means for receiving a formatted message, including a plurality of words~ each word including start, stop~ and parity bits and an information character one of which characters may comprise a command and for detecting said start and stop bits, checking the par;ty condition and storing the received character in a character stack;
means for checkincJ the contents of the character stack to vertify correc-t receipt of a command and Eor setting control bits to effectuate a command if said contents are correct;
means for formatting and storing an acknowledgement message in response to de-tection of an alarm condition by said detecting means or in response to a correctly received command; and means for transmitting a said acknowledgement messaye out of said test and control unit.
Further in accordance with the invention there is pro-vided a data communication system comprising~

- 3a ~3~

a plurality o~ clat.l mo(~ems; a central processor mealls for generatin~ formattecl messages for transmission over selected transmission links; transmission means connec-tiny said modems to said processor; and a test and control unit for use in conjunction with each data modem, each said test and control unit comprising:
means for detect:ing a plurality of alarm conditions in the operation of its associated modem and setting corres-ponding alarm bits;

means for receiving a formatted message, including a plurality of words each of which contains an information portion, one of which portions is a command, and for storing the received information portions in a stack;
means for checking the contents of the stack to verify correc-t receipt of a command and for setting control bits to effectuate a comrnand of said contents are correct;
means for formatting and storinq an acknowledgement message in response to detection oE an alarm condition by said detecting means or in response to a correctly received 0 command; and means for transmitting a said acknowledgernen-t message out of said test and control unit.

- 3b -13L38~

i ~IEF DESC~IPTION OF TIIE DR~WINGS
The preerred embodiment and best mode contemplated for practicing the just summarized invention will now ~e de~cribed in conjunction with the drawing~ of w~ich:
FIGURE 1 is a generalized ~lock diagram of a portion of - a 6ystem configured according to t~e preferred embodiment of the invention.
FIGURE 2 iB a block diagr~m of a two-tier modem Byatem incorporatiny the preferred embodiment of the invention.
FIGURE 3 illu~trates the dat~ format for transmitting a command according to the preferred emfiodLment of the invention.
FIGURE 4 illu~trates the format of a concise status word.
FIGURE 5 is a ~lock dia~ram of a dial ~ack-up technique of the preferred embodLment.
FIGURE 6 is a c;rcuit diagram of a test and control unit according to t~e preferred emfiodiment, FIGURE 7 i~ a circu;t diagram of receiYe-transmit cir-cuitry of the preferred embodiment.
FIGURE 8 i~ a c;rcuit diagr~m of the po~er fail detect circuitry of the preferred emfiodLment.
FIGURES 9 - 21 are flo~ diagrams illustrating th~
operation of the test and control unit.
FIGURE 22 illustrate~ the receive function of t~e te~t and control unit.

' 1~3~
F,T~\ILED DE:SC~XPTION OF TIIE
PREFERRED EMOBDIMI:NT OF T~IE INVl~NTION
Fig. 1 illustrates in functional block form a portion of a system according to the preferred embodiment of the invention providing xemote test capabilitles for modems. Such modems may ;be, for example, in four wire controlled carrier muitipoint or continuous carrier point-to-point data communications networks.
Testing is accomplished according to commands sent 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* T~sting and control of the xemote modem 13 is accomplished undex 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 contains 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. As discussed later, the test and control unit 21 is preferably configured around a microprocessox, which, by way of example~ may be a Fairchild F8 CPU and PSU. The test and control unit 17 decodes addresses and commands from the system controller 15~performs a specific test if addressed, frames and formats the test results, and transmits these;results back to the system con-troller 15. Some tests arq a~complished without interfering with ~ormal network operation while other ~ests temporarily interrupt il portions of the network~ , ¦
~ n addit~on to responding to certain test~ inltiated by ~the controller 15, the tes~ and control unit senses certain j PDP-ll is a trademark Ii ............ . . . . . .... . . .
.. 5 ~3~

anomalous conditions in the modem 13 a~d transmits suitable alarm messages back ~o the system con~roller 15~ Because it ha~ the ability to address a particular ~est 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 2~
operates asynchronously at a relatively low speed data rate such as 75 bits per second. The modulation technique utilized by the secondary transmitter and receiver 19 is frequency shift keying (FSR). 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 modemO Tones other than 392 Hz and 447 ~z 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. A number of modems, for example~ from 1 to 254, can be associated with each `
of these ports l...N. Each central site ~.~rJrt l~...N communicates with a central site modem 11, which in turi~ communicates with a number of remot~ site modems 13 acro~s a four wire multi-drop line 27. ~hese modems l3 are typic~lly synchronous operatins but may also be a~ynchronous types. The ] to 254 modems can be either centrally or remotely located. I
One of the modems 13 is illustrated communicating with a ' ;remote controller ~9 and a digital mixer 31, which communicate with a number of additional modems 33, 35. Typically, the modems 33, 35 will be asynchronous and arq separated by the remote controller 29 from the ~;ynchronous modem 13. Central second tier modems 33 communicate ,~cross a four wire multidlop line 34 with remote second li ,, , ' 113Y9~

tier modems 35. The digital mixer functions as an OR-gate 31 to provide a path around 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 asynchronous 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 communication between a number of terminals using modems 33, 35 and ; the modem 13.
i 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 from the system controller 15 by the central modem 11 supplies a modu;Lating signal to the frequency shit key (FSK) secondary channel transmitter 19 in the central site modem 11.
This transmitter 19 places the system controller commands in `
proper analog forma~ 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 demodulates the analog signal to pass an asynchronous bit stream to the system controller 15 a~ the FSK cha~nel data rate, e.g. 75 bps. The central site test and control unit 17 does n~t .. I .. . .

, .

~3~

monitor this recei~ed 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 syst~m controller in an asynchronous data stream. This data stream is applied to the FSK modulator in the transmitter 19 where it is converted to an analog format for transmi~sion 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 controll.er 15 which are in analog format and converts lthese commands to a digital data stream, for example, at 75 bps. I:E the da~a 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- i 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, . I

1. ~
1 , . . .. .
_ ~ J

. . .

~3~6 .' ' ' ' ,, mod~m 13 in digital format. This digital data itre~m, for example, at 75 bps, is then connected to the FS~ 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 activated (on). ~lso in response to the marking condition, the secondary channel transmitters of each modem 13 are ~et 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) ~t i8 necess~ry for the remote site to send an alarm message back to the central controller o~ its own 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 ~ controlIed carrier mode on their , secondary channel. In implementing this function, secondary DCD from the asynchronous modems will be OR-gated by the gate 31 ; l~ to the synchronou~ remote modem 13 to serve as secondary ~TS
, (request to ~end). No connection like this in the opposite ii ,, `, I
.. . .
; !

.. . ...

_ 9 _ ~

:`

direction is needed because the secondary channel of 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 st~tus reporting and testing functions in response to commands from the system controllex 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 perform desired combinations of tests may be provided by the system controller 15. These test mo~es 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 sequence, 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 i~
i length and ;ncludes a start bit (logic 0), seven information~bits, an even parity bit and two s~op bits (logic 1).
; The seven bit delete character DEL is composed of all ones and serve~ to allo~ the receiver to synchronize to the asyn-chronous dat~.
If bit To is a logic zero, the message is a command I 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 .
~, .
.

n36 chec~ character cont~ins a longitudinal even parity value geilerated ~y computing ~he exclusive OR for each bit position of the two address characters and the command character. For e~ample, the first bit o~ the block check character is the value obtained by computing even parity for Ao~ and C0. Replies from the test and control unit 17 have the same format as commands from the eentral eontroller 15 with the exception that the first DEL~TE
eharac~er i5 replaced b~ a ~RIC character (all ll bits ar~ loyic l's). ~pl;esare transmittecl on a one sho-t basis. ~s discussed " below, thc command word is r~placcd by either one or thre2 in- j formation words. Maydays and concise status require one informa- ;
tion word. Extended status and error counts require three in~orma-tion words. The test and controi unit 17 will respond to a com-mand from the system controller 15 if the following conditions are met: (l) one D~LETE character is detected; ~2) an address deeode is obtained; (3) parity is correct for each word, and (4) the block check character is correct.
For most of thc commands r the test and control unit 17 provides an acknowledger,~ent back to the system controller 15.
This acknowledgement consists of an echo of the original command with one control bit To chanc3ed in Address Word l~ Comm~nds which are not echoed are Squelch Primary Transmitter d Simula~e Power Fail, Analog Loop, and all commands which require a response from the test and control uni~.
i In addi-~ion ~o respondinc~ ~o it.s own address, a particular test and control unit 17 has the a~ility to respond ' to a yroup addre~s (represente~ ~y all ~ercis~ This address 1' 1 , addresses each modem on a gi~n portO A~y command ean be used~

!
. 1 ` ~ ' ' '~

., ~ I

~3~

Return to normal (~TN) clears an e~istln~ test or alarm Mode and stops the 'r. anSmiSSlOn Of all alarm messages. It further resets all storage registers in the test and control unit 17 and causes the associated modem and its ~est channel ~o revert to normal operation~ RTN will aiso clear an existing RSI stateO
~owever if ~he alarm condition persists, the control unit will again transmit another mayday~
The return ~o normal storage and inhibit (RSI) instruc- !
tion clears an existing test mode~ stops the transmission of all alarm messages and causes ~he modem to revert to normal operation.
~Iowever, the existancel and t~pe o~ ~he particular mayday is saved.
R~I inhibits the transmission o~ an alarm even if the condition still persists, until it is reset by an RTN command. RSI may be ~sed by the central control 15 to clear the network of alarm messayes before the central con~roller institutes diagnostics to isolate a problem. RSI is also used in t:he situation where mayday messages are transmitted simuitaneously from two or more locations.
Simultaneous maydays result in the central controller receiving a noncoherent data stream Wi ~}l continuous parity and framing errors. The controller will then transmit an RS~ with a group address to all modems on tha'c lineO A11 mayday messages will be inhibited but their existencc will be stored. ~rhe controller can then selectively address each modem on that line with a dump stored mayday command (DSM) to recover all of the alarm conclitions which had previously occurred simultaneously.
The various test modes which may be entered and performed in conjunction with the above command format will now be discussed.

These include two types of staLus checks, concise and expanded.
I' . ' The test functions available include selr test, end to end test, analog loop with cest pattern~ digital loop with test pa'ctern~ ' ~analog loop and digital loop~ ¦

~13~
1, Tl~ irst of thc s'-atus moni.or mod~s i5 co~cisc i~tatus r.lolli~cor. In ~his mode, -~Ihe cci~'cral eontroller 15 scans a group or all of the modems in the syst~m, sequentially addressing eaeh modem with a transmit eoncise scatus (TCS) eo~ and. In response, eaeh modem sequencially cransmits back '.o the system eontroller 15 a eoneise status word. The eoneise status ~ord has the format illustrated in FigO 4. The eoneise status ~ord eontains the Z
following information:
(A) DCD - ON/OFF (remote) or RTS - ON/OFF ~eentral~
(B) DSR - ON/OFF
(C) DTE: Po~ler - ON/OFF~ The voltage level of the RrrS
! lead from the da-ta terminal equipment is eontinu-ously monitoredO Speeifieation RS232C requires that the voltage of any DTE interface lead be bet~7een -~3 and ~25 vol-ts or between -3 and -25 volts~ The status monitor yields an off eondition for this parameter if the monitoring eireuit senses a voltage level becween 3 volts and ~3 . volts ais an open eireuit~
(D) Modem in either Loeally or remotely înitated analog o.r digital loop - YES/NO
(E) Modem is either a Central Site or Remote Site ; (~or multipoint) RTS - ON/OFF (remote)~ DC~ - O~/OFF (Central) [point to poin'c (F) Logie 1 indieates that m.odem is eonnected to dial lines and logie 0 indieaces that modem is eonneeted i to dedicated linesv i ~3~

(G) Sigrlal Quality - ON/OF~ n O~ condition corres-ponds to either the case where there exits a lo~7 probability ~or error on the primary channel .. (GOOD Q-U~LITY) or DCD is off when the signal ~uality lead is checked. An OFF condi~ion corres-ponds to ~he case where DCD is ON and signal quality is unacceptable.
In the preferred embodimen'c~ the controller does not evaluate the . status reply response. A second type o. status monitor available is the expanded status~ In -the expanded mode~the syst~m controller 15 transmits a Dump Modem Status (D~S) command. ~eceipt of this command 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 s~atus mode~ The format for the second and third words is as follows:
: Status Word 2 , ' '.
Bit 0 - DCD (ccntral or RTS (remote) - current state [multi~oint]
Logic 0 (point to point) Bit 1 - Receive Clock - transiti.ons are occurring at the data rate of the modem Bit 2 - Transmit Data - current state Bi~ 3 - Receive Data - current state Bit ~ - CTS - current state Bi.t 5 - Transmit Clock - tran:;.tions are occurring at least at the data rate OL the modem Bit 6 - Digital Lcop - the modem is in either a local or remote di~ital loop modeu Bit 7 - Parity ., Il . I

.' ` ' , ~3~

Status l70rd 3 . . _ . . it O - DCD Transitions ~central mul-tipoint~ - at least one one transition has occurred since the last DMS command.
RTS Transitions (remot~ multipoint) - Logic O for point to pOillt Bit 1 - Not Used Bit 2 - Transmi~ Data Transition Pit 3 - Receive Data Transition Bit 4 - CTS Transition Bit 5 - Modem Type Bit 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 remote modem. When 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 pseudor2nd0m test pattern generator is connected to the moclulator input and a pattern detector and error coun~er is connected to the demodulator output.
Errors are accumulated for txansmission back to thc 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 prima~y channel data traffic is affected only in that ~he transmission from the modem under test is inhibited. To ~erfo~m this ~est~ ~Lhe system controller transmits in sequence a self test enable (STE) command~ an enable il i -~5-, error counter irEc~ commnd, a c~ump error count (D2C) command and a return to normal (~TN) co~mand.
In response ~c the s~r~ command, ~he test and control uni. 17, causes the modem transmi~ ou.pu~ to be connected to its receive input, .urns internal RTS o-E the modem on, and enables the test pattern generator and de-~ector. The ~EC command resets the error counter and ini~iates ~he accumulation of errorsO
The delay between the STE and E~C co~nand provid~ sufficient time - ; for the scrambler and descrambler .o synchronize.
~fter the STE and FEC commands have been performed, I~the DEC command initiates reply back to the cen~ral system con-troller 15. The system cont~oller performs -the timing function to determine the length of the self test run. An approximate error rate (errors in 106 or errors in 105 can then be computed by the system controller 1~.
The format of the response of ~he test and control unit to a DEC command is as follows:

Error_Count Word 1 Bits 0 - ~ - Character error count - binary coded number which represents the num~er of parity or rraming errors in all messages received from the system controller since the last DEC command.
Bit 7 - Parity , -16- i, .

Error Count ~ord 2 Bi~s O - 3 - Low order 4 bi~s oE the pxi.mary channel test error count Bi~s 4 - 6 - Logic O's Bit 7 - Parity Error Count Word 3 Bi~s O - 3 - High order 4 bits of the primary channel ~I test error count ,; , Bits 4 - 6 - Logic O's . , Bit 7 - Parit~
' ~ A total of 8 bits in a binary coded format are utilized for the : primary channel test error count~ Hence, it is possible to count i up to 255 test errors~ All error counts are reset to zero after being reported. Character errors are tabulated for all messages received by a particular test and control un:it whet}leL~ or not that modem has been addressed~ ~n incorrect block chccl~ character is counted as one character error~ Proper framing for a received character is determined in the following manner: .
(1) A MA~ to SP~CE transition is recognized as the beginning oi a start ~ito t2) The center or the start bit i.s chec~ed to see that it is still a SPACE. If it is notO one character error is countedO

(3) The 9th bit after the start bit is checlced to determine it is a proper stop biJL (MA~K~ If this is not the case, a fra~.in~ error i5 reco~nized and the character , j ~3~
.

error coun-Ler is increnlented by one.
The character error count ~or a particular modem in the system i., . ~
provides an indic~tion of the ~uality of the secondary channel data received a~ that location~
After the error information is ~ransmitted 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 i; i embodiment is an end-to-end ~est be~ween a central modem and a . , ; liremote 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 i5 inhibited during this test for all the ' modems branchin~ 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 t ' the central system controller 15 transmits in sequence the following commancls test pattern enable (TPE), EEC, DEC, and RTN.
The system controller 15 will sequentially send the TPE command first to ~he central modem and then to the remote modem. The TPE instruc~ion enables the pseudo random pattern ; ~enerator in the modem transmitter and the pseudo random pattern detector in the modem receiver. Internal RTS o~ the modem is forced on. A delay is provided before the nex-t EEC command to allow the scramblers and descramblers in the two modems to synchronize~
¦I The other commands EEC~ DEC~ RTN , cause the test and control unit to perfo~ as discussed above The EEC command !, ¦

18- i ~, .

is sen~ first ~o the central modem and then to th2 remote modem, as is the D~C co.-i~nand.
Another form of test enabled ~y the subject preferred e~bodim~nt is an analog loop with test pa-ttern, which is always conducted between a central and remote modem. This test is operator-controlled and may take ad~ntage of the ~act that the i central system controller 15 can have the network structure stored l !
- in its data base. For example, the operator of the system ; jcontroller 15 presses in se~uence first an analog loop key and then a test key. Then, it is only necessary for the operator to ; key in the address of the remote modem. Since the system con- ', , jl troller has the network structure stored in its data base, the address of the central slte will ~e implici-~ in the commandu The system controller 15 can then address the necessary analog loop test commands to the proper site~
~ o 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 control unit is desiyncd such that even though placing the modem in an analog loop mode causes loss of carrier, a subsequent receive line fault mayday will not result.
The sequence of co~nands utilized is discussed in the following paragraFh~
Xn performing 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 ~ack to its transmit output through a stage of gain. Next TPE is s~nt to ~he central site, enabling I its scrambler and descrall~leru ~n EEC com~and is also sent to ¦ the central site to initiate the accumulation of errors. Next DEC , li ' . .

is sent to the central site where it elici~s a reply, including the error count at ~he end of the test in an 8-bit binary format.
~TN is then sent to all modems on the central line, utilizing the group address previously discussedO The sequence of commands emanating from the central controller during this test may be summarized as follows: i (R~T ADD 3) (ACL), (CEN ADD) (TPE), (CEN ADD) (TPE) ack. back to CSC 15 (CEN ADD) (EEC), (CEN ADD) (EEC), , ack. back to 15 ?
(CEN ADD) (DEC),(CEN ADD) (ERROR COUNT), (GROUP ADD) (RTN) response to CSC 15 he repertoire of commands for the test and control unit includes ' a Stop ~rror 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 of it:s own channels. SEC
can be used to hold an error count while~ the modem is removed from an analog loop mode. l ;~ A digital loop with test pattern mode is also available. I
This test follows the same format as the previously described `
analog loop test. Central sita modems are not placed in a digital loop mode. The test is conducted between a central site ,and a remote site. ~he scrambler/descrambler and error counter are enabled for the central modem~ The remote modem is placed in a digi~al loop mode. ~n this mode, recei~ed data becomes transmit ' data, received clock becomes an external transmit cloc~ DCD becomes ~S. j I The DTE loop serves to isolate the DTE from the modem. DSR will be j I off a~ the inter~ace ~o i~lica~e to ~he controller that a test is in progress.
. 1l ` ' ~
1, -20- ~

.', .
digital loop con~la~d (DCL) is first scnt to the rcmote _it~ which is under test. TI'~ is ~hen sent to ~e c~l-`cral site, fol]o~ed by EEC, follGwed by DEC
af~er t:he central system controller 15 has timed out the appropri-ate length of the error count. Finally, RI~N is transmitted by means of the group address.

: . , A final test mode available is the ~naloy loop, or digital loop mode wherein the scrambler and descrambler and error counter i of the central modem are not enabledO The central site modem operates in its normal modeu The test is performed according to the following sequence of co~nands: ACL or DCL to remote site under test, and RTN by grcup 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 lS. The format for the transmitted messages is the same as ~, , for the conci~ie status messase. Each possible alarm condition ¦is assigned a bit position in the mayday word. Prcsence of the mayday is ind:icated by a one in the corresponding bit position.

~ll other bits are logic O's. The alarm bit assignmer.t is as ; follows:

Bit O - Customer Alarm Bit l - Streaming ¦ 1~ Bit 2 - Receive I.ine Fault Bit 3 - Modem Power Failure Bit ~ - Dedicated Line Not Restored Bit 5 - O

Bit 6 - O , Bit 7 ~ Parity ll A mayday message is continuously transmitted by the test .1 , , . . ~

and control unit 17 until it receives an RTN or RSI command from the central system controller 15. The RTN con~and stops transmission of 'che 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 command. A single exception to 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, diaynostic proce~ures can be undertakcn either with the aid of the system controller 15 (using the Dump Stored Mayday command) or by other procedures, to dctenmine the cause o the problem.
Thcre 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 ~ystem controllcr 15 wi-th tcst or status in~ormation.
In either case, the result will be that the system controller 15 will detect framing errors on its receive data line. After a certain nùmber of framing errors are counted, the system con~trol-; ler will cease issuing test commands. If the raming errors per-sist, the system controller 15 will transmit an RSI command with a group address. This RSI inhiblts all mayday messages from the group, and causes their storage at the sites which havc ex perienced an anomalous conditions. The system c~ntroller 15 ~ay then poll each site on the central line with a DMS command to dump the I stored mayday status. Receipt o~ this command at a remote site ~, , , -22-`
which has stored a mayday message re.sul~s in retransmission of the mayday message back to t}le central system controller 15.
In this manner no mayday message will be lost. The following parasraphs describe mayday messa~es provided according to the preferred embodinlent of the invention.
If the request to send signal from the data terminal equipment DTE associated with a particular modem is held in an "ON" condition ~or an excessively lon~ time, preventing other modems on a multidrop line from transmitting, the test and control unit 15 will send a streaming àlarm (STR) back ~co the system controller 15. An "excessive" period o time may be identitied according to a strap selection. RemGte site test and contxol units 17 sense ~TS 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 the stream-ing condition is due to RTS being ON for a long period of time, the remote modem will always mayday first, and ~or a period of time there will be no multiple mayday signals. If a streaming mayday is only received from a central site, then it is known that the condition has been caused by a m~dem 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 RSI command from the system controller 15. RSI always squelches a mayday. RTN
will not, if the mayday condition is still present upon RTN receipt A customer alarm message (C~ may also be provided in response to an extra input signal fLom the customer. In response to an ON condîtion, a mayday messa~e is transmitted back to the sys~em oontr~ller. Again, the ~larm is s~uelch~d by receipt o~ either R~N orRSI.

. , :
-~3 3.~

-~ The failure to detect a carrier on the primary data channel of a remote modem or RTS being off at a central modem for a prolonged period of time will cause the test and control unit 17 to transmit a receive line fault (RLF) mayday message back to the central system 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 be terminated by a command from the system controller 15 because the re-ceive line to the modem has failed. After 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 operates in a contin-uous 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 will transmit maydays simultaneously and framing errors will occur at the system controller 15. The DSM command can then be used to recover both mayday messages. If a tele-phone 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 faultO This condition results in the transmission of simultaneous multiple alarm messages back to the system controller 15.
The existence of simultaneous messages on line may prevent S the system controller 15 from decoding a unique alarm.
Only after the receive lines have been restored can - 24a -, the DSM command be used to determine which modems have previously 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 deteets the MPF tone and transmits an alarm message with its own address to the central eontroller 15. The eentral eontroller may then conduet a sean of the modems assoeiated with the central modem whieh transmitted the MPF mayday. The results of this scan are then analyzed by the eentral controller to determine whieh modem in the network has the failed power supply.
Sinee the only line of communica~ion 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 seeond tier eentral si.te modems 33. This digital alarm is neees<;ary beeause the interface betwee~ll the two~ ms 13, 33 is of a digital nature. Hence, if a second tier cen~ ~. modem 33 ex-perienee.s a power supply failure, it will present a digital alarm signal aeross the DTE interface to its associated modem 13.
This modem 13, whieh is actually a remote site modem of another central line will deteet the digital power failure alarm eondition and will send an analog power fail signal with its address to the system eontroller 15. The digital alarm signal lead is bi-direetional. 13y strap seleetion it will be an output if the synehronous modem is a eentral site modem 11 and an input if the ' ~' ~3~

synchronous modem is a remo~-e site modem i3.
Finally, a dedicated line not restored (DNR) 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 modcm 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 ~e automatically initiated i 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 squelc~l 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 ~enerated by the central controller 15. These commands have the same format as the tes-t commands previously discussed. Ccmmands provided according to the preferred embodiment are discussed in the succeeding paragraphs.
The squelch primary transmitter (SPT) oo~d 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"
~tate 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 ~SI command, the next step is for the central controller 15 to transmit an SPT command.

.. Il . I
I ` !

~ --26-- i Receipt of this command stops the streaming condition and also causes DSR 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 turnd 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.
The SPT command also provides a diagnostic tool in the situation when two or more sites respond to the same - primary channel 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 SPE. Upon receipt of this command, 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 command may also be used as an aid to the central controller 15 in checking the actual network configur-ation. In its data base, the system controller 15 may have the entire network configuration stored. Each remote modem is associated with a particular central line, as already discussed.
The accuracy of the configuration information stored by the system controller 15 may be checked by causing the remote modem 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 through the central moaem which the system controller 15 thinks it is.
As illustrated in Fig. 5, the preferred embodiment provides an advantageous, automatic 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-known multiline adapter 71, a number of data access arrangements (DAA) 73, and their associated telephones and a dial back-up unit 77. The adapter 71 provides an AC bridye to connect the transmit and receive line pairs of the central site modem to the associated remote 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.

:.

~,~

~3~

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 sites which must be dialed depends upon the location of the telephone line fault. If a fault occurs hetween the central site modem and the telephone company AC
bridge 75, all of the remote sites must be dialed. IE a fault occurs on one of the lines from . .

- 28a -~13~ J~

the bridye 75 to a remo-te site, then o~ly that location must be called. For this case, the dedicated lines to the telephone company brid~e 75 must also be connected to the multiline adapter 71.
Once a remote site modem has been dialed, the system controller 15 is utili~ed 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). ~ 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 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 from the dial lines back to the dedicated lines occurs. If the test and control unit 17 does not detec~ a transmit concise status (TCS) command on the dedicated channel within a fixed interval, for e~ample 3.4 seconds, the test and control unit 1 71 will send a control signal to the dial back-up unit 77. This - control signal causes the dial back-up unit 77 to switch hack to the dial lines. Over the dial lines, the test and control unit 17 will send a dedicated~line-not-restored (DNR) mayday back to the central controllex~
If the dedicatecl 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 quality. If the error 11i ' i ,, Ij .

, ` -29- !

. .

rate is satisfactor~, thc system controller 15 will transmit a discon~eet dial back-up (DDs) command over the dedicated line to the remote site. Upon receipt of this command, the test and control unit 17 sends a signal ~o the dial back~up unit 77 which causes it to hang up the dial lines. If the error ra~e, 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 eontrol signal is transmitted from the test and eontrol 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 eontrol unit 17 is in the dial mode and it detects a receive line fault, it will transmit the required mayday and will generate a disconneet dial pulse. This is the same pulse that is generated in response to a DDB command.
Reeeipt of this pulse by the dial back-up unit 77 will cause it to hang up the dial lines and switeh the modem to the dedieated lines. If the dedicated line has not been restored, the operator at the site of the system controller 15 can again plaee the required calls to establish the dial back-up connection. If it were not for this protocol, it would not be possible to reestablish the dial eonneetion because subsequent calls would encounter~a busy signal (dial baek-up still holding the dial lines).
Aceording to the preferred embodiment, it is possible to use the seeondary channel as a data ehannel. For this purpose, the repertoire o~ commands includes an Inhibit Test and Control (ITC) command. Upon reeeipt of this command from the system controller 15, the test and control unit 17 will not monitor its ~
!
Il .
i -30- i .' ' ' ;
. , receivcd secondary channel da~a ~or possible ~esk and control con~nands. I~en~e, it will not inadvertently go into a tes~ mode by deeoding a co~mand in a random data stream. I~ an alarm con-dition occ~rs while the modem is in an ITC mode, the test and eontrol 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 ehannel is preferably subjeet to the following limita-tions in the preferred embodiment:
1. No seeondary CTS.
2. 4 Wire opexation.
3. Only seeondary RTS eontrol. No reverse channel operation under eontrol of primary RTS.

4. If the modem is a eentral site of a multidrop networ~, it must operate in a eontinuous earrier mode on the seeondary ehannel.

5. If the modem is a remote site of a multidrop network, its seeondary ehannel is operated in a eontrolled earrier mode but seeondaxy DCD will not be present at the DTE interfaee.

6. The data transmitted by the secondary channel cannot inelude either the RTN or RSI commands. For a cen~ral site modem, if the reeeived data is in a SPACING condi-tion for greater than 300 msee, it wi]l be elamped at the DTE interfaee to a MARK until a SPACE to MARK
' transition occurs.
When operated in a data mode, the secondary channel aeeepts 0-150 bps asynehronous data.
.
. !

-31- i ,~ ~
.

' ~ ~ 3~?~

Fi~ure 6 illus~rates a particular structu~e for a test and con~rol unit accordinc3 ~o tile preferred embodime~t of the invention. 1`he test and control uni~ includes four multiplexcrs, 55, 57, 59, 61, a microprocessor central processing unit (CPU3 63, and program storage unit ~PSU) 65. The multiplexers 55, 57,59, 61 serve to double the num~er of possible inputs to the micro-processor CPU 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 l), the Bn inputs to the multiplexers are gated to the n~ultiplexer outputs Yn, while if no select signal (the select lines are a i 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, pri~ary DCD of a central site,r a loyic high or low. If the modem is operating as a remote unit in a point to-point confiyuration, 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 of the central unit. If the modem lS 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, -32`

, the ~1 input indicates whether the modem is a xemote or central multipoint uni~.
If ~he moden. is operatin~ as a rcmote unit, the A2 si~nal is DCD; ~nd, if the modem is operating as a central unit, tl~e si~nal is ~TS. For a relnote unit, DC~ should always be on, as should ~TS ior a cen~ral unit. An off condi~ion at ~2 thus indicates a rcceive 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 speed select code is used to program the particular data rate at which the secondary channel o~ the system is to opera~. ', The A3 input signal is primary DSR and the B3 input signal is the second bit of the speed select code, either a logic 0 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 ~rom primary DCD and the signal quality level produced by the associated modem. The modem ,., I
signal quality indication is inverted and serves as an input to an AND gate. The other input to the AWD gate is primary DCD, ¦ and the output of the AND gate i5 the A4 input. An off condition ¦ at the output of the AND gate indicates that DCD is on and the ,i I
signal quality is poor. The B4 input is binary logic level, which serves as a one bi~ of a stream time code STL. The Y4 output is alternatively a signal quality indication or the STL bit.

i .
I !
Il -33- 1 "
i. !j The ~5 input is primary r~Ts ~rom the da~a tcrminal equipment DTE. Primary RTS is preferably supplicd to the circuits of Fig. 4 by a window compara~or which monitors the voltage of the RTS lead cir~uit. 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 ST~. 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 54.
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 the~irst bit of a nodem t~pe code. The Y2 output signal is then either a dial mode indication ' ~ or a modem type indication.
The A7 input signal in a r~note modem is a digital power l fail pulse from an associa~ed second tier central modem 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 fail-ure, then it will transmit a tone corresponding to a S~ACING con-dition on the secondary channel. Detec~ion of the S~ACING 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.
, i ~3~

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 bit stream representing 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 B~ input. The Y8 output is either a customer alarm or an error signal.
The A9 input is either DCD (OFF in test) in central multipoint modems or RTS in remote multipoint modems. In point- i to-point modems, the A9 input is grounded. The Ag 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 Bg input is a first bit ADo of 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 siynal is developed by feeding the receiver clock to a retri~gerable monostable multi-~ibrator. 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 sisnal to the input terminal Blo is the second bit of the test and control unit address ADl. The output siynal Ylo is the receive clocXing indication or the second address bit. The A~l input is primary channel transmit data of the modem.

-3~5-., ~ .

~3~

The input to th~ t~rminal sll is th~ ~hird bit of ~he microprocessor address ~D2. Thc output Yll is either an indica-I tion of the state of modem ~ransmit data or the ~hird address b i t A3 2, The input signal to terminal A12 is the receive datasignal while the Bl~ input i5 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 input to terminal B13 is the fif~h address bit AD4. The output Y13 is then either the Clear to Send signal CTS or the fifth address bit AD4.
The input to Al~ is a transmitter clocking signal. This ~signal is again produced from the transmitter cloc~ utilizing a retriggerable monostable circuit, as previously described for the receive clock. The ~14 input is the sixth test and control unit address bit AD5. The output Yl~ is an indication of the transmit clock operation or the sixth address bit AD5.
The input A15 is an indica~ion of whether or not the modem is in the digital loop test mode. The digital loop signal is tapped from the digltal loop control output of the micropro-cessor PSU 65. The inpu~ B15 is the seventh test and 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 `36-.
.

~3~

control unit address bit ~D6. Therefore, the output Y15 is eithex 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 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.
~ ormatted data including commands are received by the microprocessor PSU 65 at a receive data input 64. The formatted data is then ~ransformed by the microprocessor, as later discussed.
The microprocessor PSU 65 supplies a number o~ control signals to its' associated modem, as well as trclnsmit 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 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. ~t the same time, the inverse of the breaX-time loop control signal RCC disables the primary channels test pattern signal from appçaring on the telephon lines. , The secolld~ry channel may then be used to transfer the self-test results back to the modem. The S~T 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 seconclary 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. ~t remote sites, data outputted from the message out channel is applied to the modulator for transmission on the secondary FSK channel. At central sites, the ~essage 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 of checking out t:he 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 capacl~or 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 ,. .
,.
!
` -38 3~

volta~es and opens on the scco~dary powcr sid~. When a powcr failure is detected, r~lay 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 I 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 !l i 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.
~ ig. 8 illustrates the preferred 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 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 fil~er centered i: , :' !`

at 130 IIz. The output of the low-pass filter is applied to a second comparator 99, the output of which is secondary channel demodulated data. ~he secondary channel carrier is detected by a c~rrier 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 AND gate 103 is jsecondary channel received data which is supplied to the input 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 j receiving secondary carrier. The output of the second comparator ¦99 and a delayed form of the carrier detect signal are applied ; 'Ito a second AND gate 105. AND gate 105 accomplishes the same jfunction as AND gate 103.
In remote modem, the output of the AND gate 105 is otuputted to the receive data input 6fi of the microprocessor ,and to the secondary channel receive data OR gate 10~. 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 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 linput 110 in the central site modem. The 300 millisecond space ¦jinhabit timer limi~s the propagation of the power fail space tone to a single tier of the network.

I .

~ t remote sites, ~ mcssage out of ~he microprocessor is applicd to a di~iLal modulator 119, preferably on ~he mo~em LSI chip, and then to a band-pass filter 15 for transmission across the transmission line cha~nel~ If the mod~m 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 Pig. 9, the processor normally runs in an idle loop, monitoring various system parameters.
First, the processor tests the powex failure indication and receive line fault indication and resets respec~ive timers i~ !
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 i~ no streaming condition is indicated. The next test in Iig. 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 indicate character rece~tion. Once the start bit has been detected, a character will then be received. Next, 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 counting test errors condition is checked. If errors are being counted, and an error has occurred, a counter is incremented. The idle loop is then xe-peated.
Every 3.348 milliseconds, 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. At 75 BPS, rour clocks per bit time are provided.
. ' l ' -41-~3~

The first test in the interrupt chain is to ascertain if a character is being received according to the flags set during i the 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-I 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, 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 necessaxy. Next, a test for DNR
' timing is made. If DNR is timed out, the processor se*s the DNR bit in the alarm storage register and pxoduces 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 Fig. 15 is per-i~formed. If not, a test is made to indicate whether any alarms Il 42-?g~

have been set If no alarms have b~en set, the idel condition is rcentered. If alarms have becn set, it is necessary to send an alarm message back to the Ce~ntral System Controller 15, if the alarm condition is not one which has been inhibited by a received RSI command. In such case, the ITC 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 transltion 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 uti].izing a second counter which decrements on each sampling interval and is idle during 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 messaye word, and if the parity is good, the receive process is cancelled, saving i! ' . . , 1 ~.3~ ?~
'` . .
th~ new word in a receive stack. I~ it is not time for a stop bit, the new bit ls sampled, detected, and shi~ted into a regis-ter storing the current data word. The receive routine is then exited.
If a new word was saved in a stack according to the flow 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 charactcrs are rcceived. Thus, thc first 5tep in Fig. 13 is to check the top o~ the stack (the oldest character position) to determine if it is a "DEL" character. I so, a '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 nex~ 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 address 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 command set. If it is 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 hranches to the commanded routine through a table, as indicated.
Fig. 14 illustrates the Acknowledge procedure. The command number saved during the flow of Fig. 13 is placed in ' l ii I

the data byte location of the transmit stack. Parity is then calculated for the data by-te, and the Block Check Code is yenerated and isaved in the transmit stack. In the case of a single data byte 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 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 routine of Fig. 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 character time. This delays the second character, a sync byte, until the corresponding xeceiving 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 subseqllent charactcrs. The receive line fault counter is therl 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. First, a test is made to de~ermine 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, the start bit is sent. If not, a test is made to determine if it is time to send a stop bit, and if so, a mark ~is sent. If it is not time for a start bit or a stop bit, a ~ ' I

, ~.~3~

test is made to determine if the end of the character being i transmitted has been reached. If the cnd of a character has not becn reached, a data bit is sent and the proccssor returns to the idle mode. If the end of 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 1' 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. i6 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 Fiy. 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, tlle 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 .. ' i 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 Fiy. 15 is re-entered at point 5 ~ he 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 R~N
command, all mayday inhibits are cleared. The SPT, TPE, BLL, ACL, , -46-"

~3~

DCL, and any active alarm bi~s are cleared as well as ITC mode, (Not a bit) before returning to the Acknowledgement flow. Thc SDB command sets a bit w~ich 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 AC~
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 Ack~owledge flow of Fiy. 14.
Fig. 18 illustrates the performance of the DEC and DMS
con~ands, which require the processor to configure a 3-data byte messase. 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 ~low 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 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 is stored. In response to the STE command, the AC 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 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 ., ~3~

and the only step in responding to the DDB command is to drop the dial line and return to the ~cknowledgement routine.
As may be appreclated 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 prac~iced other than as specifically described above.

.

. .

., ` ' .
: ~ 48-

Claims (3)

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

1. A test and control unit for use in conjunction with a data modem comprising:
means for detecting a plurality of alarm conditions in the operation of said modem and setting corresponding alarm bits;
means for receiving a formatted message, including a plurality of words each of which contains an information portion, one of which portions is a command, and for storing the received information portions in a stack;
means for checking the contents of the stack to verify correct receipt of a command and for setting control bits to effectuate a command if said contents are correct;
means for formatting and storing an acknowledgement message in response to detection of an alarm condition by said detecting means or in response to a correctly received command; and means, for transmitting a said acknowledgement message out of said test, and control unit.

2 . A test and control unit for use in conjunction with a data modem comprising:
means for detecting a plurality of alarm conditions in the operation of said modem and setting corresponding alarm bits means for receiving a formatted message, including a plurality of words, each word including start, stop, and parity bits and an information character one of which characters may comprise a command and for detecting said start and stop bits, checking the parity condition and storing the received character in a character stack;
means for checking the contents of the character stack to verify correct receipt of a command and for setting control bits to effectuate a command if said contents are correct;
means for formatting and storing an acknowledgement message in response to detection of an alarm condition by said detecting means or in response to a correctly received command;
and means for transmitting a said acknowledgement message out of said test and control unit.

3. A data communication system comprising:
a plurality of data modems; a central processor means for generating formatted messages for trans-mission over selected transmission links; transmission means connecting said modems to said processor; and a test and control unit for use in conjunction with each data modem, each said test and control unit com-prising;
means for detecting a plurality of alarm conditions in the operation of its associated modem and setting corresponding alarm bits;
means for receiving a formatted message, including a plurality of words each of which contains an infor-mation portion, one of which portions is a command, and for storing the received information portions in a stack;
means for checking the contents of the stack to verify correct receipt of a command and for setting control bits to effectuate a command of said contents are correct;
means for formatting and storing an acknowledge-ment message in response to detection of an alarm condi-tion by said detecting means or in response to a correctly received command; and means for transmitting a said acknowledgement message out of said test and control unit.