CA1315368C - D-5 channel bank control structure and controller - Google Patents

Abstract

ABSTRACT
This invention relates to a circuit for encoding incoming user signals into an outgoing stream of digital words. Each digital word includes an outgoing parity bit.
The current also decodes an incoming stream of digital words, each including an incoming parity bit. The circuit is comprised of a first unit for sensing deviation of parity of the incoming stream of digital words; and a second unit for developing a parity for the outgoing stream of digital words responsive to the first unit.

Description

D-5 CHANNEL BANK CONTROL STRUCT~RE AND CONTROLLER

This is a division of copending Canadian Patent Application Serial No. 520,716 which was filed on October 17, 198~.
Background of the Invention It is well known that telecommunication signals can be transmitted over long hauls in digitally encoded form.
Approximately in 1962, the Tl carrier system began to sarve the U~S. metropolitan areas in conjunction with the D1 Digital Terminal System lDTS), and since then improved and more versatile digital terminal systems have been introduced.
The basic elements of a digital terminal system are the modulation, transmission, reception, demodulation, signaling, and testing subsystems. To perform the modulation and demodulation functions efficiently, the D1 system and all subsequent ~ type systems (D2 through D4) have been oryanized in channel banks of two 12 channel groups, collectively called a digroup, with common control extended over the entixe digroup. Modulation is performed by developing pulse amplitude samples of voice ~requency signals and encoding the PAM samples into 8 bit PCM words. The PCM words of individual voice frequency circuits, or channels, are time division multiplexed and the resultant multiplexed signal is applied to the digi~al transmission medium.
Although initially the D type channel banks were intended as interface elements between analog voice circuits and digital transmission facilities interconnecting central offices, their primary use changed substantially over the years. More and more of the switching machines were converted to digital operation with stored program control, and those machines employ direct interface equipment to link the T-carrie~ systems. These developments shifted the primary application of channel banks from handling interoffice message trunks to providing special service circuits.

2 1 31 536~
One example of a special service circuit is one that allows a call to be made outside the local exchange area for the price of a local call. Another example is providing a digital computer-to-computer link.
One characteristic of special circuits (which is closely related to the nature of their use) is that they often are being rearrar.ged. Unlike the message trunks, about twenty-five percent of all special--service circuits are either installed, modified, or disconnected every year. These movements are expected to reach fifty percent in the near future.
The problem is that each of the circuits for these special service lines must be designed, set up electrically and tested before being turned over to a customer. A
substantial use of labor with the existing D4 channel bank is requlred because there are many types of circuits, many different electronic modules that collectively create the circuits, and the electrical set-ups and testing of such circuits.
It is an ob~ect of this invention to provide channel units that are capable of interacting with controller 10. It is another object of this invention to provide channel units that are programmable D It is a still fuxther object of this invention to provide channel units that are capable of remote provisioning and testing.
Summary of the Invention These and other objects are obtained with a channel unit D5 that comprises a user port and a carrier port for interfacing users with a carrier system by converting user signals to a PCM format adapted for the carrier system, and vice-versa. The channel unit further comprises a metallic access port for disconnecting the channel unit from the user and connecting test signals to the user port, and a digital provisioning port for providing operational parameters to the channel unit or verifyin~ the operational parameters of the channel unit. In addition to the check-out available through the metallic access port, the operation of the channel unit is 3 1 3 1 536~
checked by an arrangement that determines the parity of incoming PC~ words and generates outgoing PCM words with the same parity sense. The operational parameters are supplied to the channel units via digital words that contain a bit that specifies whether reading or writing of parameters is to be executed, an address field that specifies the operational parameter to be considered, and a message field that specifies (in the case of a write request) the value of the parameter.
In accordance with one aspect of the invention there is provided a circuit for encoding incoming user signals into an outgoing stream of digital words, each including an outgoing parity bit, and for decoding an incoming stream of digital words, each including an incoming parity bit: first means for sensing deviation of parity of said incoming stream of digital word; and second means for developing a parity for said outgoing stream of digital words responsive to said first means.
In accordance with another aspect of ths invention there is provided an apparatus for receiving data from e~ternal equipment and for transmitting data to said external equipment comprising: first means for receiving data sent by said external equipment; second means for evaluating a parity sense of data received from said external equipment; and third means for developing a parity bit for each data word transmitted to said external equipment with a parity sense that is related to said parity sense developed in said second means.
A clearer understanding of the invention may be had from the following detailed description and the accompanying drawings where:
Brief Description of the Drawinq The present invention taken in conju~ction with the invention disclosed in copending Canadian Patent ~pplication Serial No. 520l716 which was filed on October 17, 1986, will be described hereinbelow with the aid of the accompanying drawings, in which:

~ 1 3 1 536~ 1 FIG. 1 is a general block diagram of the D5 channel.
bank system comprising a number of channel banks;
FIG. 2 is a block diagram of a digroup, which is part of each channel bank, including a plurality of channel units;
FIG. 3 is a block diagram of a channel unit in accordance with this invention;
FIG. 4 is a diagram of impedance interface block 32 of FIG. 3;
FIG. 5 is a diagram of gain stage 33 of FIG. 3;
FIG. 6 is a diagram of egualizer block 34 of FIG. 3;
FIG. 7 is a diagram of signaling block 37 of FIG. 3;
FIG. 8 is a diagram of encoder block 35 of FIG. 3;
FIG. 9 is a diagram of decoder block 36 of FIG. 3;
and FIG. 10 is a diagram of parameter control block 39 of FIG. 3.
Detailed Description Channel bank 20, T~hich is part of the channel bank depicted in FIG. 1, includes a bank controller 21 that interacts with system controller lO, up to four digroups 22 responsive to commands from controller 21 and a facility interface 24 that combines the digital output of digroups 22 to form a T-2 digital output (carrier) signal. The channel bank also contains a bank test access unit 23 that provides digital and metallic access for testing the channel units ~: (also interacting with system controller 10), a clock and synchronization unit 25 responsive to bank controller 21, and a bank power unit 26. Bank controller 21 receives instructions from system controller 10 and after translation and reformatting sends instructions to other circuits in the bank.
Facility interface 24 per~orms multiplexing and demultiplexing operations. It multiplexes the output signals o~ digroups 22 and outputs the developed PCM signal onto line 241. It also demultiplexes the PCM signal incoming on t 31 536~

line 241 and distributes it to digroups 22. In addition, facility interface 24 converts, as appropriate, the format of signals flowing between digroups 22 and PCM line 241.
More specifically in connection with the format conversions, facility interface unit 2~ is designed to operate with one, two, or four digroups in a channel bank; providing an output rate of either 1.544 Mb/s, 3.152 Mb/s, or 6.312 Mb/s, respectively. When transmitting to line 241, facility interface 2~ o~tains its clock from bank clock and synchronization unit 25. It converts the unipolar bitstxeam from digroups 22 to a bipolar bitstream and clocks it onto line 241. When more thar. one digroup is handled by facility interface 24, it also adds control, stuffing, and framing bits, performs encoding of the signal, and scrambles the bits to improve t~e statistical properties of the PCM signal. This accounts for the fact that the above~mentioned rates are not multiples of each other. Facility interface 24 also provides cable equalization circuitry at the output to line 241, to compensate for cable lengths in the connection of interface 24 to cross-connect frames in the telephone company central office. Selection of the specific equalization is achieved through supply of operational parameters by bank controller 211, as directed by system controller 10.
When receiving from line 241, facility interface 24 recovers the clock from the recei~ed line signal. It then converts the bipolar signal into a unipolar signal and applies it to digroup 22. When the incoming sig~al is destined to more than one digroup, facility interface 24 also unscrambles and demultiplexes the signal into two or four bitstream signals using the stuffing, control, and framing bits inserted at the far end.
FIG. 2 presents a more detailed block diagram of digroup 22. It includes a digroup controller 221 and a digroup formatter 222 r which make up the common equipment, and twenty-four channel units 30. Controller 221 is responsive to control signals arriving from bank controller 21 and it distributes the control signals to formatter 222 and channel units 30. Controller 221 is also responsive to bank test access unit 23 and it distributes test signals to channel units 30.
Digroup formatter 222 provides framlng, performance monitoring, and rate conversion between the line rate going to facility interface 2~ and the internal data rate.
In its interaction ~ith channel units 30, digroup controller 221 provides the following: a) channel counting sequence to enable the various channel units to communicate with facility interface 24 via controller 221, digroup formatter 222, and line 223, and b) access to the PCM ports of channel units 31 by the bank test access unit 23.
In accordance with this invention, a channel unit 30 may be provisioned, or arranged, to perform many different functions that heretofore required specially designed channel units. Consequently, the number of different D5 channel bank units is much smaller than the number of different prior art (D4) channel units. Still, there may be different types of channel units 30 connected to a digroup controller 221.
FIG. 3 illustrates one such channel unit. This channel unit is commonly known as a four wire unit because it contains two pairs of transmission leads going to the user:
that is, T and R, and Tl and Rl. In a context of this disclosure, a user may be a customer's t:elephone line or a switching machine's (e.g., a central office or a PBX~ trunk line. The FIG. 3 channel unit is adapted for users that provide voiceband analog signals. Signals going to the user are provided by the channel unit in a balanced -fashion across the Tl and R1 leads, and signals coming from the user are offered to the channel unit across the T and R leads, also in a balanced fashion. Also interfacing with the user is signaling bus line 2~2 that communicates signaling information between the user and the channel unit, such as on-hook/off-hook information.
Other than the interface to the user, the FIG. 3 channel unit interfaces with test access unit 23 via a metallic testing port embodied by bus 243 in FIG. 3, bank 7 1 3 1 5 3 6 ~
controller 21 via a provisioning port containing message line 244 and enabling MNP-MNQ selection leads, and digroup controller 221 via a data port ~or communicating PCM encoded words to the carrier system. The data port includes Transmit and Receive PCM lines, Transmit and Receive clock lines, and Transmit and Receive leads.
The signal path for information ~lowing from the user to the carrier system includes leads T and R, metallic access (MA) block 31 for inter~acing with test access unit 23, impedance interface block 32-1, gain stage 33-1, equalizer 34-1 and encoder 35. The signal path for information flowing the other way includes decoder 36, gain stage 33-2, equalizer 34-2, and impedance interface block 32-2. The output of impedance interface block 32-2 is applied to MA block 31, wherefrom the signals are connected to leads T1 and R1. Signaling information to and from users, on bus 242, communicates via MA block 31 to signaling block 37 which interacts with encoder 35 and decoder 36. Blocks 31-37 are controlled by parameter control block 3g through common bus X and enabling leads A through F. In accordance with the principles o~ this invention, parameter control block 39 is able to dictate the operational parameters of the blocks interacting therewith in xesponse to information supplied by bank controller 21, as well as to ascertain the value of the parameters resident in the channel unit and communicate them to bank controller 21. Parameter control bloc~ 39 communicates with bank controller 21 through a message bus 244, which is a serial bus, and two enabling control leads:
MNP and MNQ leads.
Encoder 35 communicates with digroup controller 221 via the Transmit PCM digital line, the Transmit Clock, and the Transmit P and Q selection leads. Decoder 36 similarly communicates with digroup controller 221 via the Receive PCM
digital line, the Receive Clock, and the Receive P and Q
selection leads.

1 31 536g To better understand the operation of the FIG. 3 channel unit the following, in conjunction with FIGS. 4-10, describes each of the elements in the FIG. 3 channel unit in greater detail. Of course, the designs described are illustrative in nature and many modifications can easily be implemented by those skilled in the art.
Impedance Interface Block 32 Impedance interface blocks 32-1 and 32-2 can be identical in structure. Their function in the FIG. 3 channel unit is to present a balanced port for the user leads (T, R, T1, and R1) with a characteristic impedance controllabie by parameter control block 39. FIG. 4 depicts one embodiment in impedance interface 32 and, for the sake of simplicity, includes the relevant portion of MA block 31.
Lead T is connected to terminal 1 of the primary winding of balanced transformer 321 through normally closed contact 312-1 of relay 312 and normally closed contact 311 1 of relay 311. The R lead is connected to terminal 2 of the primary winding of transformer 321 via normally closed contact 312-3, of relay 312 and normally closed contact 311-3 of relay 311. Also connected to the T lead is lead 313 through normally open contact 312-2 of relay 3:L2 and the R lead is connected to lead 314 through normally open relay contact 312-4 of relay 312. In a similar fashion, lead 315 is connected to terminal 1 of transformer 321 through normally open contact 311-2 of relay 311, and lead 316 is connected to terminal 2 o~
transformer 321 through normally open relay contact 311-4 of relay 311. Relays 312 and 311 (and their contacts) are part of MA block 31 and their function is to isolate the T and R
leads from the channel unit, allowing thereby signals to be sent to the customer through leads 313 and 314 and separate signals to be sent to the remainder of the channel unit through leads 315 and 316. Leads 313-316 are part of signal bus 243 running from MA block 31 to metallic access unit 23.
Transformer 321 has a secondary winding with terminals 3 and 4, and connected to terminals 3 and 4 is impedance converter circuit 322. The center tap is grounded. Impedance converter 1 3 1 536~3 circuit 322 causes a specific impedance to appear across the T
and R leads as directed by parameter control block 39 via bus X. The output signal of impedance con~erter 322 appears on line 323 and is applied to the appropriate following stage, e.g., gain element 33-1. Impedance converter 322 can be realized in a number o~ ways, e.g., merely with resistors switched in or o~t by means of relay contacts not unlike contacts 311. One particularly advantageous realization is described in U.S. Patent No. 4,476,350.
Bus 324 provides the necessary signals to control the impedance presented by converter 323 (via transformer 321) to the T and R leads, and those signals are derived from parameter store block 325, which is connected to parameter control block 39 via bus X and enable line A. Bus X contains an MSG line, a gated clock, and a Read/Write lead (R/W). The MSG line is connected to an FET switch 328 which, when enabled by line A, connects the MSG line to register 326. Register 326 is clocked by the gated clock signal. The parallel output of register 326 is applied to bus 324, while its serial output is connected to AND gate 327. Gate 327 is enabled with the R/W lead of bus ~. In operation, when new information is sent to parameter store 325 from parameter control block 39~ it is enabled with lead A, lead R/W is low, and the digits appearing on the MSG line are serially clocked into register 326 through closed switch 328. When parameter control block 39 wishes to be informed of the contents o~ register 326, parameter store 325 is enabled with lead A, the R/W lead is high and the gated clock shifts the contents of register 326 through AND
gate 327. That information is clocked back into register 326 and is also cast upon the MSG line, to be read by parameter control block 39.
Gain Stage 33 Gain stage 33, as depicted in FIG. 5, is a generic representation of gain stages 33-1 and 33-2. It comprises an operational amplifier 330, resistors 331 through 335, relays 336 through 338 (with associated normally open relay contacts 336-1 through 338-1, respectively), and parameter lo 1 31 536~ 1 store block 339. Resistor 331 connects the input of gain stage 33 to the negative input of amplifier 330, while the positive input of amplifier 330 is connected to ground.
Resistors 332 through 335 are serially connected and interposed between the negative input of amplifier 330 and its output. Relay contacts 336-1 through 338 1 are connected in parallel with resistors 333 through 335, respectively. The state of the relay contacts is controlled by bus Kl, emanating from parameter store 339, which controls relays 336 through 338. Parameter store 339 is identical in construction to parameter store 325, described above, and like parameter store 325, it is responsive to bus X. Parameter store 339 is enabled by lead B.
In operation, the resistance of the series connection between the output of amplifier 330 and its negative input is based on the particular combination of closed relay contacts, and the value of that resistance determines the gain. Thus, by controlling relays 336 through 338, the gain of blocX 33 is determined.
Equalizer Block 34 Equalizer 34 may be any conventional e~ualizer that is adapted for electronic control of its operational parameters. Channel units often use equalizers known as "bump equalizers~', and that is the type of equalizer depicted in FIG. 6. It comprises operational amplifiers 342-1, 342-2, 342-3, 342-4, and 342-5 with various resistors and capacitors arranged in accordance with well-known design approaches. In addition and as part of the design, amplifier 342-1 has a series connection of resistors 343-1, 343-2, 343-3, and 343-4 interposed between its output and its negative input.
Normally open relay contacts 344-1, 344-2, 344-3, and 344-4 are connected in parallel with these resistors, respectively.
Also connected to the negative input of amplifier 342-2 are resistors 346-1, 346-2, 346-3, and 346-4. Connected in series with these resistors are normally open relay contacts 348-l, 348-2, 348-3, and 348-4, respectively, which effect connection of the resistors to ground. In a similar manner, resistors i') l 5 3 6 ~
347-1, 347-2, 347-3, and 347-4/ are connected to ground through normally open relay contacts 349-1, 349-2, 349-3, and 3~9-4, respectively, and arranged to affect the negative input of amplifier 342-4. The state of the relay contacts is controlled by bus K2, emanating from parameter store 341, which connects to relays 344, 345, 348, and 349 and communicates with parameter control block 39 via bus X and enable lead C. Parameter store 341 is identical in structure to parameter store 325.
Signalinq Block 37 In addition to the message signals, users provide and also expect, signaling information. An example of signaling information is the off-hook/on-hook condition.
This signaling information is communicated between the users and the carrier system via signaling bloc~ 37, whose block diagram is depicted in FIG. 7. For the sake of simplicity, ~IG. 7 also includes the relevant portion of MA block 31.
The FIG. 7 signaling block provides to users the si~naling lines MA, MB, EA, and EB. Line MB is connected to current limiter 376 through contacts 373-1 of latching relay 373. Current limiter 376 limits the current o~ an applied -48 volt supply. Line MA is connected to a battery detector circuit 377 via normally closed contacts 379-1 and 380-1 of relays 379 and 380, respectively. Battery detector circuit 377 is also connected to line MB through normally open contact 371-1 of relay 371, and still further connected to ground via a series connection of normally closed contact 371-2 of relay 371 and contact 374-1 of latching relay 374.
Line EB is connected to ground through contact 375-1 o~ latching relay 375, and to ground detector 378 through normally open contact 372-1 of relay 372. Line EA is connected to ground datector 378 through normally closed contacts 379-3 and 380-3 o~ relays 379 and 380, respectively.
Line M~, battery detector 377, line EA, and ground detector 378 are connected to bus ~43 through normally open contacts 379-2, 379 4, 380-2, and 380-4, respectively.

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The output of detectors 377 and 378 is applied to logic block 383. This output comprises the signaling information supplied by the user. Logic block 383 is a combinatorial logic block that develops an output signal which is placed in register 384 and sent to encoder 35.
Signaling information from decoder 36 is clocked into regisker 385 and the output of register 385 is applied to combinatorial logic block 382. The output o~ logic block 382 controls relays 371 and 372 which provide signaling information to the user via lines MA, MB, EA, and EB.
Parameter store block 381 controls logic blocks 382 and 383 as well as relays 379, 380, 373, 374, and 375. Parameter store bloc]c 381, which is identical in structure to parameter store block 325, communicates with parameter control block 39 via bus X and enable lead D.
Encoder 35 FIG. 8 shows a block diagram of encoder 35. Sampler 351 connects the analog signal from equalizer 34-1 to A/D
converter 352, where the signal is converted to pulse code modulated digital words. Sampler 351 and A/D converter 352 may be of any conventional design. The output of A/D
converter 352 is applied in parallel to shift register 353, whose parallel output is connected to parity circuit 356.
The serial input of register 353 is connected to shift register 354 which is f~d by register 38~ within signaling block 37, and its serial output is connected to AND gate 358.
The parallel output of register 354 is also connected to parity circuit 35G, and circuit 356 develops a parity signal based on the inputs from registers 353 and 354. The parity signal is stored in parity bit 357 which feeds a serial input of register 35~. Whether the parity developed by circuit 356 is even or odd parity is dictated by a parity sense signal from decoder 36. The contents of registers 353, 354 and 357 are sent to a Transmit PCM line via AND gate 358 under control of a Transmit Clock gated through AND gate 359. Gates 358 and 359 are sensitive to enable leads P and Q, which are sent by digroup controller 221.

Decoder 36 FIG. 9 presents a block diagram of decoder 36. A
Receive clock and a Receive PCM line are gated (with P and Q
leads) through AND gates 361 and 362, respectively, with the gated PCM signal being sent to parity detector circuit 363 and to a series connection of registers 364 and 365. The gated clock signal is applied to the clock inputs of register 364 and 365. Parity detector circuit 363 is of conventional design, having its output sent to encoder circuit 35. In this manner there is a uni~ue feedback from the Receive PCM
signal to the Transmit PCM signal that checks on the trans-mission path to and from the channel unit, as well as on the parity detection hardware within the channel units. Whatever parity is detected on the incoming PCM words by circuit 363 and that same parity is generated by parity circuit 356 within encoder 35. That way, digroup controller 221 and bank controller 21 can check on the operation of the equipment by merely looking at the outgoing parity bits.
Register 364 stores arriving signaling information and register 365 stores data. Accordingly, the output of register 364 is sent to signaling block 37 and the output of register 365 is sent to D/A converter 366. The output of converter 366 is an analog signal that, after filtering through low pass filter 367, is sent to gain block 33-2.
Parameter Control 39 FIG. 10 presents a block diagram of parameter control block 39 which, in accordance with the principles of this invention, interfaces bank controller 21 (via digroup controller 221) with the various elements of channel unit 30 and provides for entering operational parameters into those elements. The interface to bank controller 21 comprises enable leads MNQ and MNP, and a bidirectional message line (MESSAGE). Enable lead MNP is normally at logic level 1 when the message line of the channel unit is not enabled. It goes down to logic level 0 to enable communications with the channel unit. Enable lead MNQ is similarly at logic level 1 when not enabling, but it becomes a gated clock when it is 14 l 31 536~
enabling. Enable lead MNQ is applied to OR gate 391, to NAND
gate 393, to the gated clock line of bus X, to the clock input of counter 392. Enable lead MNP is applied to OR gate 391 and to NAND gate 393. The output of OR gate 391 is connected to the set lead of flip-flop 394, while the output of NAND gate 393 is connected to the reset lead of flip-flop 394 and the reset lead of counter 3g2. The output of flip-flop 394 is set when both MNQ and MNP assume logic level o and is reset when both resume logic level 1. Flip-flop 394, therefore, provides an enabling signal for the bidirectional message line and is accordingly connected to AND gate 396 and NAND gates 386 and 390. The output of counter 392 (which counts clock pulses) is applied to decoder 395 which, through simple combinatorial logic, develops appropriately timed enable signals ~A through AE.
. The bidirectional message line is applied to AND
gate 396 which, as indicated above, is enabled with flip-flop 394. The output of gate 396 is applied to flip-flop 397 which, under control of signal AA, captures the first bit arriving on the message line through gate 396. This is the Read/Write bit of the arriving message, and it is fed to bus X. The output of AND gate 396 also feeds register 398 which stores the remaining bits appear:ing on the message, and parity check circuit 399. Under control of signal AB, the result of the parity check on the word coming through gate 396 is transferred to register 385 and decoder 387. The output o~
register 385 is sent back to digroup contxoller 221, through the message line, via NAND gate 386 which is responsive to gating signal AC. The output of register 398 is applied to decoder 387. Register 398 contains information that indicates both the element that needs to be accessed and the parameter value that needs to be placed in that element. ~ccordingly, decoder 387 is responsive to that portion of the register 398 contents that specifies the element to be accessed. The output of decoder 387 is enable leads A through E which are appropriately controlled by the output from parity check 399 to either inhibit or enable the transfer of data from bus X to 15 'I 3 1 536~
the addressed element within channel unit 30. That porti.on of the contents of reyister 398 that specifies the parameter value is sent to the element, enabled by decoder 387, within channel unit 30 via bus X.
When the information appearing on the message line indicates a Read request, which is a request of the channel unit to read the parameters contained in one of the elements within channel unit 30, the MSG line of bus X is connected to the message line via NAND gate 390 which is responsive to the Read/Write bit of flip-flop 397 an~ to signal AD of decoder 395. The information applied to NAND gate 390 during a read operation is also applied to parity generator 383. A
parity bit is output by generator 383 and is appended, through gate 384 with enable lead AE, to the information being read.

Claims

Claims 1. A circuit for encoding incoming user signals into an outgoing stream of digital words, each including an outgoing parity bit, and for decoding an incoming stream of digital words, each including an incoming parity bit:
first means for sensing deviation of parity of said incoming stream of digital word; and second means for developing a parity for said outgoing stream of digital words responsive to said first means.
2. The circuit of claim 1 wherein said first means develops a sense output that is at one logic level when said parity of said incoming stream of digital words is even and at another level when said parity of said incoming stream of digital words is odd, and wherein said second means develops even parity when said sense output indicates even parity and develops odd parity when said sense output indicates odd parity.
3. The circuit of claim 1 wherein said outgoing digital words are PCM-coded representations of said incoming user signals.
4. A circuit for encoding incoming user signals into output digital words delivered at a transmission interface, each of said output digital words including an outgoing parity bit, and for verifying reception of input digital words incoming at said transmission interface, comprising:
means for receiving said input digital words from said transmission interface;
first means for evaluating parity of said input digital words;
means for developing said outgoing parity bits for said output digital words that are related to said output digital words and to the parity evaluations of said first means; and means for applying said output digital words and said outgoing parity bits to said transmission interface.

5. Apparatus for receiving data from external equipment and for transmitting data to said external equipment comprising:
first means for receiving data sent by said external equipment;
second means for evaluating a parity sense of data received from said external equipment; and third means for developing a parity bit for each data word transmitted to said external equipment with a parity sense that is related to said parity sense developed in said second means.
6. Apparatus for receiving data from sending equipment and for transmitting data to said sending equipment comprising:
first means for receiving data sent by said sending equipment;
second means, responsive to said first means, for developing a reception error signal when parity of data received from said sending equipment is other than expected;
and third means for developing a parity bit for each data word transmitted to said sending equipment with a parity sense that is related to said reception error signal.