Herald 4 port 4 wire Terminal Card

1A1/SA20006- YELLOW Handle

This card can run 4 4 wire terminals or 4 2 wire POTS phones

The D4071 crosspoint ICs

This card can switch audio from any one of the Herald's 20 2 wire speech highways to any of the attached 4 telephones. It does this with 4 integrated circuits, each able to connect 2 of the phones to 10 of the speech highways. Each D4071 is passed commands from the processor card, via the backplane and finally from the control IC via the XPT lead.

 Crosspoint IC Highway
 a 1-10 1 3
 b 1-10 2 4
 c 11-20 1 3
 d 11-20 2 4

The Ring Relay

Conventional POTS type ringing is used to alert a two wire POTS  phone extension user to an incoming call.
The ring current is not carried on any of the Herald's 20 highways, instead the 25 Hz ring current, generated by the tone card is carried to all the left hand slots, each card has a 4 ring relays to direct the ring current to the appropriate extension telephone.

The D4073 Control IC

The relays and crosspoint on the terminal card itself and the LEDs, tone caller on the 4 terminals attached to the card are controlled from the    Herald's processor card located on the right hand side of the top shelf close to the power supply.

Terminals equipped with loudspeaking amps and headsets can also be controlled via the above method fro the LEDs

Part of the processors address bus:- R/W,A0,A1,A13 
and a CE (Card enable) signal and serial DFM (data from micro) signal are processed by the four Control ICs pictured below.

The state of the terminals hookswitch and press buttons as well as the loop and earth recall detectors on the card are also communicated via these control ICs and via the the serial DTM (data to micro) signal back to the processor.

There are 2 pairs which form the  interface between the terminal card and each terminal
1 pair carries (the speech pair) the speech and ring current, the other pair carries the push button and hookswitch states from the terminal and LED states to the terminal.

In the event of a mains power failure an exchange line is switched directly to a terminal via the speech pair, so that the signals on the speech pair are very similar to a POTS line.


The terminal interface card is supplied with the following DC  power via the backplane connector
-50VSpeech,-50VData,-12V,+12v,+5v,+1.5, ov

When the herald is powered the terminal card supplies DC to the terminal over the data pair that all times except when a short on that pair is detected.

The things I don't know
When the herald is powered the terminal card supplies DC to the terminal over the speech pair that all times (inc when ringing) ?
When the herald is powered the terminal card supplies DC to the terminal over the data pair at all times  ?
When the herald is powered both internal and external ringing is signalled to the terminal by ring current or over the data pair ??? 

When the herald is powered dialled digits are signalled back to the terminal card over the data pair or by 10 pps on the speech pair ???

The Data Transformers

Both the data pair and speech pair need transformers, below are the four data transformers on the card

Back Plane Connector

Thanks to Mike for sending me some pictures on later versions of this card

SA20557 4 port 4 wire  Terminal Card (featurephone)

This card allows up to to  four system phones  or POTS telephones  in the Pentara System is provided by this 4-circuit,  4·wire  Extension  Card.  

Connections to system phones are made via a speech pair line and a 
data  pair  line,  POTS telephones are connected to the speech 
pair lines  only. Display data is provided for the use of 
system phones having  the Display  feature. 

4 x 4 wire extension card block diagram

Digital Communications with system phone

There are four Data Circuits on each 4-wire Extension Card.,  one  to  each Terminal. The circuits are identical  

Data  Transmission

Data sign all ing between the Extension Card and the Terminals is by biphase serial data transmission at 1.2kHz. LED data or display data is transmitted to the Terminal and keycode data is received from the Terminal.

The data pai r   (Data Line 1)   to  a  Terminal is coupled to  the  Terminal  Interface  Data  IC  (lC15)  via  trans­mitter  circuit  IC2(a).  TR5  and  T3.  The  transmitter circuit  combines  the  outputs  from  IC15  (TXOA  and TXOB)  into  a  biphase  signal  by  differential  amplifier IC2(a).  This  differential  mode  allows  the  first  transition  to  be  half  that  of  subsequent  ones,  thus  preventing  an  initial  voltage  offset  which  would  cause the  receiver  at the Terminal to miss the first few transitions. 
Herald digital comms with featurephone


Data transmission to a Terminal is initiated by a "Control Load" command from the Processor. The
Data IC reads the LED data from the appropriate locations in the RAM and sends it to the Data Circuit from the TX outputs. If the Terminal requires display data this is transmitted after the LED data, refer to para 4.6.2. If the Retransmit Request Latch (RRL) in the Data IC is set, the control word sent before the LED data is set to 111 1, the R R L is then reset.

Figure  4.2  shows  a typical biphase waveform. 
The  output  of  IC2(a)  drives  TR5  to  apply  the  signal to  the  data  line  via  T3,  maximum peak-to-peak  volt­age is 0.85V and the transmission rate is  1.2kHz. 

LED/DISPLAY DATA transmitted to terminal

A LED data transmission consists of a preamble byte (6 Hex), a control byte'and four LED data words of 2 bytes each, the transmission is completed by a post-amble bit (0). The control byte consists of two hexa-decimal characters, the first character is either 0 for a normal transmission or F (Hex) for a re-transmit request, the second character is 4 to indicate that 4 LED data words follow (see Figure 4.8).

A display data transmission is similar to a LED data transmission, but has the second character in the control byte = C (Hex) to indicate that there are 8 display words (2 bytes each) in addition to the 4 LED data words. The transmission is completed by a post-amble bit (0) (see Figure 4.9).

format of data sent to herald feature phone

Data Re c e p t i o n  

The  incoming  biphase  data  signals  on Data Line  1 from  a  Terminal  are  connected  via  T3  and C6  to the Schmitt trigger circuit IC9(al. R24, R32 and R40.

The Schmitt trigger circuit has a hysteresis of approximately 0.2V. R 10, R13 and C3 provide the reference voltage for all four data circuits. The detected data signals are output on the RXO line from IC9(a) to the Data IC (IC15).


After the LED data (and display data) has been sent to a Terminal the Data IC waits for keycode data from that Terminal on the RX input. Should the preamble and control byte not be received within 2 frames the terminal type counter (TTC) will have reached a count of 2 and the code T000 0000 will be sent to the Speech IC (where T = 0). When the preamble and control byte are received the TTC is reset,

the Data IC then waits for the keycode and the key-code complement_ Should the keycode not be received or should the keycode not match the complement, the R R L is set and the code TOOO 0000 will be sent to the Speech IC (where T = 1) _ When the keycode data is correctly received the code TKKK KKKK will be sent to the Processor (where T= 1 and KKK KKKK is the keycode).

Herald digital comms with terminal

how herald terminal keypresses are encoded

Data Power  Supply  

The  data  pair is   f e d  via  Darlington transistor T R 7  and T3  secondary  windings.  Signal  DP1  active  from  the Speech  IC  (IC14)  turns  on TR3  to  drive TR7 on and connect  the  -50VD  supply  to  the  data  line   via   T3 winding  4/5.  OV  is connected  to  the  c  leg  via  T3 w inding  1/2.  Diode  D13  protects  TR7  against  the back  emf across T3 when TR7 turns off. R62 and R64 
protect the data pair against over voltage. 

Data Power Supply Short  Circuit Detection 

The current through  the  secondary windings of T3 is monitored by resistor network RU1(a) and level comparator IC1(a). Should the current exceed 90-250mA, IC1 (a)  output  switches  low  to  generate  signal  SC1 active  to  the  Speech  IC  (IC14).  IC1 (a)  output  goes high when the current  is not greater than 90mA.

Hywel Clatworthy,
23 Mar 2012, 07:41