Exchange Line Interface Card
SA2008 - Blue Handle
This card allows any of the Herlald's Extensions access to 2 exchange lines.
Ringing is detected here, as well as Hold, Pulse Dialling and Earth Recall
The D4071 crosspoint ICs
It can be seen how the Crosspoint ICs pick up connection to the 20 speech highways on the backplane connector
This card can switch audio from any one of the Herald's 20 2 wire speech highways to either of the Exchnage lines. It does this with 2 crosspoint ICs , The first IC is able to connect either of the exchange lines to either the first 10 of the speech highways. The second IC is able to connect either of the exchange lines to the speech highways 11-20. Each D4071 is passed commands from the processor card, via the backplane and finally from the control IC via the XPT lead.
The D4071 crosspoint IC is also used on the 4 wire Extension line card
Exchange Line relays
Hold, Off Hook,Pulse Dialling and Earth Recall are performed by this fine set of relays.
The coils in the relays are driven by the following relay driver circuits
Which in turn are controlled by the microprocessor via the backplane and the Control ICs
The D4072 Control ICs
Each D4072 receives commands from the processor card on the following lines.
R/W
A0, A1 , A13
a CE (Card enable) signal and
serial DFM (data from micro) signal
When the A13 signal is 0 and R/W is low(o) it sends the data it receives from the processor onto the crosspoint ICs
When A13 signal is 1 and R/W is low(0) it receives data to send to the relays
When A13 signal is 1 and R/W is High(1) it sends data from things like the ring detector and dial tone detector to the processor
Examples of Commands sent to the card by the processor
each command is 1 byte long
12 Release Seize relay
Examples of reports sent to the processor from the card
each report is 1 byte long
5c
a9
1c
Start of Ring detected
Seized /Digit store empty - Processor card can now send digits to be dialled
End of Loop - Processor card can now operate PFL relay
Power Fail Relays
When Power is lost to the Herald system, it can be arranged that one 2 wire or 4 wire terminal is attached to each exchange line.
The power fail relay on the exchange line card needs to intercept the conection between the Terminal Card and terminal. This is done in Box Connection 101A or 102B as follows
Note
Not all 4 wire terminals are suitable as Power fail telephones
Not all exchange line cards have a power fail relay
see bottom of this page http://www.britishtelephones.com/heralda.htm
Back Plane Connector
Thanks to Mike for sending me some pictures of other versions of this card
SA20558 Trunk card 2 Port Full feature
This has fewer big relays, but looks more complex than the SA2008
below is another example of a SA20558
