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.
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

 B9Connect Line 1 to highway 9
04send digit 4
 0D Pause Digit
 2A loop Dis mode IDP 840 ms
 15 operate power fail relay- This command is needed to maintain any calls that may be in progress after power returns

 12 Release Seize relay

Examples of reports  sent to the processor  from the card
each report is 1 byte long

 5cStart of Ring detected
a9Seized /Digit store empty - Processor card can now send digits to be dialled
 1cEnd 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

Not all 4 wire terminals are suitable as Power fail telephones
Not all exchange line cards have a power fail relay

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