[Elecraft] KPA500 Serial interface question

[Andy Durbin]

I started down this path because I'm trying to help a Scottish op who seems to have a failed KPA500 XCVR port.  (Scope shows no indication of polling when Ser Poll is on.)

KPA500 has an IO board with two serial interfaces.  One is for connection to a PC and the other for connection to a transceiver.   The data out line of the XCVR port passes through relay K1 and I assumed this relay would be open when SER POLL is set to OFF.  My scope measurements show this line is actually pulled low when SER POLL is Off which implies that K1 is not open.  With Ser Poll On I see normal polling with IF;FA;FB;  My interface works ok although I'm currently using the PC port not the XCVR for KPA500 control.

I suspect that Jack is the only one who will know the details but I'm asking here so others can learn if interested.  My question is:

Serial IO board K1 is controlled by "SBYB" which traces to signal "BYPRLY".  The names suggest "bypass relay" but under what circumstances is this signal set/cleared?  In other words, when does K1 open the data out line?

Thanks and 73,
Andy, k3wyc