[Elecraft] KPA500 Serial interface question

[Andy Durbin]

"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?"

Well I found the answer thanks to another KPA500 owner running a similar test.

It turns out that setting SER POLL to OFF and remaining in the Menu stops polling but leaves the relay closed with -12 V on pin 3.

Only when the menu is exited does the relay get deactivated and pin 3 goes open circuit.

Similarly, opening the menu and setting SER POLL from Off to On leaves the relay open and the relay only closes when the menu is exited.

I wonder how many KPA500 owners have been caught by this.

73,
Andy k3wyc