Starboard GMSK modem CD and SQL signals

Some of the Moencomm Star*Board signals are not very well documented so this brief article documents some of the information I received from Jim Moen:

The Carrier Detect (CD) / Squelch (SQL) output from a radio can be used by the Star*Board but Jim Moen's standard board setup does not use the radio to detect a valid carrier. Instead, it uses the RX SN signal which is generated by the GMSK modem chip on the board. 

RX SN is a logic level signal which, when averaged, indicates the signal to noise ratio of the received signal.

The related jumpers on the board are labeled RSSI. SW3 enables this function and SW4(2+3) selects the RX SN signal from the modem chip to drive an input of the PIC to indicate that data is being decoded (by the GMSK modem) and needs to be processed.

Jim Moen calls this the "Dutch*Star Soft COS" technique which is a more accurate method than using the radio's squelch or "hardware" COS.