I’ll confess that I am new to this part. It was dumped on me without warning and that’s OK. Time to learn a few things. I have a GM300 that is going to be a remote base and a cable purchased from eBay that is prewired for the GM300 side. I understand (I think) that TX goes RX, RX goes to TX, GRND to GRND – but now for PTT, COR and CTCSS. I wonder if some could push me over a cliff – I mean in the right direction. I have no documentation on the RTCM. Thanks, R. Wayne The RTCM connections are: DB9-Female 1 Vin (12v) 2 TX Audio 3 RX Audio 4 CTCSS 5 GND 6 GND 7 PTT 8 GND 9 GND The GM300 cable (and radio) connections are: Pin # 2 MIC BLU 3 PTT WHI-BLU 4 PROGRAMMABLE OUTPUT BRO 5 FLAT TX AUDIO GRN 7 GROUND WHI-GRN 8 PROGRAMMABLE I/O (COR ?)ONG 11 RECEIVE AUDIO WHI-ONG 14 PROGRAMMABLE I/O WHI-BRO 15-16 Jumpered to enable internal speaker