Interfacing Motorola GM1200

I had an unused GM1200 that should be interfaced to a SVXlink node. Turns out that it is not as easy as I thought. I wanted to use the CTCSS decoder and a hardware squelch signal. The CPS offers different options that can be programmed per FM channel. The essence is the option “Data Channel”. That enabled unfiltered raw output on pin 25 of the accessory connector. The audio input can be connected to pin 24. The hardware squelch can be used on pin 8. It carries +4.5V if the squelch is closed and drops to GND if the squelch opens.

GM1200 internal connection for CTCSS squelch signal

The audio output on the ACC connector are all DC loaded (about +5V) so you have to use a capacitor for coupling the NF. Also I found out that the squelch signal is obviously not tied to the CTCSS decoder so it also goes to low level if there is a signal without valid CTCSS tone.

So I configured the channel without “Data Channel” enabled. Now you have to use pin 11 for filtered audio output (disabled if data channel is enabled and also DC loaded) and pin 23 for feeding the NF into the transmitter (using a 1µF capacitor). The drawback is that now the squelch signal is disabled on pin 8.

But there is a nice mod I found on the SVXlink forums [See [1]). That pin is even controlled by the CTCSS setting. It only drops to GND if there is a signal on the RX carrying a valid CTCSS subtone. Nice, that is what I was looking for. I cut the small track connecting pin 7 and used that pin to feed the “CTCSS squelch” signal to the ACC connector.