Hi William, I’ve been using GM300 radios for some years and I love them.
About the accesory connector, I usually use the “General I/O” setting and then I customize the pins by pressing “F9”. Then, for about the 99% of the situations I only change function of pin 8 to “PL/DPL & CSQ Det” which provides an indication either of carrier present or PL/DPL decode which that depends on how you configured the mode (channel) “RX Squelch Type”. The active level depends on your interface circuit.
Lastly, the RX audio is taken from pin 11 and there are two options, flat (deemphasized and unmuted) or filtered (deemphsized and muted, the last means that is controlled by the squelch circuitry). These two options are selected by means of JU551 (bottom board of the radio). Please check the following link: http://www.repeater-builder.com/motorola/maxtrac/gm300-info/gm-jumpers.jpg
Hope this will help.
Regards.
Gustavo - LU7WA
(By the way this is my first post so thanks for letting me in!)