I concur with jim202, but if you are commited to plugging into the control head, here are the pins:
2 12vdc power (1A max)
3 ground
4 mic input
5 mic ground
6 aux input 1
10 speaker
11 speaker
Go to EDIT>FUNCTION PORT>INPUT>HEAD1 AI1 and program EXT PTT (for pin 6).
Enable OFF HOOK DECODE if you choose, but do NOT enable MIC MUTE W/PTT as this is for data applications only!
In the FireCom, set receive leve to your liking and the PTT delay (monitor function) to minimum time delay.
Set the 2 dip switches for the lowest mic drive level. Turn mic drive pot CCW, transmit and turn pot up until transmit volume just comes up to max volume and no more.
Big gotcha is the KMC27 mic for the 790. Its mic element is hot during any PTT source which means if you key the radio with headset, the palm mic is sitting there picking up engine noise, siren or whatever. Makes the transmission sound lousy. Two fixes: leave the palm mic off, or do the hot mic modification that is not for the faint of heart.
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