I am looking for any guidance on interfacing an RTCM to a TKR-750. In particular: 1) should I use TA or TD for feeding in the transmit audio (I am using TD)
You should be using the TA input. TD is the low-speed data input. It is a wideband input with response down to near-DC. It has no preemphasis. Even with the softpot cranked all the way up you still need several volts of audio to deviate to 5 kHz. Also, if you enable PL on the transmitter, it will disable the TD input (i.e. you can't both encode PL AND use the TD input concurrently), so if you have the TKR configured to encode PL, you're not going to hear anything coming in the TD input.
2) how should I configure the channel ('duplex'?)
Yes, duplex.
3) what about other settings like PTT priority (currently local mic, external ptt, repeat ptt)
Shouldn't matter since you won't have more than one PTT source active anyway, you can leave it at default. Also be sure to disable TOT on the external PTT, disable CWID, etc.. --- Jeff WN3A --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus