Mobilinkd interface with TM-942A / Shack

A while back I bought the excellent Mobilinkd TNC - a device that interfaces amateur radio with smartphones and PCs via Bluetooth.  There are a ton of useful applications for this, not the least of which is being able to place my radio where I want it, and not be cable-tethered to it.  I've used the Mobilinkd TNC with a Kenwood TH-F6A as a mobile device, and that worked very well - I can leave my radio and the TNC in my backpack and type messages on my smartphone.  If I'm doing something mobile/portable like parade/race support, I put the radio/TNC and a battery pack up where it can get good reception and use my smartphone or PC as the interface - as long as I'm in Bluetooth range, I'm good. 

I've long been a fan of the old Kenwood TM-742A and TM-942A tri-band mobile radios.  I own several of them, and have become fairly well-versed in the art and science of repairing them.  I've always been curious about a somewhat unique feature of the radio, which is the Receive Data (RD) line on the microphone 8P8C connector - it's a direct feed of the receiver audio intended for packet radio that bypasses the final audio shaping/amplification stage, with a 100 mV pk-pk signal across a 10k load. 

I run a TM-942A (which is just a TM-742A with the 1.2 GHz band module in third slot) in my shack, which is interfaced to an MFJ-1263 microphone switch.  I thought it would be interesting to wire the Mobilinkd into the MFJ-1263 switch, so I could use the TM-942A for audio or APRS/packet at the flick of a switch.  The RD line makes this really simple to wire up.  Critical for APRS/packet applications, the RD line is tapped off before the squelch and CTCSS circuits - so I can monitor the audio for debug, or mute it as needed, without having to worry if the squelch and CTCSS settings are screwing with the RD line.  (Of course, this requires me to run DCD on the TNC software, otherwise it will never transmit while it sees the channel as "busy".)  Volume control also has no effect. 

I started off by ensuring that the MFJ-1263 was jumpered properly to bring the TM-942A connector out to the switch's front panel 8P8C jack without re-ordering the pins.  I put this onto the switch's A-side, and left my voice microphone on the B-side.  I had previously built a 3.5mm (1/8th inch) TRRS cable with breadboard pins soldered to the ends for my test kit.  I also have an 8P8C breakout board in my test kit.  Both pieces were jumpered together on a small breadboard.  Once I had the wiring confirmed, I made the cable permanent.  It might sound like overkill to proceed this way, but the TM-942A's microphone jack also has an 8 VDC (@ 100 mA) voltage source, and I wanted to make sure I wasn't injecting a voltage into my TNC. 

I'm really happy with this set up - I can use the Mobilinkd TNC at home, or take it mobile by simply unplugging the TRRS connector and USB power cable.  My Mobilinkd stays charged via USB when it's in the home configuration.  My only gripe is that the Receive Data line from the TM-742A is a fixed level which is slightly lower than the Mobilinkd's software wants to see, but it doesn't appear to be affecting decode on APRS.