The other day Silvia DM9KS sent me parts and PCB for a NucleoTNC by mobilink. This is a nice option to get QRV in 1k2, 9k2 packet radio or even use it for M19. The original was a breadboard version (see [1]). But the sources for board and firmware are open (see [2]). So after a short soldering session there was a nice board running the latest firmware. A few days later also case and front and back panels arrived and this went into a nice box. The pinout and more is described in an article on [3].
So the next thing was to wire this up to the IC-705 radio. The schematic shows that the hardware is capable to control PTT via MIC line and a simple resistor (audio is decoupled using a capacitor). So it is basically just wires from the 3.5mm 4-pin jack of the TNC to 2.5mm 4-pin jack for MIC and 3.5mm 3-pin jack for PHONES. The wiring is here:
For the 2.5mm 4-pin jack on the IC-705 I used an old headset from Nokia mobiles (type WH-101) and scraped the connector along with some wire.
To use this cable with multiplex PTT set the TNC parameters as shown in the picture. These can be configured using a simple Python program (see [4]).
The rest of the settings can be left at default for a first test. Audio level from the IC-705 can be as low as 5%. Decodes are fine for example in Xastir using it as a KISS TNC.
References
[1] http://www.mobilinkd.com/2019/06/24/nucleotnc/
[2] https://github.com/mobilinkd/NucleoTNC
[3] https://nbviewer.org/github/mobilinkd/NucleoTNC/blob/master/Build/NucleoTNC.ipynb
[4] https://github.com/mobilinkd/tnc1-python-config