The layout is straight forward, not much to be said here. The audio signals are separated through 600 Ohm 1:1 isoltion transformers which can be salvaged from old modems. There is a provision for a series resistor to the trimmer which allows attenuation of the signal to mic level in case the rig does not have a line level input.
The PTT and keying lines expect RS232 level signals which can originate from a USB dongle. The signals are opto-isolated with CNY17 optocouplers which works great on most modern rigs. In case of grid block keyed rigs (like the TS830), make sure to exchange the left optocoupler (which handles keying) with a model that can handle large voltages. The one I found to be suitable is the Sharp PC725V. In most cases, the keying voltage will be negative so please make sure the optocoupler is connected in reverse (emitter to key, collector to ground).
If the signal level on both ends is at line level, use a 10k trimmer and place the jumper that shorts the series capacitor. If the signal needs to be converted from line to mic level, remove the jumper, choose a 50k series resistor and a 1k trimmer. In my version I have used a 10K trimmer and a 100K series resistor which works perfectly fine. In this case the configuration can be determined by the jumper only.
The LEDs should be low current models (5mA or so). Some of the schematic (the idea with the monitoring LED) was taken from prior work from local club chapter F11.