My first official KiCAD project. My old ICOM 725 HF transceiver uses a 2-wire serial connection, I wanted to be able to add CAT (Computerized Automatic Tuning) to this radio. It was accomplished using a very simple 4-transistor circuit, which I laid out on a PCB to fit into this small Hammond enclosure.

Resources can be found on the tabs below.

This ICOM serial port uses two wires, data and ground. In the idle state, the data line is clamped at +5V. The data format is one start bit, 8 data bits and one stop bit.

Data transmissions are from +5 to ground, commencing with the start bit. The interface circuit converts this to +/-12V, sourcing voltages from the data leads. The receive data signal is connected to a single transistor which turns on when it is at 12V, bringing the data to the radio to ground, generating the start bit. On the transmit side, the data is first double-buffered, then it goes to a PNP transistor which turns on when the ICOM data is at a low state. It sources the +12V from one of the DTR or RTS signals, which must be high, and the negative from the transmit data, which should be low. Full duplex operation is therefore not possible.

A simple but effective way to convert serial data to a single 2-wire pair.

You can download the CAD files here:-

One caveat: this layout adds an LED that was supposed to show data activity, however I could not get it to work correctly. The best practice is to short it out, or you could try lowering the value of R4 to about 330 ohms to source current for it.

Contact me for the gerber files, as I cannot upload a zip file here.

The enclosure for this project is an off-the-shelf Hammond 1455D601 extruded aluminum box, which is available from several distributors. The PCB was designed to slide into it, and the DB-9 connector was mounted end-on, so any type will probably work. For the two-wire serial connector, you can roll your own, I chose to use a CUI-MJ1-2503A connector, which is 3mm.