WIP The ICOM IC-728 has no serial port, which is of course common for older rigs. It does however expose two “ACC” ports, one of which has exposed lines for modulator input (Tx), audio frequency (Rx) and transmit control (PTT). These lines are on the ACC1 port respectively labeled MOD, AF, and /SEND. The IC-728 also exposes a serial CI-V interface, which can be driven by UART.
We will start by analysing the Tx section. J3 is a normal stereo audio jack. The left audio channel of Tx is fed into a 600:600 ohm transformer, providing galvanic isolation. The circuit expects 400 mVrms and a voltage divider 3k:1k is used to present a 100 mVrms signal to the MOD line.
The Rx section begins at the AF pin of the ACC1 port. This is fed into a 600:600 ohm transformer, providing galvanic isolation. The output is fed to both channels of the J2 stereo jack. The AF output is around 350 mVrms, perfect for consumer line level.
The UART section features a breakout board of the CP2102 UART adapter. The CP2102 presents a serial interface at 3.3V. The breakout board I am using exposes 3V3, GND, Serial and DTR. We use the DTR pin to provide PTT. DTR is active low - therefore the U1 opto is connected to DTR and 3V3. When DTR goes low, the opto shorts /SEND and rig ground. This provides an isolated PTT.
The ACC1 section describes how to connect the circuits to the ACC1 port. The port is nominally DIN-8, but we are able to use DIN-5 as we only use pins that are present on such cheaper connectors. This does create a risk of connecting the interface to ACC2, which would certainly lead to trouble - hence the DIN-8 option is the *safer* one.
The serial point are fed towards the optional CI-V section. U2 and U3 optocouplers isolate the serial interface from the CI-V interface. Note that /CI-V is active low, while UART serial is active high. Therefore, the optos also serve an inverting purpose. CI-V is fed via a 3.5mm mono cable, but for simplicity's sake J5 is a stereo jack identical to J2 and J3. Only sleeve and tip are fed, we leave ring floated as it will just be shorted to sleeve in a mono plug.