Someone asked about the different standards of 3.5mm audio connectors sometimes used as programming cables. This is what I replied:
Just a quick comment on the ubiquitous 3.5mm TRS connector. It was originally created for telephonic or audio applications. However their robust design quickly saw them being used for other unintended applications such as chargers.
The one application that nobody foresaw was serial data transfer. There are no generally accepted standards for voltage (RS232 is +/-15v) or speed for the data connection – anything goes as long as it seems to work for that manufacturer. The modern 3 volt usage is not even mentioned in the spec!
However for the physical connections there are two popular options: Either GND-TX-RX or alternatively GND-RX-TX, depending on the manufacturer. There is a fourth connection which is often used by the audio enthusiasts in a TRRS configuration so be careful not to plug that into a data port as you can short out some contacts.
Before plugging in you should check which connection your radio expects and at what voltage, not all the radios are the same. While they can occasionally be interchanged successfully, be ultra careful not to unintentionally damage the radio when using a USB-to-TRS cable. Use an oscilloscope to measure the signals if necessary, before connecting.
According to the manuals included with the Xiegu G90, the CIV Port (3.5mm Jack on the Head Unit) when used for firmware updates or CAT control is Gnd/Rx/Tx (see below, for G90 only).
