Here is the requested diagram for the FT-4. Note that the Baofang uses a different cable.
WARNING: Please do not blindly adapt this for other radios. This entire scheme can probably be adapted for any radio that uses a 3.3V setial connection, but if you plug in an older radio that uses higher voltages, you can fry you little computer, and/or your radio.
NOTE: if you use a serial port that is configured as a login port (a "console") you OS can spew lots of stuff to the port. This can in theory confuse your radio. This is true of any serial port the CHIRP uses in any OS, but it may be more important to think about this when using itty-bitty computers, since they tend to configure at least one serial port as a "console" port.
This scheme "should" work on any itty-bitty computer that can support CHIRP and that has a 3.3V serial port. On a Raspberry Pi, your OS will be richer. You will not need the X-over-SSH trick, and if you do use it you will need less setup. On a Raspberry Pi, you use pins (6,8,10) as (GND,TxD,RxD), and your OS will see the port as /dev/ttyS0).
My NanoPi Duo is running the sun8xi linux distro provided by FrendlyELEC, who manufactures the NanoPi. the distro is based on Ununtu, so you use apt-get to install packages as needed. To support CHIRP, I installed xorg, since the supplied image uses something strange. To actually use CHIRP, I use ssh -Y from my Linux desktop computer to allow CHIRP to present its X window to my desktop computer's screen via the magic of X-over-SSH.
I'm connecting the radio to NanoPi's second serial port, which is on pins labelled RX1 (pin 20) and TX1 (pin 22) and of course GND (pin 6) on the NanoPi headers. This port is known to the OS as /dev/ttyS1.
Here is he requested diagram:
NanoPi FT-4 MIC plug
pin 6 (GND)---------------------------------------------------------- sleeve
pin 20 (RX1)----------------+-----------------------------------------tip
|
pin 22 (TX1)---2K resistor--+