Making a DIY programming cable for the Tait 8105 Radios

I am currently building a project that utilises two Tait 8105 UHF radios. I needed to upgrade the firmware and configure the radios, but couldn't find a programming lead in the UK for sensible money. After a bit of research, I managed to build my own using a FTDI TTL-232RL USB adapter and a DB9 Breakout board.


The DIY cable worked for me without any issues, however anything DIY increases the chances of something going wrong. I will not be held responsible if you cook your radios. If in doubt, please contact an Authorised Tait Service Centre or Dealer.

Required Parts

  1. FTDI TTL-232RL USB adapter (see note)
  2.  DB9 / RS232 Breakout board
  3. USB cable, in my case it was Mini USB.
  4. Jumper wires to connect the Serial adapter to the breakout board.
Note- This requires a genuine FTDI USB adapter, as the process requires editing the settings which seems to be impossible on the clones. I purchased mine from Ebay a long time ago, and it's since disappeared. I believe it's one of these. Unfortunately the majority on Ebay today seem to be counterfeit clones.

Inverting the FTDI adapter signal

Its possible to invert the serial signal on the FTDI chips using their FT_PROG utility. 

You can find the FT_PROG utility on the FTDI website. Its for Windows only, however there's an untested version for Linux here

Once you have installed tool, plug in the adapter into a PC and open the tool. You should see the following screen-

Hit F5 on your keyboard to scan for devices. Once your device is found, you should see the following-

In the Device Tree, click Hardware Specific > Invert RS232 Signals, and check the boxes for TXD and RXD so it looks like this-

Press Ctrl + P to flash the updated configuration to the FTDI chip. To confirm its worked, press F5 to refresh the programmer and check that the Invert boxes are still checked. If they are unchecked, you may have a counterfeit FTDI chip that doesn't correctly save the settings.

Building the Cable

Use the diagram below to assist you making up the cable. Carefully read the notes before proceeding.

If you've followed the instructions above, you should now have a functioning programming cable for the Tait 8105 radios, to use with the programming and calibration software.


Thanks to G8KLC for the hint about inverting the signal!


  1. Thanks for the time you spend leaving the information so that others can see your work, you solved a headache, I replied using a USB to serial TTL adapter Prolific PL2303 3.3V
    Is what I had on hand, I work at first without any problem, I will see your block frequently, greetings from venezuela south america

  2. Great post.
    Although the manual for the TM8100 series state that the TXD and RXD ports are 3V3 CMOS, if you look further in the manual they will also work with 5V TTL and RS232. This is the reason you needed to invert the signal as RS232 is inverted compared to TTL. The TM8100 series are backwards compatible with the Tait Serial programming cable, this uses RS232 Levels in and out. Although it has a MAX232 IC it is just used as a "Buffer/isolation" not for level conversion as the RS232 signals are inverted through it and then corrected as it is fed back through the MAX232.
    Indeed the TM8100 does put out 3V3 on the TXD pin if you look at it on an oscilloscope, however most modern serial interfaces will easily see that signal if it was wired direct (without using a MAX232 as a buffer)

    de 2W0ODS