I communicate with a serial device using a FTDI FT232R usb -> serial cable created by the manufacturer of the device. Everything is ok I can communicate with the device.

Now I have another cable, also using the FT232R chip.

Both cable use the same:

  • FT232R chip
  • Serial connector
  • Driver

And I've also duplicate the information contained in the EEPROM, so the custom cable use the same information (just in case). And I open the serial communication at the same baudrate.

But unfortunately if I write a serial command (using the custom cable) I got no answer from the device.

What can still be different between the manufacturer cable and my custom cable ? Is it possible to create a perfect copy of a specific cable ?


The first comment suggest that some PIN could be swapped and it will indeed explain why I got no answer from the device.

But the compagny FTDI propose a small software (FT_PROG) that can be use to reprogram the EEPROM (picture below):

enter image description here

There is an option where the pins can be configured.

Does that mean that I could swap the pin programmatically ? (I've not a lot of hardware knowledge) Or the only solution is to swap the pin with soldering ?

  • 3
    \$\begingroup\$ Rx and Tx might need to be swapped at one end of the cable. \$\endgroup\$
    – Andy aka
    May 15, 2019 at 11:58
  • \$\begingroup\$ I know that some manufacturers actually provide close to the standard pin assignment cables but introduce small changes. E.g. sometimes the pin which is normally used as DTR is used to signal an interrupt which can mess up the whole communication. Your best bet is to check the documentation carefully for deviations from the standard. If there is no documentation you could try to figure it out using an oscilloscope. \$\endgroup\$ May 15, 2019 at 12:18
  • \$\begingroup\$ Thank you for your kind advice, I appreciate this sharing of knowledge ! I've edited my question with a new suggestion, could it work ? \$\endgroup\$
    – obchardon
    May 15, 2019 at 12:32
  • \$\begingroup\$ There are "hardwired" and "configurable" functions. TX and RX are of the former kind. Can you provide a datasheet or model identifier for your device? \$\endgroup\$ May 15, 2019 at 12:42
  • 1
    \$\begingroup\$ Well as we dont know about which hardware we are actually talking and as do not provide any documentation we can hardly help you. The information about the type of connector is not helpful at all. \$\endgroup\$ May 15, 2019 at 19:44

1 Answer 1


Ok I solved my problem, the pin were not swapped but the signal was inverted !

It looks like the FTDI chip can invert the output signal on each pin.

So using the FTDI FT_PROG software I've inverted the signal of the following output:

enter image description here

It now works flawlessly


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