TReX Jr with USB-to-serial problem

Hi everyone,

I’m trying to connect a TReX Jr to my computer with Pololu USB-to-serial adapter with no success. My TReX configuration utility gives the error : “unable to connect to a TReX Jr through COM port 3”.

  • First, the TReX Jr and it’s configuration utility are working fine if connected to a real serial port, so the problem is not with the TReX Jr.
  • I’ve installed the USB/Serial drivers for windows XP and everything seems to be OK under the device manager. (COM port 3, no warning of any kind, red LED on the adapter.)
  • So I’ve done a test using the Pololu serial transmitter application : I’ve wired the RX and TX on my USB-to-serial adapter and sent data. I would see : Send 0X81, Received 0X81. This is getting confusing as it seems the adapter is working fine too.
  • Finally I’m connecting my TReX Jr TX to the adapter RX, and vice versa, and the GND with the GND.

Do I need any other equipment for it to works? Anyone encountered a similar problem? Do I need a service pack 3, or something? Sorry for the vague question, I’m kind of out of options. :stuck_out_tongue:

Thank you!

Hi Mike.

Thank you for the thorough troubleshooting information in your post! I think I see your problem. The Pololu USB to Serial adapter is a TTL-level serial device, so it won’t work if you connect it to the same TX and RX pins you were connecting to your RS-232 COM port. Insted, you’ll need to connect it to the TTL serial pins on the TReX Jr.

You should be connecting the adapter’s TX pin to the motor controller’s SI (serial in) pin, and the adapter’s RX pin to the controller’s SO (serial out) pin, and ground to ground of course.

Did that get things working? What are your plans for your USB-controlled TReX Jr?


I was having this same problem and it took me a while to figure it out. The problem was that I was following directions instead of what was implied, I guess… I needed to start up with the program jumper on, remove it with power on (the manual explicitly says NOT to do this!) and then attach the serial… basically the firmware flash directions, BUT without removing the mix jumper.

To clarify, here are the exact steps to connect the configuration software via serial connection.

  1. Turn off TReX Jr.
  2. Short SO and GRND
  3. Put on mix jumper
  4. power on TReX Jr.
  5. Remove SO and GRND (leaving power on)
  6. Connect serial cable
  7. Press connect on configuration software.