I can’t connect 21v3 to PC after upgrade firmware

I plugged the 21v3 to PC ( windows 7 64) with USB cable, windows recognized the board by adding one pololu device attached to COM7.
When I started the Pololu Configuration Utility, the application appeared “Not Connected” but the device was up on windows 7 and the board had a green light.
Trying to connect the board, I upgraded the firmware with “umc02a_v1.3.pgm” to COM7, and the board turned on the orange Led for couple of seconds. After that the Device disappeared from my windows device and the board doesn’t turn on any lights.

Is it possible to reset the 21v3 with the original configuration?
(Taking into consideration that the 21v3 disappeared from windows devices)

Thank you and any help will be appreciated!


If your jrk had the correct firmware on it, you should have seen two COM ports and a “Pololu USB Device” when you first plugged it in. Do you remember the name of the COM7 displayed in your Device Manager? Was it “Pololu umc01a Bootloader”?

You accidentally loaded the firmware for the Jrk 12v12 (umc02a) on to your Jrk 21v3 (umc01a). Now your jrk has invalid firmware on it. But the bootloader should still be there, so we can fix it.

To get your jrk in to bootloader mode, please follow these instructions:

  1. Make sure the jrk is powered off. It’s best if you can just disconnect everything from it.
  2. Using a screwdriver or some other conductive tool, short the two exposed bootloader pads together. The pads are shown in this picture: pololu.com/picture/view/0J3102
  3. While the pads are shorted together, connect the jrk to USB. Usually the easiest way to do this is to have one end of the USB cable already plugged in to the jrk, and connect the other end to the computer after you have shorted the pads.

This might take a few tries if you have trouble shorting together the pads. But once it works, you should see the green LED do a double-blinking pattern, and there should be an entry for the “Pololu umc01a Bootloader” visible in your Device Manager. Then you can stop shorting together the pads and try the normal firmware upgrade procedure described in the jrk user’s guide, starting at step 6:

If all goes well, the jrk will behave correctly after you follow this procedure.

Please let me know if you have any problems.


Thank you for your answer David!

I shorted the two pads and the board is working again, then I upgraded with the correct firmware and all the devices appeared and the configuration utility works fine.


Thank for the help.
Ariel Sandez

I’m glad you were able to get your jrk working!

Could you please tell me the serial number of the jrk? It is displayed in the upper left corner of the Jrk Configuration Utility Window, in the box labeled “Connected to:”. It should be an 8-digit decimal number.


Serial number: #00010892