Jrk21v3 + LACT4P-12V-20 - USB connection lost during runtime


I’m glad you are experiencing fewer dropouts. It sounds like you are able to recover from Mode 2 and Mode 3 automatically, so we should focus on fixing Mode 1. The next time Mode 1 happens, could you try shorting the jrk’s RST line to its GND for second? This should reset the internal state of the jrk and to the computer it should look like you unplugged the jrk. After you stop shorting those two pins, hopefully things will work again.


Hello David,

Just an update on this, adding the capacitors connecting each motor terminal to the case of the motor (as your guide suggested) got rid of the noise causing this issue. Only the next few weeks will tell if it is completly fixed but at the moment we cant induce a "drop-out"
We also found that using 0.1 uF and 47 nF capacitors in paralel cut out more noise.



Great! I’m glad you fixed the issue, and thank you for letting me know. Were you inspecting the power lines with an oscilloscope to see the level of noise?


the initial tests seem to show a major improvement (this was done with a test rig clamped to the desk and me trying to stop it to induce noise). after adding the capacitors it wasnt possible to make the system drop out.

i have copied the same set up into the robot and now the dropouts are back. It appears there is still noise and 6 of the pololus attached to a hub seems to drop out the USB. We have a few more tests to try on monday to make sure we are having the same issues, or if this is a fun new issue.

is there a reason the pololu would disconnect itself or would it maybe induce noise down the USB line that would make the computer unhappy? having the 5Volt line disconnected still allows it to drop out. this implies it is the whole usb comms part going wrong, not just the power line.

i have seem that a few others have had this issue which seems to be fixed by using the RX and TX lines, i guess these use a different area or is the usb port just a USB to serial port adaptor and the serial ports connect after it?


I’m sorry your problems have returned. Please let me know what the results of your tests are if you do them.

What do you mean when you say you have copied the same set up? Did you just move all the components (including the computer) onto a robot chassis, or have some of the components in the system changed?

Previously you said that only one jrk would drop out at a time, but now all 6 of them are dropping out. Is that correct? What are the LEDs doing and do you see any messages in dmesg? Does the hub itself drop out, i.e. is the hub still visible when you run lsusb?

The jrk will not try to disconnect itself from USB (unless you send it the command to get it into bootloader mode). I think the problem is electrical noise from your motors interfering with the hub or the Beagleboard. You might be able to figure out which is happening if you put 3 jrks on one hub and 3 jrks on the other.

My understanding is that you only disconnected the 5V line between the hub and the computer, not between the hub and the jrks. You might want to try disconnecting the power line between the hub and jrks, but you would have to supply 5V from somewhere to the USB power line so that the jrk detects power from USB and does the right thing.

The jrk has a native USB connection; there is no USB-to-serial chip on it.


We kept having trouble with the native USB connection. For the benifit of future users it is far easier to use the serial connection. With a TTL serial cable (you can buy nice cables which have the USB to Serial built in the USB plug) it works perfectly all the time. So if you are struggling with the same problem you might want to consider just leaving it and switching. There are no real disadvantages of using serial coms.