I have a custom USB cable, USB type A male on one end and a DB-9 connector on the other end (only using 4 pins). The shielding of the cable is connected to the USB connector but not the DB-9 connector, and when connected to a tablet (using the reverse cable DB-9 to MicroUSB), it causes the tablet to restart (but only if the computer used to connect is changed, plugging it back into the same computer has no effect).

After connecting the shielding to the DB-9, everything works fine. Any idea as to what could be causing the restarts?

  • 1
    \$\begingroup\$ Restarts are hard to tell without schematics of the tablet. However, the shielding is commonly connected to ground to give a path for the noise to be shunted to. It is possible the tablet uses the shielding as a ground reference. \$\endgroup\$ Oct 24, 2014 at 14:48
  • \$\begingroup\$ A DB9 is a very poor choice for USB signals! Someone is likely to cross-connect it with a 12v serial port sooner or later. \$\endgroup\$ Oct 24, 2014 at 17:17
  • \$\begingroup\$ The DB9 is used as the tablet is enclosed in a casing. Of the 9 pins, USB takes 5 (1 pin for host mode) and 2 for DC power from an external source. There are separate cables coming out of the DB9 connector to make sure power doesn't interfere with USB. \$\endgroup\$
    – EnG
    Oct 24, 2014 at 20:07

1 Answer 1


I've had a similar experience connecting a USB to a micro controller development system. In my case the problem turned out to be a momentary surge. Basically, "ground" on one device was not the same as "ground" on the other, likely because of other equipment I had connected to the controller. For me, adding an extra ground wire, separate from the USB cable, solved the issue. Fortunately for your case, the DB-9 shielding is accomplishing something similar, diverting any differential between the grounds to point within the tablet's circuitry less sensitive to such grounding differences. Its good you found this, but I'd dig a little deeper too. For example, is the +5V output from the USB contacting something it shouldn't on your custom D9 (like the +5V from the tablet)? Even if its near the same voltage, you don't want both your ground and +5V source from both machines joined. I'm curious if your tablet is connected to anything else, such as a powered speaker system, which may be picking up an external ground. If so, it would be interesting to measure any voltage (both DC and AC) that exists between the D9 shield (pin-1 I believe) and its mating contact. Sometimes the metal case of a D9 connector (if it is metal) is connected to the shield too. If you see nothing measurable there, do the same with the other pins, and make sure any and all voltage differences you measure are explained and expected. It seems you've solved your problem with the shield, but its worth doing the additional investigation here. You just might spare yourself possible damage to tablet's USB port, or just as bad, other circuits that sometimes share the USB on a tablet (like the battery charging circuit).

  • \$\begingroup\$ The tablet isn't connected to anything else. Right now theres a usb micro to db9 male, and usb type A to db9 female. Ive attached the shielding from the USB type A to the DB9 female and it helps suppress some of the resets, but the shielding on the USB micro to DB9 male terminates on the cable (not connected to the connectors). My guess is if the shielding was connected throughout it should help as thats what a normal usb cable would have and using a regular cable, there are no restarts. \$\endgroup\$
    – EnG
    Oct 24, 2014 at 20:14
  • \$\begingroup\$ <sigh> Perhaps the least troublesome combinations are the ones that make it likely for the ground and/or shield to connect first. Often when it comes to things like this, truth is stranger than fiction. \$\endgroup\$
    – Randy
    Oct 25, 2014 at 18:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.