Problem: USB resets computer

General Questions
#1

I have assembled a new computer and this problem has me stumped.

Background information: A USB connector has four “pins” in line. The two outside ones are a bit longer and make contact before the two inside ones. The two outside ones carry the supply of zero and +5V and the two inside ones carry the data. Additionally there is the exterior metal housing which is connected to ground for the purpose of shielding.

The problem: The mainboard has two USB ports on board and a header for two additional ports off board. The case I have has two USB connectors in the front connected with flat cables to the USB header on the mainboard. When I connect a device to the USB connectors placed on the rear of the case (directly on the mainboard) everything works fine but when I go to connect a USB device to the connectors in the front, just when the shield touches the shield, and before any of the main connectors touch, the computer resets itself like I had pushed the reset button. It does not happen every time but it happens often enough to be annoying as I have to wait for scan disk, etc.

When I started looking into this I noticed the shield of the USB connector was grounded through the 0V USB pin to the mainboard. I thought some static might be enought to send something through the mainboard so I disconnected the shield and connected it to the chassis a couple of inches away. And it still does the same thing! Touch the USB connector shield with a metal object (not any of the four pins) and it often resets the computer. Touch the screw two inches away where the USB shield is grounded to the chassis and nothing happens. It makes NO sense.

When I had the computer without proper grounding it happened a lot. I have properly grounded the case an it happens much less but it still happens ocassionally. It has me completely stumped. The only thought I have right now is to replace the USB connectors in the front of the case with connectors in the rear which would have shorter cables. Anyone have any other ideas?

#2

Well… you know oodles more about electrical and grounding issues than I ever will, but from past experience when the system is re-setting like this upon insertion of a connector, I generally (not always) trace stuff like this back to IRQ conlficts or motherboard grounding issues. In the case of MB problems I have found that using the little reddish non-conductive washers (they usually come with the MB screws) on both sides of the screws that affix the MB to the case and/or using plastic (vs metal) standoffs often cures grounding reset problems (not sure why).

If it’s a cabling issue your best bet might be to ditch the front connectors and just get a little powered USB hub and feed off the rear connectors.

Sometimes USB stuff will also do this (reset) if the power demands of the device you are connecting exceeds the power available in the USB line feed, but if it’s doing it with just a USB cable attached and nothing sucking power and nothing else demanding power it’s unlikely to be the problem.

As a stray thought you might want to check the solder pin joints under the MB to make there are no extra long pins brushing the case (or anything else).

Although it’s a PITA - pulling the MB slightly out of the case and hooking it up to the PS (it’s a clumsy jig true) and after inserting whatever minimum of cards are necessary to boot (video usually) then seeing if the front USB resets with it not grounded to the case, will generally tell you if it’s a MB grounding issue or specifically a USB grounding/cabling issue.

#3

Astro, note that the computer resets even before any of the four USB pins have made contact so it is not anything to do with IRQs power requirements or anything of that sort. Touching the USB conector shield with the tip of a knife will do it but touching the grounding screw a couple of inches away will not. I was thinking of trying one of those USB connectors which go in the rear and see what happens but I don’t have one handy.

I may try insulating the MB from the case and se what happens but that is quite a PITA. Also, because the problem is random, I have thought several times that I had solved it only to find it ws still happening. I’m scratching my head over this one.