Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!

 


Forgot your password?
Close
typodupeerror
×
Hardware

Grounding a Rack-Mounted Motherboard? 29

Posted by Cliff from the that-wasn't-on-the-circuit-diagram-before dept.
MadCow-ard asks: "Here is one for the Electrical Engineers among us: I have a rack-mount case that I had installed an Intel D845GBV and P4. I had to switch them out with an Asus A7V333 because of incompatibility with my other hardware (which is a long story for another day), but they were functioning fine otherwise. In doing so we found that the new motherboard would not work in the system. We checked everything: multiple motherboards, video, RAM, power supply, cables, you name it. We were getting AOK POST sometimes, others not. It would randomly boot and other times it had no video, or a partial boot. After going nuts (in the field with a couple of clients) we switched out the risers the motherboard sits on. Voila. The risers appear to have grounded the system board. Not the Intel, just the Asus. I spoke with another tech associate who claims to have seen the same issue recently. Now grounding I understand, but it seems that it wasn't the risers specifically. It was their height. We tested two, the bad ones were 2mm smaller. It could be the alloy, but I thought that motherboards would shield the screw points from grounding. It appears to have been a field that built between the case and motherboard due to the smaller risers. It wasn't actual motherboard contact with the case, I checked. Does this make sense? Has anyone else seen this? Is it some sort of capacitance with the case that is generated specifically from the board design and layout?"
This discussion has been archived. No new comments can be posted.

Grounding a Rack-Mounted Motherboard? More

Grounding a Rack-Mounted Motherboard?

Comments Filter:

  • Did you try putting the old ones back? (Score:5, Insightful)

    by Eagle7 ( 111475 ) on Tuesday November 05, 2002 @08:45AM (#4598618) Homepage
    I'd be curious if you actualyl fixed it with the higher risers, or if the real fix was some sort of connection problem that was rectified by the act of reinstalling the motherboard when swapping the risers.

    Just find it hard to believe (and it sounds as if you're skeptical as well) that anything could have been grounded without touching. If this was the case, there would have been arching, and you would have seen marks. Even some sort of magnetic or eletrical field generated between the board and the case (Is this possible? I have no idea) doesn't seem like it should cause such a problem - I'd imagine that there are fields of all sorts overlapping everywhere in the average case.

  • Lower risers - cards not fully in their slots ? (Score:5, Insightful)

    by Sherloqq ( 577391 ) on Tuesday November 05, 2002 @08:54AM (#4598654)
    Have you made sure that with the lower risers all expansion cards (particularly the video card) were fully going into their slots? Sounds to me like your problem is right there, especially since you say that with the new risers the motherboard is 2mm higher than previously. 2mm is a *big* difference, and very well could have prevented your video card from working and/or made it work only some of the time.

    Just my CDN$0.02
    • Eh, just realized something else.

      Not all enclosures use metal risers for mounting motherboards. Some use plastic pegs. Plastic pegs don't conduct electricity. Motherboards have to work even if anchored with plastic pegs. Motherboards can't rely on being grounded via pegs/risers.

      Motherboards require power to be supplied to them. Power supply cables have 'ground' leads in them. Motherboards get grounded via power supply cables.

      Grounding of the motherboard most likely is not an issue here.
      • Wouldn't the motherboard be grounded as well through the PCI cards that contact both the MB and the case?

        --Mike
        • Wouldn't the motherboard be grounded as well through the PCI cards that contact both the MB and the case?

          Not necessarily. Have a look at this [theregister.co.uk] picture, from an article [theregus.com] at The Register, recently featured on Slashdot in the most dangerous server rooms [slashdot.org]. There's no enclosure for the cards to ground the motherboard to, but that "server" most is running. Therefore the motherboard manufacturers can't rely on grounding the mobos that way, either.

          Incidentally, haven't you ever tested a motherboard that was *not* mounted in a case? :) You know, rest it on the foam it ships with, shlop in a video card, a stick of RAM and a CPU, hook up a power supply, keyboard and monitor... :)

      • I'd like to add that not all holes in a motherboard are made for metal risers. Some motherboards have holes made for plastic risers. If you look closely, the holes for metal risers seem to have a small metallic ring around them.

        Hope that made sense. Anyways, I'm seriously doubting that a 12V maximum difference between ground and the motherboard would result in the problem you think. I'm going with the poster that thinks the PCI cards might not be inserted all the way.

  • Shorting out? (Score:4, Insightful)

    by redelm ( 54142 ) on Tuesday November 05, 2002 @09:48AM (#4598902) Homepage
    Normally, the mounting holes are surrounded by solder rings and obviously supposed to be grounded. Additional capacitance is negligable because the board usually has a ground plane [layer] that is much closer. 2mm is a big difference -- are you sure the motherboard wasn't shorting against the tray with the short ones? Mobos frequently have some thru-soldered devices where the pins can protrude 2-3mm. Short one of these, and you're hosed.

    The other thing to check is if the holes on the mobo matched the case perfectly. It isn't unusual to have the holes 0.5mm off, and then the standoff or screwhead could short a trace.

    • the mounting holes are surrounded by solder rings and obviously supposed to be grounded

      I'm not so sure they're supposed to be grounded... IIRC, the solder rings aren't actually connected to other areas of the motherboards in a lot of cases (though some may be), so not much of motherboard grounding here. I think the solder rings are there to provide cushioning, if you will, for the screw / peg / riser that will be anchoring the mobo -- IOW, to dissipate the pressure across a larger area, like a washer would -- to avoid damage (e.g. cracking) of the circuit board. Also forces the mobo designers to route leads away from those holes, so screw heads and the like won't damage / short-circuit them.

      The above statement was brought you by the nit-picking dept.
  • i had a similar problem in a desktop case...

    if the motherboard was sitting on the table, the computer worked.... in the case nope

    After returning one motherboard (to Fry's, so it goes on the shelf again anyhow :) i figured it out, and just attached the foam it shipped with to the bottom. That fixed the short that was causing the system to not start.

    • I've had exactly the opposite problem. One motherboard would not boot unless it was actually installed in the case and using a metal riser peg. I came to the conclusion that this particular morherboard required a case ground to work properly. However there was nothing in the motherboard to indicate that was the case and it took me several evenings to get it working.

      On a related note I had a firewall whose case was live with 120V AC. Even touching the table near the computer was enough to feel a tingle. The system ran just fine with no problems.
      • >On a related note I had a firewall whose case was live with 120V AC. Even touching the table near the computer was enough to feel a tingle.

        Was it hooked up to a UPS?

        Cheap UPSes with dead batteries often put a "buzz" on the case. Dumb, and for the life of me I can't figure out why CSA approves them, but I've had more than one do this, and I've concurred this with other people.

        Either that or your Power Supply cable is wired in reverse with a broken/poor ground connection.

        The system will (likely) run fine forever. Just don't use 10 base-2, RS-232 or anything else that requires a ground between that machine and another...
  • I'm on my third MB of the same make and model =(

    The #1 memory slot was bad in the first one (still under 30 day warranty). The first thing the tech looked for was a grounding problem with how the MB was mounted. Seems ASUS has advised the authorized techs about a mounting problem that can lead to a ground shorting things out. Can't recall which mount it was, but think it was one of the ones near the back of the PC, near the external connectors between the PCI slots and the CPU socket.

    New MB #2 fixed the bad slot, until it mysteriously died 30 days later.

    Understandably, I'm not happy with Asus MB quality thus far and still have new MB #3 in it's RMA box while that cheap Soyo keeps chugging away. At this point "quality" has lost out to operability.

  • Right Solution, Wrong Problem (Score:4, Informative)

    by GoRK ( 10018 ) on Tuesday November 05, 2002 @11:55AM (#4599669) Homepage Journal
    What you have done here is fix your problem without realizing what it was. Don't worry, though, the Higher-Ups will probably buy your grounding/capacitance bullshit anyway if you have to justify the downtime to them. It sounds impressive.

    Motherboard replacement involves handling, reseating, and resocketing every single component in the system, except sometimes drives. Anyway, here are the problems you might actually have been having:

    1) Grounding something that wasn't supposed to be grounded: If you didnt get the new board on exactly square, or you accidentally left a metal riser where the old board needed one but the new board didn't, you might have been grounding something that shouldn't have been. How many times did you take the new board out and check before changing the risers?

    2) Card seating. Your Asus board might have had PCI/AGP slots manufactured by a different company than your old motherboard. Perhaps the 2mm difference made a huge difference in the way cards seated in them (The AGP slot would be a likely contender to exhibit weird behavior on a 2mm height difference since it has two rows of pins.) This happened sometimes to me on EISA boards where I would not get a card seated all the way (You had to push HARD) This explination seems the most likely

    3) Regular gremlins. Reseating everything: processor, cards, cables, etc. can sometimes do wonders for a system that's causing you to bang your head on the wall. If it just so happened that whatever device was giving you the problems was twiddled in just the right fashon when you switched the risers, then it would seem only natural that the risers fixed the problem, only they probably just forced you to unplug everything looking for it, and as a side effect you got some contact to scrape up against some peice of metal just so..

    Hope that answers your question.

    ~GoRK

  • Grounded by other means just as bad (Score:5, Informative)

    by kriston ( 7886 ) on Tuesday November 05, 2002 @12:45PM (#4600077) Homepage Journal
    Having built over 100 systems by hand this is nearly always the case of the chassis coming in contact with pin lugs that are hanging out the back of the motherboard, or the motherboard mounts contacting circuitry.

    I have built a number of systems which have stamped motherboard mounts which are quite wider than the usual screw-in mounts--so wide that they contact the motherboard well past the screw holes and into the circuitry. Not good! Worse yet, these stamped mounts can't be removed (obviously) and if the motherboard doesn't have a mounting hole where the mount is located you have to insulate it with something that cannot be punctured by the pins hanging out the back of the motherboard.

    This has been a problem more than I like to say but it is always corrected by using non-conductive washers on the mount points and using rubber feet or cut-off plastic standoffs for spacing. The problem for me was prevalent on many MicroATX chassis, and especially thin steel cases of the sub-$30 variety which had a tendency to flex and bend in normal use.

    Of course others have mentioned cards not seating properly (AGP cards are terrible for this) so to solve it you don't use the screw mounts adjacent to the AGP slot. This allows the motherboard to flex towards the AGP card and not "pull back" on it and unseat it. Newer AGP cards have retainer clips but I haven't run into those yet.

    Kris
  • I had to help out a lame-brained friend who tried to build their own system, and then discovered it wouldn't work properly. Crashed sporadically, yada yada.

    The problem? He'd screwed one corner of the motherboard into the backplane with no post. Direct contact, warping the board. I was extremely surprised the board survived after being remounted.
  • I've seen this happen on a shuttle motherboard. Extremely frustrating (put motherboard in case, press power button, and nothing happens, take motherboard out of case, put on table, assemble there, press power button, and system works. put back in case, press button, nothing happens. scream. yell. rinse and repeat)

    The trick is to make sure that the holes line up perfectly, use as few mounting points as possible (4 is usually the magic number), put rubber washers on the screws, or use plastic screws and mounting points. Otherwise, certain boards won't work. Some better quality boards have several screwholes with no traces nearby.
  • Addmitedly it's not actually a rack mount but here goes...

    I remember having problems installing a CD burner in my home PC because the CD Burner was longer than conventional CDRoms. THis had the effect of shorting out the mobo mounting screw against the side of the excess length of the CD Burner.

    Sometimes the machine would boot, sometimes it wouldn't. Some electric tape between the mounting screw and the side of the cd burner, and voila, problem solved
  • Yes! Same board, same problem! Mine was in a desktop case, and it was impossible to see whether the mobo is actually touching or not.
    So what I did was this (after calling tech support):
    Once I made sure that the mobo was working fine outside of the case, I took a folded over piece of paper, and put it over each riser, taping it into place with scotch tape (imagine putting a paper towel over a cup, taping the corners to the desk).
    Then I put the mobo on that - worked. Then I screwed it in, two screws at a time (screws went through the paper), and booted fine each time. That solved the problem, the box is now rock-stable. Just hope the paper doesn't catch on fire... I have a ways to go till Fahrenheit 451. :)

Slashdot Top Deals

No man is an island if he's on at least one mailing list.

Close