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Grounding a Rack-Mounted Motherboard?
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?"
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.
_sig_ is away
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
Have EVDO, will travel.
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 motherboard is already grounded through it's power supply connector. One of those is ground. It should be tied to earth ground in your power supply.
i had a similar problem in a desktop case...
:) 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.
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
Need a Catering Connection
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.
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
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
Kriston
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.
Asus is junk. The Intel board is much better. If you other hardware doesn't work with it then get hardware that does.
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.
-- If you try to fail and succeed, which have you done? - Uli's moose
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.
"If you could only see what I've seen with your eyes..." - Roy Batty
is that those mechanical thingies inside a PC are not very well standardized.
Did you check to see if some insulating washers came with the MB?