Testing PCIe connector for faults - Page 1
Forum section: electronics / beginners

author: Buriedcode
author_rank: Super Contributor
timestamp: November 24, 2024, 03:50:53 am
content: Hi,Posted in beginner because I don't know much about PCIe's electrical specs, and its an oddball question.Got an old motherboard with what appears to be a flakey PCIe slot - I'm guessing years of having a heavy card in there has either warped it, or affected the pins. The system works, but any movement of the card during operation will crash the system.I'm not after verifying signal integrity because it seems to me there's a connection breaking somewhere rather than drivers/software - but I'm not sure if its the connector itself, or the board, or even the CPU socket. Is there a way to test each PCI lane, passively, whilst the CPU is in system, unpowered? That way I can get a yes/no for each lane as it has a direct connection to the CPU, so will check from the connector, to the board, then in a seperate test, from the board to the CPU socket to the CPU. That should also pick up any trace cracks or solder joint problems.Do PCIe controllers (CPU) have built in termination on their IO? Every motherboard schematic I have seen just has series caps for each differential pair going from slot to CPU, so I can probe between those caps and connector, but I'm unsure about probing from cap back to the CPU if its not powered, and don't want to risk destroying the CPU when it is powered.I realise there are cards specifically for testing for open/shorts and signal integrity, but the system is so old it isn't really worth spending money on, plus it's an excuse to learn something new.Cheers!

author: Harry_22
author_rank: Frequent Contributor
timestamp: November 24, 2024, 07:12:46 am
content: Hi!You are not a developer to understand the PCIe electrical specification. This is not necessary. You have already diagnosed the fault. But as I understand you want to find the exact location of the contact fault and thereby 100% confirm the malfunction.Then you will need a milliohmmeter. I use a 4-wire LCR bridge with dipsticks.For guidance, up to 50 milliohms - the connection is good, over 150 milliohms is bad. Obviously on bad lines you will see the resistance bounces when touching the PCIe card.As always the most difficult thing is to get to the lines.I'm guessing you can forcefully remove the plastic case and unsolder the pins one by one.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 24, 2024, 08:04:12 pm
content: Hi,If the connector is at fault, I very much doubt I can replace it - but if the fault is merely a couple of the pins bent away from the socket due to years of "card sag" - I may be able to bend them back,If there is a crack in the board, again, that's not really repairable - unless its a power trace, or the "card detect" lines. Any signal/controlled impedance lines couldn't really be repaired.The reason I'm not 100% sure its the PCIe connector is - even though the fault happens when I move the PCIe card - when I recently refirbished it, I didn't touch the PCIe, instead I replaced the heatsink/fan on the CPU, and I have seen some aftermarket heatsinks can put excess pressure on the CPU causes intermittent faults on high speed lines - such as the memory interface. I have no idea how likely this is, I'm leaning much more towards the PCIe connector, but I was trying to think of ways to narrow the fault down.So am I looking for low resistance to GND? I'm not sure if the motherboard has any kind of passive termination, or whether these lines go directly to the CPU with no resistors to GND. If that resistance comes from the CPU, then that would be ideal because I could trace it all the way back there, with any high resistance lines being open.I don't want to go around probing this with a standard multimeter - I'll have to check the max test voltage my meters use - since I'm hoping to do with with the CPU installed. That way - hopefully, I can hook one end of a meter to GND< and the other to the signal, freeing up one hand to "wobble" the card to check for intermittent connections.

author: amyk
author_rank: Super Contributor
timestamp: November 24, 2024, 11:08:26 pm
content: I suspect there's an intermittent solder joint on one of the pins.

author: Buriedcode
author_rank: Super Contributor
timestamp: November 25, 2024, 06:02:43 pm
content: Quote from: amyk on November 24, 2024, 11:08:26 pmWell, after putting it all back together, its behaving now. I can even "wobble" the card in the socket and its fine, which pretty much rules out the connector/solder jointsI can only surmise it was a problem with the CPU socket, since I had issues with it reading the second bank of RAM before this, and, like the PCIe connections, there really isn't anything between the memory and CPU, just traces and the connectors. And both the RAM and graphics card have been tested in another system.It's a pity, I was prepared to attempt to make impedance measurements on all the signal lines using an old (stripped) graphics card PCB have I have, that was destined to be used as a coaster. Perhaps even a very low voltage (0.7V) shift register to measure 32 lines at the same time, just checking for connections to ground. But it seems my brainstorming efforts aren't needed.Cheers anyway