Let's say you know that a certain point on a data bus has a defective device that is always trying to drive the bus low; Was wondering if anyone has any good tricks in finding the defective device, other than cutting/lifting pins?
Hi Phil, That sound like a good possibility, thanks. I have a Philips PM2534 DMM that has a 300mV range with 100nV resolution. The only thing might be that if the bus is active (changing states) the readings might be jumping around too much. Often I see a tied node on a TTL bus that has transitions from .5 to 2 volts or so. Any ideas for a situation like that? Maybe try to crash the processor? :)
Does the bus point show low ohms to ground in cold state?(powered down). If so, leave a sensitive ohmmeter connected to it, hover over each device feeding the bus with a hot air blower. When you see a sudden pronounced change in ohms to ground you're hovering over the culprit. Jango
I always check for that - if I see a low resistance to GND when power is not applied, that's an easy one. I have a Toneohm with a 200m Ohm range. Also useful for finding etch/solder shorts. Thanks, though.
I don't think you understand quite what I'm asking. For sake of argument, let's say I have five ALS245 transceivers all located on a 8 bit bus. One of them should be driving the bus at any given moment, but another one has a defective output that is corrupting data bit 1. I'm looking for any tricks that would help to narrow the fault to one device. So far, Phil Hobbs has had the best idea.
Sockets? Heh. If they were in sockets it would be a piece of cake. Have you taken a look at a modern circuit board with SMD parts lately?
Once found a shorted cap by powering the Vcc with a voltage regulated, current limited source and traced to the hot spot. Once determined that there was heavy traces, fed a couple of amps and watched the bad cap smoke. Saved a $5k board that had been passed around everyone else. There were about 50 or more bypass caps throughout the board and a real pain to have to replace them all to find the culprit.
An ESR tester can be used to trace down low ohm shorts.
I used to use an 'RCA curve tracer' to troubleshoot bus problems. Sick gates usually lack a 'sharp knee' on the curve.
binary search is helpful, if you have to cut a trace, try to do it 'in the middle' (half the circuits on one side of the cut, half on the other). That way you cut the number of possible bad circuits in half, each time you cut a trace.
When you narrow it down to a few ICs, you can cut (or unsolder) pins instead of cutting traces.
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I've been lurking for a while and this is my first post here. I've been in the electronics industry for 40 years and have picked up a few tricks along the way.
Many faulty parts will heat up faster than their counterparts and can be detected with a liquid crystal sheet. Lay the sheet on a cold unpowered board and apply the power. Whatever heats up quickest is suspect. I've used this technique to find faulty ICs, Caps, and even high current traces.
Almost the weirdest fault I ever saw was like that, one PC printed fine to an old Epson dot matrix, another printed garbage.
The printer had a 74LS244 or 245 (can't remember which, was a *long* time ago) on the parallel input, measured with a meter on ohms the offending pin was 2-3 ohms to ground. The old Zenith 8088 drove it fine, the newer 386 machine failed miserably.
One way to trace is to get some thermochromic labels but if it's just one input then it may not work and can be fairly random depending on bus load etc.
Another way that's worked well for me in the past is with a component tester, the sort that injects a signal into the device and displays the 'curve' of the device under test, Huntron Tracker or Hameg 'scope. You quickly get used to the shape of the trace produced by various different chip input/output pins and it's extremely easy to spot a dodgy one.
Okay, that's something to work with. Here's how I'd tackle the problem: Hook your CRO channel 1 to one of the 74x245 enable lines, & use that as the trigger, while using the 2nd channel to look at the faulty data line. Probe each of the 74x245 enable lines with channel 1 until you see a stuck line on channel 2.
I guess I'm showing my age here. ;^)
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