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From: Bill in Co. on 17 Dec 2009 23:00 Bill Cunningham wrote: > "Bill in Co." <not_really_here(a)earthlink.net> wrote in message > news:u3NS9U3fKHA.1112(a)TK2MSFTNGP04.phx.gbl... > >> Of course you could. We were talking about semiconductor devices (like >> ICs = integrated circuits) in this discussion, not transformers, >> resistors, etc. But just FYI, a transformer is also a discrete (i.e. not >> integrated) device. :-) > > Isn't the motherboard, system bus, address bus and so on made of ICs? AND some discrete components. > Can a person test memory with a multimeter via the motherboard and its > bridges? No.
From: Paul on 18 Dec 2009 10:19 Bill in Co. wrote: > >> If you buy "generic" DIMMs by the barrel full, one good test >> to run, is measure the rail to rail resistance. Some failed >> cheap bypass caps are a dead short, and can cause the motherboard >> socket to get burned. (I've seen reports of this on Newegg.) > > Again, that is a pretty limited test. Extremely limited!! > The purpose of that test, is to prevent a motherboard from burning a contact, due to a dead short across the DIMM. I can't say I've done this test myself, but it would have saved a few people from a mess, after they purchased extra memory. I would reserve such a test, for memory products with a dodgy history (memory that comes in a baggie, from Ebay). Paul
From: Paul on 18 Dec 2009 10:44 Bill in Co. wrote: > Paul wrote: >> Pegasus [MVP] wrote: >>> >>> The multimeter test might destroy the RAM chip, like the tap on the head >>> with the hammer. I find it difficult to think of a more inappropriate >>> test for a RAM chip. Remember - it contains a hundred million or more >>> transistor gates operating at extremely low currents, less than the most >>> sensitive multimeter can detect! >> You can safely test silicon devices, if you use a multimeter with >> "low power ohms" setting. It applies a voltage not intended to >> cause forward conduction in the silicon chips. > > Paul, this only applies to checking *discrete* devices, like diodes and > transistors, it does not apply to integrated circuits. > So what exactly does an integrated circuit have inside it, fairy dust ? Golly, I see two transistors here. It is just a big package full of transistors, diode, resistors etc. http://en.wikipedia.org/wiki/File:CMOS_Inverter.svg Neither the level of voltage excursion, nor the level of current used for testing, is an issue. The picture in Wikipedia, is the circuit inside the chip. It would be considered a logic primitive. On the pad ring, the pads have clamp protection like this. There are various kinds of networks of components, intended to prevent input excursions above the top rail voltage, or below the bottom rail voltage. (I had another example network I wanted to show up, but I can't find a picture right now.) http://www.circuitstoday.com/wp-content/uploads/2009/09/cmos-device-protection-circuit.jpg So when you're using "low power ohms", you would be avoiding forward biasing one of those things. It is to make the input of the IC "transparent", so you can measure something which is driving that input. Nobody particularly wants to detect the clamp diode inside the IC. (And as we've already agreed, there isn't much point to ohming across a couple data pins for example. That is pointless.) And even if the low power ohms did happen to trigger forward conduction, because the test current doesn't go above 1 milliamp, there is still no danger to the device. The input diodes are rated for 10mA continuous. Paul
From: Bill in Co. on 18 Dec 2009 13:46 Paul wrote: > Bill in Co. wrote: >> Paul wrote: >>> Pegasus [MVP] wrote: >>>> >>>> The multimeter test might destroy the RAM chip, like the tap on the >>>> head >>>> with the hammer. I find it difficult to think of a more inappropriate >>>> test for a RAM chip. Remember - it contains a hundred million or more >>>> transistor gates operating at extremely low currents, less than the >>>> most >>>> sensitive multimeter can detect! >>> You can safely test silicon devices, if you use a multimeter with >>> "low power ohms" setting. It applies a voltage not intended to >>> cause forward conduction in the silicon chips. >> >> Paul, this only applies to checking *discrete* devices, like diodes and >> transistors, it does not apply to integrated circuits. >> > > So what exactly does an integrated circuit have inside it, fairy dust ? > > Golly, I see two transistors here. It is just a big package full of > transistors, diode, resistors etc. > > http://en.wikipedia.org/wiki/File:CMOS_Inverter.svg > > Neither the level of voltage excursion, nor the level of > current used for testing, is an issue. > > The picture in Wikipedia, is the circuit inside the chip. It would be > considered a logic primitive. > > On the pad ring, the pads have clamp protection like this. There > are various kinds of networks of components, intended to prevent > input excursions above the top rail voltage, or below the bottom > rail voltage. (I had another example network I wanted to show up, > but I can't find a picture right now.) > > http://www.circuitstoday.com/wp-content/uploads/2009/09/cmos-device-protection-circuit.jpg > > So when you're using "low power ohms", you would be avoiding forward > biasing > one of those things. It is to make the input of the IC "transparent", so > you > can measure something which is driving that input. Nobody particularly > wants > to detect the clamp diode inside the IC. (And as we've already agreed, > there > isn't much point to ohming across a couple data pins for example. That is > pointless.) > > And even if the low power ohms did happen to trigger forward conduction, > because the test current doesn't go above 1 milliamp, there is still no > danger to the device. The input diodes are rated for 10mA continuous. > > Paul The point is, a typical integrated circuit like a memory chip has thousands of transistors inside, and there is no way you can test them, as they are all interconnected within the IC. This is NOT the case for testing a single discrete device, like a diode (has 2 leads) or a transistor (has 3 leads), or a resistor, capacitor, etc. And even with that simple CMOS inverter, you couldn't check it with a multimeter (except to see if there were shorted pins, a pretty limited check). The only way to check it out would be by applying a logic LO and a logic HI input voltage to the input, and monitoring the output voltage in a test circuit. Or better yet, by applying a pulse input waveform while simultaneously monitoring the output waveform on an oscilloscope (a dynamic check)
From: Paul on 18 Dec 2009 16:20
Bill in Co. wrote: > > The point is, a typical integrated circuit like a memory chip has thousands > of transistors inside, and there is no way you can test them, as they are > all interconnected within the IC. This is NOT the case for testing a > single discrete device, like a diode (has 2 leads) or a transistor (has 3 > leads), or a resistor, capacitor, etc. > > And even with that simple CMOS inverter, you couldn't check it with a > multimeter (except to see if there were shorted pins, a pretty limited > check). The only way to check it out would be by applying a logic LO and a > logic HI input voltage to the input, and monitoring the output voltage in a > test circuit. > > Or better yet, by applying a pulse input waveform while simultaneously > monitoring the output waveform on an oscilloscope (a dynamic check) > I don't see a point to probing LSI devices *at all* with a multimeter. But there will be situations where you're probing other elements on the PCB, and the wiring just happens to be connected to a large IC as well. By using the low power ohms scale, you might avoid interference from the large IC. If it is "transparent" and not affecting your measurement, then you may be able to get a better reading on something, say, next to that chip. Again, in the lab, I didn't spend a lot of time probing boards with ohmmeters. I would typically use a multimeter for checking VCC voltage feeding some chip, to see if the chip is powered and that the voltage is within spec (+/-5%). Much more of my time would be spent with a four channel storage scope, or a logic analyzer. The logic analyzer on some occasions would even be tied to the storage scope, so I could take an analog voltage snapshot of something happening when a certain digital event was captured by the logic analyzer. One of our analyzers, even had a storage scope built in, for those kinds of mixed captures. There isn't much point in adding a stimulus to a modern PCB, because the LSI chips are more than capable of generating useful patterns you can verify. I worked with a guy programming the LSI chips (FPGAs) - he'd cook up a new design file, and later in the day, I would verify the patterns coming out of the chip, then leave him some notes on things that needed fixing. That would be a typical scenario now. If the FPGA designer is good with testbenches, in fact his design might work on the first try. (You can simulate everything on the computer, leaving nothing to chance.) Years ago, we had an ancient kit in the lab, perhaps made by HP. It consisted of a logic pulser (would sink up to 0.5 amps to ground for a period down in the nanoseconds). The second item in the kit was a logic probe. The intention was, to test simple logic gates, like a 2-in NAND. The logic probe would touch the output pin, while the pulser probe would drive an input low (and drive it so strongly, as to overrule the previous chip in the chain). Since the pulser 0.5 amp pulse was so short, there was no damage to the circuit it was strong-arming. So that is a toy we'd have used in the "jelly bean era". I think I played with it once, just to see "history in action". But for modern LSI components, that isn't useful any more. It is more a matter of observing the system with instruments. Frequently a test stimulus comes from some software program being run on the hardware. Like writing a register in a chip somewhere, to create a sequence on some I/O pin. If your logic analyzer is set up to catch the write to the register, you can then trace in time, the response coming from the LSI. Paul |