NVRAM floppies, resetting machine state |
September, 25, 2002 4:18 AM |
avit |
I'm wondering about the PowerLogix (or similar) floppy. Does anyone know exactly what it does? Does it just turn off speculative processing? Or does it actually "flash reset" the NVRAM too, or anything like that? The reason I ask is that in other threads these NVRAM floppies (e.g. from Sonnet) are mentioned as possible cures for otherwise dead machines. I'd like to know if my PowerLogix floppy will save the day if I ever need it to do so (I kinda doubt it), or if I need something else for that. When it comes to Open Firmware and NVRAM and PRAM, it's a black art to many--or dare I say all--of us. We remove the battery and press the CUDA and hold that magic key sequence for $count chimes without really knowing what's going on. Do these steps really work out *all* the bugs? Is there a way to "truly" reset a Mac's primordial brain to its factory settings? Can all the chip's registers be nulled to get a clean slate, or will that just lobotomize the Mac? What's in there? What *has* to be in there? Should I even worry about the cruft? I guess the bottom line is, is there a flasher utility out there that can do what I'm looking for? I should stop now, I think I'm wearing out my question mark key. |
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RE: NVRAM floppies, resetting machine state |
September, 27, 2002 6:20 PM |
jonsaw |
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I've fixed just about every Mac model, and I've found that the best way to reset NVRAM is to first follow Apple's instructions for each model (available online in Apple's Knowledge Base/Applecare), which varies--for instance, for some models, you first unplug the power supply from the motherboard, which is a step few people describe when they tell someone how to reset NVRAM, but you have to do it on some Macs since capacitors in the power supply can keep the NVRAM alive. This may be why some people find they need to leave the battery out overnight--it may be the capacitors in the power supply still keeping the chip alive, and it takes time for the power supply capacitors to drain--but I've worked with at least a couple motherboards where even with the power supply unplugged, the battery had to be left out overnight (or at least for some hours--I didn't bother to get up in the middle of the night to see if they were working again, but they did when I checked them the next day). On some Mac models, the next step is to press and hold the Mac's power button for 30-60 seconds, instead of the CUDA/PMU button (at least not yet). If that doesn't help, the next step is to press the motherboard CUDA/PMU reset button. But with most Mac models, Apple says to simply press the motherboard CUDA/PMU reset button, though Apple usually says nothing about removing the battery. I've found through experience that the best approach, in addition to first following Apple's procedure for a particular Mac model, is to remove the battery and then press the CUDA/PMU reset button to drain the chip's residual voltage to ground; I hold it down for several seconds, but usually just a brief tap will do the job, though this seems to vary from one Mac model to the other, or even within the same model group (each Mac is an individual!). Apple says if you press the button twice in succession on some recent models such as the G4, it will scramble the CUDA/PMU, which can also cause it to "free-run" and drain the battery in a couple days, but I suspect this is true only if the battery is left on the motherboard when you press the button, since the presence of the battery causes the CUDA/PMU chip to be reset to "proper" values immediately after you press the button rather than after you reinstall the battery, and my guess is that this reset process takes several seconds, and if you interrupt it by a second press too soon when the battery is in place, the NVRAM/PRAM will get scrambled. I've found that it's rare that a Mac needs to have its battery left out longer than 5-10 minutes when pressing the CUDA/PMU reset button doesn't help, but I have seen it, and many other people have too, so that's an additional step you sometimes need to follow. Also, before I reinstall the battery, if I don't know whether the battery is good or not, I measure its current with a digital meter (Radio Shack, etc.). Measuring a lithium battery's voltage won't do, since these batteries will usually retain their voltage even after their current-delivering capacity is too low to reliably run the PRAM/clock chip. I've found that some weak batteries that will still run the clock/calendar are too weak to reliably hold the PRAM/NVRAM. The right amount of current appears to be anything above .08 amp for about three seconds; if you leave the meter's leads attached longer than that, the current value will gradually drop lower as the meter drains the battery, so you don't want to leave the meter leads attached too long. Anything lower than 0.8 amp, and I install a new battery. When you need to reset the NVRAM in a Mac that contains a third-party processor board, some manufacturer's boards require you to remove the third-party processor and reinstall an original Apple processor, then run the manufacturer's NVRAM-patching utility before reinstalling their processor, since apparently there's something in the Apple processors that the NVRAM reset process sometimes requires, without which the Mac won't start up after an NVRAM reset. I don't know why this sometimes isn't the case, even with non-Apple processor boards that are supposed to require it. Processor boards from Newer Technology never had this problem; they often trumpeted this fact. This is where the NVRAM resetting utilities are used--with an Apple processor installed, you run the utility, and it patches NVRAM so you can remove the Apple processor and reinstall the third-party processor. I haven't committed to memory what these utilities are actually patching, but the manufacturers sometimes have tech notes about this which I've read in the past. As far as I know, this is the only situation where resetting NVRAM can cause trouble--normally, completely resetting NVRAM won't hurt the Mac. When I reset NVRAM from the Open Firmware prompt (reset-nvram), I often wonder how thorough a job it's doing; wish I knew of a way to tell! |
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RE: NVRAM floppies, resetting machine state |
September, 27, 2002 5:00 PM |
OSXGuru |
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Those boot floppies will write a patch to NVRAM. I don't think that they reset anything in NVRAM aside from writing that patch. So far as I know, the most thorough way of resetting NVRAM is the "remove the battery" procedure. As far as I know, this resets the NVRAM to factory defaults. (Actually, it probably resets the NVRAM to random values, but then it will fail a checksum test at boot and at that point revert to factory values. At least, I think that is what happens.) The TechTool utility for Mac OS 9 did have some NVRAM-resetting features in it. It is a bit of a black art for me, too :-) |
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