Why would an unmatching RAM module prevent a computer from booting?

Question

I recently came across a Power Mac G4 that I was tasked to find out if it worked. The computer had been slightly stripped, having no optical disk drives and all cables inside disconnected. After I connected everything related, I then turned on the computer, that didn't boot, as indicated by the absence of any graphical output.

After some fiddling (checking the graphics card was OK, getting a keyboard, etc), I then remembered that in these old computers RAM could be a factor. There were 4 modules, all of them 256 MB. Except one was slightly taller than the rest. The module in question is similar to this one I found online:

The numbering on the sticker, after the PC100 designation, was different on that module. I removed that and another one, so as to have a pair. The computer booted and everything ran fine.

The modules were, as far as I can remember, all equal (except for the differences stated previously). Why would that particular module prevent the computer from booting if at first sight they matched?

Also, does the numbering after the PC100 designation have some specific meaning?

Answer 1

The PC100 is a speed rating of 100Mhz.

The timings on the memory are probably different. For example you can't mix 5-5-5 timing memory with 8-8-8 without going into the bios and setting the timings down to 8-8-8.(RAS,CAS,and other timings exist).

The 8 memory takes 8 clocks cycles to for each element and if you try to do it 5 the memory values will be corrupted and your system won't function or it might not even turn on.

Answer 2

There's also a possibility that the taller RAM module was ECC memory, like the ones shown here, which is often used for servers, and which consumer Macs, from the G3 through G5 PPC lines at least, did not like. A G4 Mac won't boot with ECC RAM, as far as I know, although there may have been server versions that would take it.

I believe that if the taller module was ECC, it would have had nine chips per side, though, not the eight in that picture above.

Also, yay me, for answering a question that's a good 6 1/2 months old which already has an accepted answer.

Answer 3

Well,

The most obvious ? would be whether the RAM you removed was any good to start with! :}

If this Mac was partially stripped, it is conceivable that it was stripped BC it wouldn't boot up before you got to into it....and, the RAM you removed (maybe NOT even the taller module), was TU when you got the Mac...

"HelpDesk: Miss--please check the wall plug, then turn your computer on again..."

TC