This is the third computer question I’ve posted here, but don’t worry, this should be it for a while. 
I’ve heard that simm memory chips for Mac’s will work in PC’s and vice versa. I’ve also heard that some Mac memory is only for Macs. I’m asking because on eBay I’ve seen 16mb 30 pin simms being sold. Like I said in other computer posts, besides this computer I’m using now, I have a 486 also. I’d like to buy the 16 meg chips if they’ll work. I know that 4 meg chips are usually the highest, but I think I’ve heard of 8. Maybe not. Anyway is there any way to tell if a simm will work for a PC and a Mac, or if it will only work for one or the other. Please let me know. Thanks.
Your question is too vague. There are some types of RAM that work in some models of both PCs and Macs. But you’d have to get more specific about which models. You have to check your computer’s specs and see if the memory works in that specific model.
I personally wouldn’t be buying anything as low as 16mb simms lately, they’re almost worthless.
Well, you have to remember that I’m talking about a 486 motherboard with 8 slots for 30 pin simms. A set of 4 16mb simms is a lot of memory in this situation. I’m sure for 72 pin simms it’s no big deal, but from the prices I see on eBay, for 30 pin simms, it is a big deal. Anyway, one of the auctions says it’s for a Mac or PC, so I guess they can be used. Sadly, there arn’t many specifics, other than they’re 16mb and 60ns each. But if I had 8 of them, that would give me 128 megs. Not bad for an out of date computer.
If you’ve got a machine that’s old enough to use 30-pin SIMMs, it’s likely to be picky about how the SIMMs are configured in the slots; i.e., if you’re not filling all the slots with SIMMs of the same size, there may be only certain configurations that will work. For example, it may be that all of the SIMMs in a given bank have to match (in 486-era machines, the most common SIMM slot configuration was two banks of four slots each). If there was a difference between the size of the SIMMs in bank A and those in bank B, frequently the larger-capacity sticks had to be in bank A. Some machines, including some 486s, didn’t support SIMMs larger than than a certain size (8MB, commonly). If you have or can obtain documentation for the motherboard in your machine, you should be able to find out what its specific quirks in this area are; otherwise, it’s trial and error.
The main difference between Mac and PC RAM of this vintage was that PCs typically required “parity” SIMMs (i.e., SIMMs with an extra chip on the stick to provide error checking and correction), while Macs could use either parity or non-parity SIMMs. In any given Mac, however, all of the SIMMs had to be of the same flavor (i.e., all parity, or all non-parity).
A lot of the 68000 and 68030 based Macs used 8-bit (non-parity) 30 pin SIMMS. As long as the timing rating is low enough, they’ll probably work. You may need to check the parity settings in BIOS on the 486, though.
I’ve had 30 pin SIMMS that saw duty in Macs, PCs and Amigas and worked fine in all three systems. 30 pin SIMMs were about as generic as memory ever got after discrete chips died out. Parity, timing, and size were pretty much the only variations.
Your second post brings up another issue – the speed of the SIMMs. Typically, it’s possible to mix SIMMs of different speeds (provided they’re not too far apart), but all of the SIMMs in the machine will operate at the speed of the slowest. 60ns SIMMs were pretty fast for 30-pin days.
So if I get simms that are 60ns and have parity, then chances are good that they’ll work. Ok, thanks. That helps. Seeing how epensive they are, even though they’re used, I want to make sure I don’t blow my money on chips that don’t work. But it seems that if I’m carefull, that won’t happen. Great!