Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!asuvax!hrc!valley!pfluegerm From: pfluegerm@valley.UUCP (Mike Pflueger) Newsgroups: comp.sys.mac Subject: Re: Nix on mixing memory speeds? Summary: It's OK. THE DEFINITIVE ANSWER (technical) Message-ID: <45d9e852.15840@valley.UUCP> Date: 25 Sep 89 15:48:19 GMT References: <11979@cit-vax.Caltech.Edu> <4002@phri.UUCP> <2040@leah.Albany.Edu> <34977@apple.Apple.COM> Distribution: usa Organization: gte Lines: 57 In article <34977@apple.Apple.COM>, dwells@Apple.COM (Dave Wells) writes: > In article <11900@polya.Stanford.EDU> kaufman@Neon.Stanford.EDU (Marc T. Kaufman) writes: > >I rise to Roy Smith's defense. That IS a constructive comment -- to the > >effect that: the folklore that says you can't mix memory speeds (subject to > >the requirement that each memory module is itself adequately fast) -- is > >Bullshit. > > > >Now I know that one doesn't have to be an EE to own and operate a computer. > >But we don't have to be afraid of them, either. There is a lot of tolerance > >in the design of a computer just so spot shortages of a particular chip, or > >a low-tolerance part, won't make the machine unbuildable. That's why so many > >(IBM) clone makers can push nominal 16 MHz machines to 20MHz -- as long as > >you don't get them too hot. [some stuff deleted] > It is possible for mixed speed RAM in a bank to cause malfunctions. In a > former life I was in tech support for a small accelerator company. While > I was there, we discovered that combinations of different speed (and sometimes > manufacturer) RAM would cause the Mac to go out to lunch. This wasn't a > one time incident, and all of the RAM in use was rated at or above spec. The > Mac's would occassionally die when we mixed some brands/speeds of RAM within > banks. (Hey, don't worry. I'm not an EE ;-] ) Well, I *AM* an EE, and the way this memory is being used, there should be no problem mixing it. In fact, even though all chips on a SIMM are marked with some speed, in real life they vary - the speed indicated is the guaranteed MAXIMUM access time. Even if you use 150nS SIMMs with 40nS SIMMs, there is no problem - if the CPU hardware is designed for 150nS memory. This is because the CPU puts out its read (or write) request on the bus, which is in turn received by the 150nS and 40nS memory simultaneously. The CPU then waits for the data to be available. If it was designed for 150nS, its going to wait 150nS. If some faster memory makes data available sooner (e.g. 40nS), it doesn't matter because the CPU is waiting for a "fixed" 150nS. CPU's generally have a "wait" line which essentially halts the processor while slower peripherals (such as memory) get their data on the bus. Hardware external to the CPU (a divider) holds this wait line for some number of clock cycles ("wait states") to do this. It is usually fixed in hardware. I suspect Dave's experience with RAM problems were due to some other problem, maybe due to the Mac (Plus, II) borderline power supply designs/poor cooling. I have heard of higher power RAM causing problems due to power supply overload and overheating, especially in the Mac Plus. And if this was on an accelerator board, this would only aggravate the problem. I recommend a fan to prolong the life of your Plus. Hope this clears this issue up. -- Mike Pflueger @ AG Communication Systems (formerly GTE Comm. Sys.), Phoenix, AZ UUCP: {...!ames!ncar!noao!asuvax | uunet!zardoz!hrc | att}!gtephx!pfluegerm Work: 602-582-7049 FAX: 602-581-4850 Packet: WD8KPZ @ W1FJI