Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!wuarchive!uunet!visix!amanda From: amanda@visix.com (Amanda Walker) Newsgroups: comp.sys.mac.hardware Subject: Re: Memory speeds can be critical (was: SIMMs for IIsi - what do I need?) Message-ID: Date: 20 Dec 90 19:48:45 GMT References: <2915@ux.acs.umn.edu> <2924@ux.acs.umn.edu> Organization: Visix Software Inc., Reston, VA Lines: 34 In article <2924@ux.acs.umn.edu> dhoyt@vx.acs.umn.edu writes: >The lower speed ones tend to have higher impedances. The higher speed >chip will have it's downbeat a bit before the slower chip. So far I follow you; I have in fact run into some weird speed problems (for example, mixing FAST and LSTTL chips), and I can see the possibility that using memory that is a whole lot faster than the design could cause a problem. However, I still do not see how the speed of one SIMM can affect the others in its bank, even with brain-dead driver circuitry. I mean, if using all 100ns works, and using all 80ns works, how can using two of each cause a problem. All of the SIMMs are still within tolerances--just in different places within them. >If you put 4ns memory in your mac most likely it wouldn't work. The advice of >Apple is sound. Don't mix if you don't have to mix. And if you do mix, don't >mix in the same bank. We're not talking putting bipolar SIMMs (an amusing concept) into a IIsi. We're talking about putting in SIMMs that fall into different spots in the "safe zone." If the problem really was speed slew of some sort, you'd have to have actual matched sets of SIMMs, not just ones all stamped with the same speed. A 100ns SIMM may well be an 80ns SIMM that got marked as 100ns because the memory company had more orders for 100ns SIMMs that month. The rating is simply the worst case--the best case is always undefined. -- Amanda Walker amanda@visix.com Visix Software Inc. ...!uunet!visix!amanda -- "I wouldn't be surprised if the architecture of Intel's microprocessors were eventually linked to the eventual fall of mankind." --Steve Gibson