Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!bloom-beacon!eru!hagbard!sunic!news.funet.fi!assari.tut.fi!assari.tut.fi!n67786 From: n67786@lehtori.tut.fi (Nieminen Tero) 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 15:25:30 GMT References: <2915@ux.acs.umn.edu> <2924@ux.acs.umn.edu> Sender: news@assari.tut.fi (USENET News System) Organization: Tampere Univ. of Technology, Finland. Lines: 32 In-Reply-To: amanda@visix.com's message of 20 Dec 90 19:48:45 GMT In article amanda@visix.com (Amanda Walker) writes: 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. Only reason for this I can think of is that the logic doesn't properly check the memory ready lines from all simms in a bank but instead just uses one single sim for that purpose. Now if the simm in question happens to be faster than the others in the bank the other simms might not be staedy at the time the cpu issues the read. This all comes down to the gray areas in signal timing charts, ie. transition periods. If mixing different speed simms cause the signals to fall out of those areas you may be in trouble. The timings (and the ram chip speeds also) are probably designed so that this should not be a problem within same speed chips. Amanda Walker amanda@visix.com Visix Software Inc. ...!uunet!visix!amanda -- Tero Nieminen Tampere University of Technology n67786@cc.tut.fi Tampere, Finland, Europe