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From: fjo@ttrde.UUCP (Frank Owen )
Newsgroups: comp.sys.mac
Subject: Re: SIMM RAM for the MacII
Message-ID: <252@ttrde.UUCP>
Date: Tue, 8-Sep-87 14:08:34 EDT
Article-I.D.: ttrde.252
Posted: Tue Sep  8 14:08:34 1987
Date-Received: Thu, 10-Sep-87 01:33:41 EDT
References: <20389@ucbvax.BERKELEY.EDU>
Organization: AT&T, Skokie, IL
Lines: 37

in article <20389@ucbvax.BERKELEY.EDU>, korn@cory.Berkeley.EDU (Peter "Arrgh" Korn) says:
> put the 256K SIMMs into my MacII.  Trouble was, these SIMMS were the 
> older 150ns SIMMs.  But, lo and behold, THEY WORKED.  I haven't had a 
> chance to benchmark 'em, ...
> So, it seems that 150ns DRAMs are perfectly kosher for a MacII.  

This is a dangerous thing to do, although the results you are having are
not surprising. In many cases, rams spec'ed at 150ns will typically
have access times much less than 150ns. The 150ns spec means that under
a certain set of operating circumstances the rams will exibit an access
time less than or equal to 150 ns. However, in practice, you will rarely
reach the limits of the conditions that would bring that access time
close to the 150ns spec.

Also, some manufacturers "over-spec" their parts more than others.
(Manufacturers will often call an "x"ns part an "x+margin"ns part to
cover themselves.)

Also, there is a certain amount of uncertainty in the access-time from
part to part. Some parts from a certain "good" lot may greatly exceed
the 150ns spec, whereas others may be right at the borderline.

So, if you use a 150ns part where a 100ns part is specified, you are
walking on thin ice. You might be lucky and get an overspeced part,
and you might not ever approach the limits of operation (like high temperature)
but if you do, you could be in trouble.

Also, using a slower or faster part will not in any way affect the speed
that the MacII will operate. If you were to use a 2ns access part, the
memory cycle time of the MacII will still be governed by the same
timing constraints as if you were to use a 100ns part.

So my advice:
   Buy the part specified, but don't pay more for a part with
   greater speed. You will be wasting your money.

   Frank Owen (..ihnp4!ttrde!fjo)