Path: utzoo!utgpu!attcan!uunet!dino!sharkey!mailrus!purdue!bls
From: bls@cs.purdue.EDU (Brian L. Stuart)
Newsgroups: comp.arch
Subject: Re: What CMOS cannot do (Re: Surges)
Summary: Transmission lines are difficult
Message-ID: <8681@medusa.cs.purdue.edu>
Date: 20 Nov 89 02:09:40 GMT
References: <7000@pt.cs.cmu.edu> <31547@hal.mips.COM> <48393@bbn.COM> <7032@pt.cs.cmu.edu> <1906@eric.mpr.ca>
Reply-To: bls@cs.purdue.edu (Brian L. Stuart)
Organization: Department of Computer Science, Purdue University
Lines: 22

In article <1906@eric.mpr.ca> hui@mprgate.mpr.ca writes:
>
>Isn't it true that 50 ohm transmission line drivers and receivers can be
>reliably and economically made in CMOS?
>Once you have a controlled impedence transmission line on your PC
>board, _any_ data rate is possible provided the internal data rates of
>your chips are fast enough, and your line drivers and receivers are up
>to it.

While transmission rates can be increased by using matched drivers
and receivers, you also need to have the characteristic impedance
of the intervening transmission line also matched.  Things can get
really weird if you have some fan-out (i.e. one driver and several
receivers in parallel).  Another drawback is that such drivers draw
more current.

In general, the idea is altogether feasable, but related factors are
one of the reasons that design of machines like Cray's is difficult.

Brian L. Stuart
Department of Computer Science
Purdue University