Building a better "old" computer

From: John Wilson <wilson_at_dbit.dbit.com>
Date: Sun Jun 25 10:39:18 2000

On Sun, Jun 25, 2000 at 09:48:57AM -0400, Douglas Quebbeman wrote:
>You can buy a brand-spanking-new IMSAI 8080 that has 1 meg
>of bank-switched static RAM and a 20mHz Z-80 (actually Z-280?)
>processor for under US$1000. I'd say that qualifies for an
>excellent "old" new computer.

Actually last time I checked, you could only *order* that machine, not
buy one, as they didn't/don't exist yet (still debugging the prototype).
And it's not an IMSAI 8080 by a long shot, it's a completely different
machine stuck in a familiar-looking box. So it doesn't address the
original poster's point of building a better-than-1979 machine using
1979 components.

Re that idea, a couple of things leap to mind:

- Backplane busses could have been done a lot more carefully, e.g.
  differential signal pairs, or at the very least O.C. with terminators
  at both ends (worked well for the Unibus).

- Doing fancy timing (e.g. RAS-CAS) using RC delays, one-shots, analog delay
  lines, lots of gates in series etc. was a Bad Idea. Using a few flip-flops
  clocked by a fast xtal clock might require an extra chip or two, but once
  you get it working it will keep on working.

- SCSI-1/SCSI-2, IDE, and probably other supposedly "modern" interfaces
  could have been done with 1979 parts. At the time, the expense of giving
  each peripheral its own CPU (or microcontroller at least) would have been
  prohibitive, but if money were no object it would have been nice to have
  some more open standards catch on, since the market was pretty fragmented
  for no good reason. Floppies were absolute hell in this regard too.

- Things might have been more stable if microcomputers had separated the
  ideas of "CPU bus" and "peripheral bus" earlier on. For the longest time,
  the peripheral bus was always just a buffered version of the CPU bus,
  which led to lots of timing problems and incompatibilities when you
  changed to a different or faster CPU. But having something that's easy
  to interface and has simple timing, like what the ISA bus became (after
  having the same problem for a while), would have been a good thing, as
  long as it's clear that this bus is for things *other* than main memory
  and the main system disk drive. It's nice being able to build a fancy
  slow peripheral (ADC, ROM burner, whatever) just once and use it for 20
  years.

- Something like PCI plug & play, only greatly simplified, would have been
  a very nice thing, so that software would always have an easy way to find
  out the hardware config by just asking. This could just be a small PROM
  with some kind of radial select mechanism.

John Wilson
D Bit
Received on Sun Jun 25 2000 - 10:39:18 BST