Rebirth of IMSAI

From: Richard Erlacher <>
Date: Mon Mar 29 19:40:19 1999

please see comments imbedded below.



-----Original Message-----
From: Allison J Parent <>
To: Discussion re-collecting of classic computers
Date: Monday, March 29, 1999 5:30 PM
Subject: Re: Rebirth of IMSAI

><signals? I guess it's no wonder people liked the 8085, with its silly
><data/address bus better than the 8080 . . .
>The 8085 had different control signals, didn't require 3 voltages, a clock
>chip, a bus control chip, was faster and allowed better memory timing.

Yes, but, moreover, it had two signals to tell you when to read and when to
write, and it had two signals telling you whether your target was memory or
I/O. The fact that it had those essentially useless (for this environment)
multi-level interrupts internally, or that it had the bizzarre (for the
time) serial I/O on chip, was relatively uninteresting in light of the
benefits of fewer power supplies, faster operation, internal clock
generation, less critical timing, and lower parts count.

><Soooo . . . the signals were named the same also, eh? pSYNC, /pWR, sMEMR,
>Follow them back to the 8080. it should answer themselves. Also the 8080
>muxed status on the data bus at the early part of the cycle so the control
>signals reflect the raw 8080 status and control for the most part.

Unfortunately, all my 8080's appear on Multibus-I cards. I've NEVER owned
an S-100 8080 CPU. In fact, it wasn't until there were some single-board
computer boards on the S-100, e.g. SD Sales' SBC-100 that I even considered
using S-100 for anything.

>That's why board with 8085 and Z80 had all sorts of screwy logic to take
>their mostly decoded controls and encode them.

I certainly agree on that one! Half the logic on most of my later CPU cards
was dedicted to making the processor create the 8080-compatible signals
required for the bus. It takes less logic to implement a floppy controller,
two serial ports, a parallel port or two, and a full compliment of DRAM on
an S-100 board than it takes to make the a Z-80 or 8085 appear to the bus to
be an 8080.

Received on Mon Mar 29 1999 - 19:40:19 BST

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