Title:  Altair 8800c
Date:  20200807
Tags:  hardware retrocomputers
========================================

A new Altair is alive!

45 years after the original Altair kit was introduced by MITS on the cover of
Popular Mechanics, another Altair has been assembled from parts.

As mentioned in my previous post[0], an Altair 8800 built today from (mostly)
new parts is referred to as an 8800c.  The original Altair was the 8800 and MITS
produced the 8800b later (which did not have a front panel).  Thus, the 8800c.

This was definitely my largest electronics project.  I had built an RC2014
previously but nothing else with significant soldering.  The RC2014 was a full
kit where as I had to source components for the Altair.  The Altair you build
can be customized quite a bit, too.  There are a number of reproduction S-100
bus[1] cards as well as new ones.  Some crazy S-100 aficionados[2] have designed
16MB RAM boards and 486 CPU boards.  On a system bus from the mid 70's...
That's the specs I had in the mid 90's.

Other than waiting on missed parts getting shipped to me, the only trouble I ran
into was with the CPU clock.  I was not getting any output from the crystal
oscillator.  The clock circuit uses an inverter to translate the crystal's
output into a square wave as well as invert the signal from the clock line on
the bus.  Turns out I had swapped a 7404 chip, needed in the clock circuit, for
a 74LS04 chip needed for interfacing with the bus.  An easy mistake as they do
the same thing, just in a different way.  I don't understand how the clock
circuit works electronically enough to understand why it matters, though.  One
of the inverters sits between the pins of the crystal in the circuit so it's
doing something.

I also had to adjust the timing of the clock by changing out a resistor until
things were in spec.  Got to legit use my oscilloscope instead of just using it
to poke at things and see what it looks like.

I have a few additional cards to build to customize the sysem a bit and I want
to migrate my current Altair software project off my Altair 8800 clone to the
real hardware.


[0] gopher://kagu-tsuchi.com:70/0/phlog/updates20200712.txt
[1a] gopher://gopherpedia.com:70/0/S-100 bus
[1b] https://en.wikipedia.org/wiki/S100_bus
[2] http://s100computers.com/index.html