Help with getting an original Altai running: 8080A clock circuitry and MITS88 2-SIO board

Robo58 robo58 at
Sun Feb 7 08:17:55 CST 2016

Hi Folks,

I'm looking for some insights in getting an original Altair running again.

My buddy asked for my help getting his Altair running again.  He went
through and replaced all the electrolytic caps, cleaned up everything and
then tried to get the MITS 88 2-SIO board to run a simple echo program.  It
wouldn't work and now I have the Altair,

I've been looking it over for about a week and I've noticed the following.

1). The 2Mhz base oscillator circuit will not reliably start up when it's
hot.  It's perfect when cool.

2). The original MITS 2-phase clock circuit was modified.  The 74123 was
replaced with a 74221 along with RC changes.  

3). The MITS 2-SIO card has two Motorola 6850 ACIA's that don't seem to do
what their data sheets say they should be doing.  

Some questions for you.  (I've searched for insights on these topics and
information is scarce).

a). Was the Altair known for hot start issue(s) ?  If so are their tried and
true fixes?

b). Going to a 74221 appears to be an excellent move.  The RC changes were
prompted by that move.  There is no longer an RC delay circuit from phase 0
to phase 1, which appears to make sense.  I don't see one-shots as a wise
clocking design choice but my buddy wants to stay with it.  Are their tried
and true fixes here?

c). I hand assembled some code to exercise the 6850.  I know it has a
software reset and then you set attributes.  I tried my buddies echo
program.  It looks for a received character and then echo's it.  I'm using a
laptop and PuTTy along with an RS-232 breakout box.  I can see characters
going in but nothing coming out.  The 6850 appears to drive the bus for a
finite amount of time and then turn off its drivers.  I don't have any
experience with the part and I don't know if that's the way it's supposed to
work or that the Altair has a bus timing issue.

Any thoughts on what I'm seeing and suggestions/fixes ?

Thanks Robo

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