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

Robo58 robo58 at
Sun Feb 7 18:15:00 CST 2016

Hi Brent, Drlegendre and Eric

Brent:  I took your advice and did a little program to output "A"'s and
monitoring Transmitter empty before looping again.  That works fine and it
will run for hours.

Drlegendre:  Thanks for the excellent document link.  My buddy has the MITS
1K static ram and the MITS 4K dynamic ram cards.  I've been doing my testing
with the 1K static ram to limit potential problems.

Eric:  This Altair has the smaller PS with the smaller Electrolytic caps.
They were all replaced and the PS brought up without cards with a Variac.
My buddy bought Kemet Capacitors on the basis of very good ripple current
rating and low ESR.

Thanks Ron


-----Original Message-----
From: cctalk [mailto:cctalk-bounces at] On Behalf Of Brent
Sent: Sunday, February 7, 2016 4:14 PM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at>
Subject: Re: Help with getting an original Altai running: 8080A clock
circuitry and MITS88 2-SIO board

On 2016-Feb-07, at 6:17 AM, Robo58 wrote:

> 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.

How about doing a program that simply repeatedly outputs a character first,
getting that going before trying to deal with input - cuts the problem space
in half.
The repeat loop should give you some consistent bus activity and observable
waveforms on a scope if you have one, rather than trying to catch events
initiated from external input activity.

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