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

Brent Hilpert hilpert at
Sun Feb 7 15:13:59 CST 2016

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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