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Old 1st Jan 2018, 2:15 pm   #1
Sean Williams
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Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,517
Default 8085 processor questions

This post relates to some problems I am having with a BT Telex Machine (schematics requested elsewhere, but no luck as yet)

The beating (or not) heart of the machine is the 8085.

I am trying to get to the bottom of the problems that this system has, so thought attacking it from the basic computer that it should be will be a start.

I am well out of my depth here, so any assistance will be welcomed.

In a nutshell, when powered up, the screen is a mass of mixed characters, this has been the case since day 1.

I have checked all the supply rails, and the power supply is fully functional.

I have noticed that the processor is getting warm, and there is a green status led that should be illuminated, but this is not happening. I reason that the actual processor chip is dead, so am awaiting a replacement. (at least substituting this for a new processor should rule it out)

Attempting to scope the clock crystal results in no activity at all.

I have popped the processor out, and powered the machine up without it, the screen still fills with random characters.

So, I wonder what is normal for the 8085 to drive a CRT monitor? is the monitor driven straight from the processor, or is there some other trickery at play here?

Cheers
Sean
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