VT101 8085 CPU Fault

Robert Jarratt robert.jarratt at ntlworld.com
Mon Jan 19 16:51:19 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Diane
> Sent: 19 January 2015 21:46
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VT101 8085 CPU Fault
> On Mon, Jan 19, 2015 at 04:39:30PM -0500, John Wilson wrote:
> > On Mon, Jan 19, 2015 at 09:26:57PM -0000, Robert Jarratt wrote:
> > >In my latest tests I found that the CPU does indeed seem to HALT,
> > >according to S1 and S0 (both go low).
> >
> > What ever happened to in-circuit emulators?  I used an 8085A ICE in a
> > microprocessors class at Boston U. in the 1980s and it was *magic*.
> > I always wondered how they worked -- is it just an 8085A with a
> > doctored clock, or some complete re-implementation of it, or what?
> > Physically it was a 40-pin DIP plug with a thick cable leading off to
> > a mass of electronics.  It's just the kind of thing that would turn up
> > on eBay, but only if you didn't need it.
> His problem is he will have to unsolder the 8085 first.
> I'd suspect RAM since the ROM is (I assume) masked ROM.

I suppose again it is likely that a logic analyser would help. I have just
skip-read the power up and self-test section from the VT100 manual you
linked for me, and realise it halts the processor if it detects a ROM
checksum error or an error in the RAM. I am not sure if this explains why I
get random characters displayed, but the halt is consistent with a RAM
failure as you suggest.

When I next get the chance I will probe the RAM chips to see if their
behaviour looks OK, but I only have a scope so it won't be a very
comprehensive test. It will be the weekend before I get the next



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