19 Jan
2015
19 Jan
'15
2:51 p.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Diane
Bruce
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:
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
opportunity.
Regards
Rob
On Mon, Jan 19, 2015 at 09:26:57PM -0000, Robert
Jarratt wrote:
His problem is he will have to unsolder the 8085 first.
I'd suspect RAM since the ROM is (I assume) masked ROM.
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.