VT101 8085 CPU Fault

[Diane Bruce]

On Mon, Jan 19, 2015 at 02:07:08PM +0100, Johnny Billquist wrote:
> On 2015-01-19 09:04, tony duell wrote:
> >>
> >> I just found something interesting, the pin activity on the address lines, RD, WR, ALE etc appears normal for a bit,
> >> and only after a few seconds does it all become inactive.

http://tkc8800.com/images/vt100/VT100_Technical_Manual.pdf

Look in section 4.2.8
The POWER-Up adn Self-Test section.

I linked this pdf before ;)

"Assuming there are no hard logic failures present on the board, "
And yes the 8085 has a HLT instruction. 
Are the keyboard lights lighting up at all?

If it *is* the ROM which is unlikely if it is a masked ROM, it might
simply be lose in the socket you mentioned. It may be sufficient to 
wiggle the ROM slightly to break any oxide on the pins. Some sockets
are notorious for allowing the chip to wiggle up due to heat and cooling down.
Your ROM may simply be lose.


> >>
> >> Perhaps this really is an indication that the ROM is bad, if it is getting into some kind of loop where it does no
> >> external activity. I don’t really know the 8085 to know whether that is possible.
> >
> > What if (due to ROM or RAM problems maybe) the 8085 executed a HLT instruction?
> 
> I didn't know it had a halt instruction, which is why I speculated 
> (negatively) about that in a previous reply.
> But if it do indeed have a halt, then this would be a very possible 
> explanation.
> 
> 	Johnny
> 
> -- 
> Johnny Billquist                  || "I'm on a bus
>                                    ||  on a psychedelic trip
> email: bqt at softjar.se             ||  Reading murder books
> pdp is alive!                     ||  tryin' to stay hip" - B. Idol
> 

Diane
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