VT101 8085 CPU Fault

Johnny Billquist bqt at update.uu.se
Sun Jan 18 18:55:39 CST 2015

On 2015-01-19 01:00, Robert Jarratt wrote:
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Johnny
>> Billquist
>> Sent: 18 January 2015 22:53
>> To: cctalk at classiccmp.org
>> Subject: Re: VT101 8085 CPU Fault
>> On 2015-01-18 23:04, Diane Bruce wrote:
>>> On Sun, Jan 18, 2015 at 09:47:35PM -0000, Robert Jarratt wrote:
>>> ...
>>>>> you more. Nice sharp edges to signals when they shift. Times that
>>>>> look reasonable... Steady levels, and correct levels.
>>>> It will never be steady with a scope because the address lines won't be a
>> regular waveform. That said they looked very different on the faulty board to
>> how they looked on the good board.
>>> A remote possibility is the boot ROM is b0rked and it's simply running
>>> around like a chicken without a head.
>>> That is something a good logic analyser could tell you.
>> If the RD pin never gets active, the contents of the ROM will not even be
>> presented to the CPU, so while it's possible the ROM is borked, that is not the
>> problem we're trying to fix right now.
> 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.
> 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.

No. That should not be possible.
The RD line comes from the CPU and is used to fetch data. The CPU should 
continue to fetch data unless it has halted. A borked ROM giving bad 
data do not stop the CPU from running. Normally all kind of data is 
executed, but it might cause funny results...

And if it is halted, more things should be at a standstill... Unless you 
have dynamic memory refresh coming from the CPU, in which case I'm not 
sure how the signals will behave. But I don't even know if you can halt 
that CPU...

But the fact that there is some activity just at the start is interesting.

This might become a case where you need to understand how CPUs work at 
low levels. I hope you are comfortable on that subject. :-)


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

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