VT101 8085 CPU Fault

sandy hamlet steemer at dslextreme.com
Mon Jan 19 13:37:52 CST 2015

Using a scope, look at the address lines and/or data lines at power
up, just after system reset.
The pulses will probably vary in width but should all be the same amplitude.
If you see something like stair stepping: where a pulse starts out at
5 volts and drops, for example to 3 volts and then drops to 0 volts,
then you might have two devices trying to access the same line at the
same time.
Devices on the same line may include a ram, rom, I/O ports, etc.
This could cause the processor to "hang up" and act erratic.
A schematic of the board would be helpfull if you have one.


On 1/18/15, Bob Rosenbloom <bobalan at sbcglobal.net> wrote:
> On 1/18/2015 12:07 PM, Robert Jarratt wrote:
>> No, I checked that and it isn't. It is an active low signal and it is
>> high.
>> Regards
>> Rob
> What about the READY pin? Stuck low?
> Bob
>>> -----Original Message-----
>>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of tony
>> duell
>>> Sent: 18 January 2015 19:16
>>> To: General Discussion: On-Topic and Off-Topic Posts
>>> Subject: RE: VT101 8085 CPU Fault
>>>> The terminal controller board on my VT101 is not working, if it
>>>> displays anything it is random characters. I have used my scope to
>>>> check the 8085 CPU. It has power, it is outputting a valid clock, but
>>>> there is no activity on the RD,WR and ALE pins. I am guessing the 8085
>>>> is bad, but before I go replacing it I wanted to check if anyone knew
>>>> of anything else that might cause the 8085 to misbehave (ie am I seeing
>>> symptoms or a cause?).
>>> What is the state of the reset input to the 8085? In other words, is the
>> CPU
>>> being held reset all the time?
>>> -tony
>>> =
> --
> Dentopedalogy is the science of opening your mouth and putting your foot in
> it.
> I've been practising it for years.
> -Prince Philip

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