VT101 8085 CPU Fault

[Johnny Billquist]

On 2015-01-18 22:47, Robert Jarratt wrote:
>
>
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Johnny
>> Billquist
>> Sent: 18 January 2015 21:33
>> To: cctalk at classiccmp.org
>> Subject: Re: VT101 8085 CPU Fault
>>
>> On 2015-01-18 22:12, Robert Jarratt wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of
>>>> Johnny Billquist
>>>> Sent: 18 January 2015 20:49
>>>> To: cctalk at classiccmp.org
>>>> Subject: Re: VT101 8085 CPU Fault
>>>>
>>>> On 2015-01-18 21:37, Robert Jarratt wrote:
>>>>> No stuck, high.
>>>>
>>>> Is there any activity on any of the address pins? If those also are
>>>> dead, either the whole CPU is dead, or else it is disabled by something.
>>>>
>>>
>>> Yes there is, but it looks odd, and very different to when I checked the address
>> pins on the working one. It looked like the voltage might be spiking upwards,
>> but it is hard to tell because I don't have a working logic analyser.
>>
>> All power pins are ok to the CPU?
>
> I checked Vcc and Vss and they appeared to be fine (5V and 0V respectively).

I can't remember (and haven't checked), but make sure that chip isn't 
one of those funny one that requires several clocks or power levels.

>> I wouldn't care much for a logic analyzer at this point. A good scope will tell
>> 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.

It should be steady between clocks.
You should essentially run up the frequency of the scope so that you can 
see each clock phase nice and wide. Checking address pins you should 
then also have the same nice long bits. You might see some initial ring 
when moving to a different level, but that should be very small and 
short. After that it should be level for the rest of the clock cycle.

Ie. You need to look closer. Just just "it's some kind of activity going 
on".

>>>> If those are showing activity then the RD pin is either dead, or
>>>> shorted somewhere, I'd guess.
>>>
>>>
>>> RD and WR are active low, and they are both high, so I suppose they could be
>> shorted high, but I don't know.
>>
>> That will be very hard to verify without disconnecting the pins. So let's focus on
>> other things for the moment.
>
>
> I have a desoldering gun now, so desoldering is no longer the problem it once was for me, but I would still prefer to avoid this if possible. I am not sure what else to check though.

That do help. :-)
But I understand you. Not much point in replacing components that turns 
out to be fine. Especially since it still might be destructive to remove 
them.

	Johnny

>
>
>>
>>>> I hope you are checking the right pins... :-)
>>>
>>> Me too, although I have checked multiple times. I am using this page
>>> to give me the pinout:
>>> http://scanftree.com/microprocessor/Pin-Diagram-of-8085-and-Pin-descri
>>> ption-of-8085
>>
>> Well, I can't offer any more help on that one. Just a reflection.
>>
>> 	Johnny
>>
>>>
>>>
>>>>
>>>> 	Johnny
>>>>
>>>>>
>>>>> Regards
>>>>>
>>>>> Rob
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of
>>>>>> Bob Rosenbloom
>>>>>> Sent: 18 January 2015 20:11
>>>>>> To: General at classiccmp.org; Discussion at classiccmp.org:On-Topic and
>>>>>> Off- Topic Posts
>>>>>> Subject: Re: VT101 8085 CPU Fault
>>>>>>
>>>>>> 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
>>>>>
>>>>
>>>>
>>>> --
>>>> 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
>>>
>>
>>
>> --
>> 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
>


-- 
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