VT101 8085 CPU Fault

[Johnny Billquist]

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 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.

>> 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 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-description-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