VT101 8085 CPU Fault

[Robert Jarratt]

> -----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 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.

> 
> >> 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.


> 
> >> 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