RE: DECstation 220 Impasse Reached

[Rob Jarratt via cctalk]

> -Original Message-
> From: Phil Blundell [mailto:p...@pbcl.net]
> Sent: 28 July 2017 18:27
> To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; General
> Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org>
> Subject: Re: DECstation 220 Impasse Reached
> 
> On Fri, 2017-07-28 at 18:04 +0100, Rob Jarratt via cctalk wrote:
> > Anyone have any thoughts on whether those data signals look right?
> 
> It doesn't look obviously wrong.  If the vertical scale is 2V per box then the
> amplitude is probably about right for 74LS logic.  I think you said this was 
> on a
> shared data bus and the little glitch might well be a benign artifact (either 
> bus
> turnaround or just a reflection of some internal logic in whatever chip was
> driving the bus at the time).
> 
> But it's quite hard to say anything very intelligent about that signal based 
> on
> just one waveform in isolation.
> 


Thanks, I wasn't too concerned about the actual levels as they seemed fine for 
74LS as you say. I did wonder if the "roughness" of the signal was anything to 
do with transmission line effects. The top of the waveform is not very flat. On 
other pins I do see a long curving rise as well. It sounds like I may be able 
to consider this a fairly normal signal.

Regards

Rob

RE: DECstation 220 Impasse Reached

[Rob Jarratt via cctalk]

So the vertical range of the signal is about 0-5V (about 2V per division) ,
I can't remember the timebase though, but I think it was about 0.1us per
division. I will do it again (later) with more info. The signal was found on
a data pin of the 80286.

Regards

Rob

> -Original Message-
> From: Paul Koning [mailto:paulkon...@comcast.net]
> Sent: 28 July 2017 18:17
> To: r...@jarratt.me.uk; Robert Jarratt <robert.jarr...@ntlworld.com>;
General
> Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org>
> Subject: Re: DECstation 220 Impasse Reached
> 
> 
> > On Jul 28, 2017, at 1:04 PM, Rob Jarratt via cctalk
<cctalk@classiccmp.org>
> wrote:
> >
> > Anyone have any thoughts on whether those data signals look right?
> 
> It's hard to guess at signals when the horizontal and vertical scales are
not
> stated, nor where in the circuit the signal was found.
> 
>   paul

Re: DECstation 220 Impasse Reached

[Paul Koning via cctalk]

> On Jul 28, 2017, at 1:04 PM, Rob Jarratt via cctalk  
> wrote:
> 
> Anyone have any thoughts on whether those data signals look right?

It's hard to guess at signals when the horizontal and vertical scales are not 
stated, nor where in the circuit the signal was found.

paul

RE: DECstation 220 Impasse Reached

[Rob Jarratt via cctalk]

Anyone have any thoughts on whether those data signals look right?

 

Thanks

 

Rob

 

From: Rob Jarratt [mailto:robert.jarr...@ntlworld.com] 
Sent: 27 July 2017 21:18
To: General Discussion: On-Topic and Off-Topic Posts (cctalk@classiccmp.org)

Subject: DECstation 220 Impasse Reached

 

Hello Everyone,

 

As I have mentioned before I have made some progress on a DECstation 220,
but I have reached an impasse. I get some signals on the data bus which I am
not sure about. They look odd, but they could be normal, I just don't know.
I took a picture of the scope trace, which is here:
https://rjarratt.files.wordpress.com/2017/07/decstation-220-data-signals.jpg
.

 

Any advice on whether that looks like a "normal" signal?

 

If anyone is interested, I have posted more details on general progress in
this blog posting: https://robs-old-computers.com/2017/07/27/decstation-220/

 

Thanks

 

Rob