Logic Analysers

[Adrian Graham]

On 03/02/2017 15:27, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:

> On Fri, Feb 3, 2017 at 8:55 AM, Adrian Graham
> <witchy at binarydinosaurs.co.uk> wrote:
> 
>> Ah yes, sorry, I'm aware of that. What I meant in this specific case is that
>> with 4 2764s right next to each other with a direct signal path between
>> adjacent address and data pins that has a resistance of 0.5 ohms pin to pin
>> surely I should be able to put a clip on each (for example) A4 address line
>> and see the same pulse at all four channels?
> 
> Yes, subject to the following unlikely cases :
> 
> 1) There is a standing wave developed between the pins. Technically that trace
> is a transmission line. I have never heard it happen between ICs next to each
> other at 8-bit micro speeds though.
> 
> 2) There is a bad connection (IC socket?) on one of the pins

That was my first thought so there's nice new turned pin sockets on there
now. I did find one connection on the data bus that was held together by
solder and luck so fixed that with a small piece of jumper wire.
 
> 3) If you have a very fast logic analyser you might be able to see the
> propagation delay as the signal gets to each pin (remember a foot is about
> a nanosecond. So you are talking 10s of picoseconds delay). You will not
> see that with the sort of analyser you or I have :-)

I wondered about that but the fastest I can go is 25Mhz, also the
missing/extra signals are across (in this case) 6 channels and don't go in
channel order, so there'll be a missing pulse on channels 1+2 or channel 4
or an extra one on 2+5 etc. I'm watching the output pins on an LS373
flip-flop (new socket+chip) and the corresponding ROMs (all new
sockets+chips) along with an LS21N in the decoding circuit.

> If you try your test with 4 of your logic analyser channels on the A4 pins of
> the EPROMs, I assume you get different traces for each channel -- that is
> what you are commenting on. What happens if you swap the logic analyser
> channels round?

I thought of that the other day, also swapped the grabber ends since they're
not the sturdiest of things. I haven't tried a PC though, my host is an
iMac.

Hopefully next week I'll have a Zeroplus to try which while still being USB
attach is 16CH+external clock and onboard RAM for storage. Could be useful.
 
> Incidentally, I'd better comment on the Logic Analysers I use. I use them
> a lot more than a 'scope, but that's because of what I generally need to do.
> 

<snip> Ah, someone else with an HP16xx. This is making me think I should
join the club. 

-- 
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?