Logic Analysers

[Tony Duell]

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

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 :-)

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?

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.

1) (Is is an LA?) The HP LogicDart. 3 Channels, 100MHz. No external clock
facility. But it is pocket sized. HP called it the 'advanced logic
proble' and that's
really what it is. A better version of the blinking-light probe I used
to use. Great for
checking clocks, power supply voltages (it has a voltmeter function),
serial data
streams, etc. Normally the first instrument I grab for an unknown logic problem
just to eliminate the 'sillies'

2) An old Gould-Biomation K100D. 16 channels 100MHz. with external clocking. I
do have the 32 channel adapter for it which can only work with an
external clock.
This was my first LA and I still have a soft spot for it.

3) An HP1630. I forget which one, probably a 1630G. It does all I
want. I was also
AFAIK the last HP LA to have a proper component-level service manual. It's also
a classic computer in its own right (6809 + 6829 MMU). Oddly the CRT is scanned
vertically, I have no idea why.

-tony