On 03/02/2017 15:27, "Tony Duell" <ard.p850...@gmail.com> wrote:
> On Fri, Feb 3, 2017 at 8:55 AM, Adrian Graham > <wit...@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?
