Hi folks, I didn't see the message at the bottom of this one arrive since I think I sent it JUST as the list software was being changed over. Gah, having just looked again I realise I've sent it from not the address I've subbed with. PEBCAK there :) Since then I discovered the -5V rail for the 4116s had dropped to -4.2V which was out of spec for both types of RAM on this box so on Chuck's suggestion I swapped the 560ohm resistor/zener combo that was powering this rail for a 79L05 regulator and the DRAMs now have all 3 voltages steady. No change in behaviour though. I'm baffled as to why the upper address bus doesn't blip once RESET goes high. HOLD is permanently pulled low so it's not that. Any suggestions? Cheers! -- Adrian/Witchy Binary Dinosaurs creator/curator Www.binarydinosaurs.co.uk - the UK's biggest private home computer collection? ------ Forwarded Message From: Adrian Graham <witchy at binarydinosaurs.co.uk> Date: Sun, 26 Feb 2017 23:34:50 +0000 To: "Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> Conversation: More 8085a oddities Subject: More 8085a oddities Hi folks, After a few days break I came back to my 8085a-powered phone system this weekend and it's decided to go on strike. By that I mean the processor locks up after only a few cycles so doesn't get as far as attempting to read anything, when it freezes the S0/S1/WR status lines are all high which shouldn't be possible since S0/S1 high should be 'Fetch' according to the manual, not WRITE. Vcc, RESET and clock are good and I can't see any other external signal which might hold the CPU. The PSU is good and putting out +5/+12/-12 as it should. CPU checks out in another 8085 system I forgot I had. Interestingly my analyser shows the upper half of the address bus doesn't change while the lower half manages a single transition, as does the ALE signal. From the CPU both halves of the address bus go directly to a 74LS373 each which both check out OK on a breadboard circuit I made up earlier. ROMs are all OK and the lines themselves back to the LS373 and CPU check out with little resistance. I'm stumped and can't help but think this is something stupid which I'm overlooking. Maybe more sleep will help. Cheers! -- Adrian/Witchy Binary Dinosaurs creator/curator Www.binarydinosaurs.co.uk - the UK's biggest private home computer collection? ------ End of Forwarded Message