A quick follow-up. I did some probing of the LS123 one-shots and I discovered something interesting. Pin 12 (Q-prime) on both U17 and U20 are free-running. The other one shots seem to be triggering properly. I don’t see any immediate cause for this, so I swapped the chips but I get the same result. Rich -- Rich Cini Collector of Classic Computers Build Master and lead engineer, Altair32 Emulator From: Richard A Cini <rich...@verizon.net> Date: Sunday, March 1, 2015 at 4:29 PM To: S100-Post <n8vem...@googlegroups.com> Subject: MiniFP build - data bus question Guys — I finished my MiniFP board this morning and I can’t seem to deposit to or read from memory using the EXAMINE toggle. The “next” part of the function works, so I can step among addresses but the data LEDs are all on all the time (unless I set JP5 SSW_DSBL jumper). However, if I use the SINGLE_STEP switch, the data bus LEDs appear to reflect memory contents. The experimental system is very simple CompuPro RAM17 (64k), IMSAI 8080 board, and the front panel. Before I say that anything is wrong, can someone confirm the jumpers? I think only the MWRT should be jumpered. The RAM17 is an IEEE-696 board so as I do for my IMSAI, I’ve isolated pins 20/70 (UNPROT/PROT) on that board. Chip-wise, I’ve used LS (including the LS123 as recommended in the Wiki) except the 7402. I don’t have any 7400’s in-stock so I’m using LS00 for now…don’t know if that’s part of the issue since those gates are used in the EXAMINE circuit. I replaced the 8212 just in case, but that had no effect. The data cable is fine. Any help, even questions to get me thinking, would be appreciated. I feel I’m pretty close to a solution here. Thanks! Rich -- Rich Cini Collector of Classic Computers Build Master and lead engineer, Altair32 Emulator |