Ongoing PDP11/34a resurrection attempts - stuck SACK?

Date: Sun, 30 Aug 2009 15:58:08 -0500 From: jplist2008 at kiwigeek.com To: cctech at classiccmp.org Subject: Ongoing PDP11/34a resurrection attempts - stuck SACK? Greetings all; In my continued efforts to try and get my 11/34a functioning I decided to set up my logic analyser and trap ALL of the 56 Unibus signal lines to see if I could spot anything that was being held asserted when it shouldn't. For reference, my 11/34a is set up like: 1 M8266 -------------------------------------------| 2 M8265 -------------------------------------------| 3 M9301 YJ-----| FLIPCHIP 4 M7856---------------------------| 5 National Semiconductor Memory Board--------------| 6 G7232---------| 7 G7232---------| 8 G7232---------| 9 M9302--------| G7232---------| I only have the basic front-console, so not the lovely button-filled operators console. All the NPR jumpers are in place. All voltages check out. The symptoms are: Unit powers on, RUN light comes on and then quickly goes out. When the BOOT/INIT switch is toggled, again, RUN light on, then out. Clearly the unit is HALTing on something, but I can't work out what and nothing seems to appear on the console. When I analyse the bus the unit follows what, from the Unibus design manual, appears to be a standard start-up procedure. - All signals float (obviously) - All signals cleared, DCLO/ACLO/INIT asserted - 773000 is placed on the Address lines (as configured in the bootstrap) - DCLO negates - ACLO negates - INIT negates, and then immediately: - Address lines are cleared - SACK is asserted I find no mention of SACK being asserted in the Unibus manual in its start-up section. Furthermore, as my understanding goes, SACK should only be asserted when a bus master has accepted a grant and is beginning a transaction cycle. There is NO activity on the request/grant lines, so nothing should be allowed to become bus master, so I think this SACK signal should not be happening. Can anyone with more understanding than me (which, you know, is pretty much everyone else who owns a PDP11) confirm I'm not completely off-base here? I have two M7856s and have switched between them, so if it's the 7856 that's holding SACK, then both of mine are doing it. Elsewise... I think the only thing that could be "sticking" is the processor boards, as the memory card shouldn't be able to assert SACK since it can't become a bus master. If this guess is correct... I'm not really sure how to begin debugging what on the processor board is getting glued. Time to break out those schematics? My thanks to all for your thoughts; - JP