PDP-11/45 Behaving Badly
Sun 09 December 2018 by Fritz MuellerWow, a year to the day since the previous post here! Not a lot of PDP-11 work this past year, with lots of other stuff like home improvements going on, but a few things worth catching up on here.
Mainly, I got brave this past year and decided to actually rent a van and take the 11/45 out of the basement to the VCF West show at the Computer History Museum in Mountain View. This was a lot more physical work than I had anticipated. Working on this thing a piece at a time, sitting in one place in the basement, you get kind of used to it and forget how much iron it actually is... But breaking it down, loading it into a van, unloading into the show, reassembling, then reversing the whole process at the end of the show is a stark reminder, both of the size of the machine and of my advancing age, ha! A huge thank-you to my workmate Brian, who selflessly gave up a weekend, a vacation day, and some mileage on his back to give me a hand. He has already informed me that "the answer for next year is 'no'." :-)
I suppose I should have expected it, but in the course of transportation to the show something shook loose resulting in a machine that wouldn't boot RT-11 when reassembled on the show floor (stupid bumpy rental van!) So my show became a two day live-troubleshooting exhibit. This was fine, and I think a lot of folks had fun jumping in and helping with troubleshooting (thanks, all!) There was a lot of interest and reminiscence about the machine and I met a lot of nice people. Still, a little disappointing, because I really had wanted people to be able to sit down and use the machine, and also because my head ended up in the machine the whole time I really didn't get to see the rest of the show or talk to other people about their exhibits! Ah well. In the end I did cajole a successful boot out of it, 15 mins. before the show closed, so at least a couple people got to sit down and play Adventure. Placed 3rd in the restoration category :-)
So, what went wrong? At the show I managed to isolate the problem to something intermittent related to interrupts from the RK11-C controller. I was still able to boot the RKDP diagnostic pack, since its bootstrap and monitor make very conservative use of processor and device interface features. Running through the diagnostics, managed to narrow down the problem to RK11-C completion polling after overlapped seeks. I guess RT-11 makes use of this feature.
I got the machine home and reassembled, and verified that the problem was still manifesting. Then many months passed, until I found some time to dig deeper into the problem just last night. The relevant failing diagnostic is ZRKK test 37, and the output is:
DRIVE 0
RK11 DIDN'T INTRUPT AFTER SK COMPLETED
PC RKCS RKER RKDS
014476 000310 000000 004713
SCP DIDN'T SET AFTER SEEK WAS DONE
PC RKCS
014526 000310
RK11 DIDN'T INTRUPT AFTER SK COMPLETED
PC RKCS RKER RKDS
014476 000310 000000 004712
SCP DIDN'T SET AFTER SEEK WAS DONE
PC RKCS
014526 000310
TIMOUT,PC=004536
And the relevant bit of the diagnostic listing:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 | 014362 2$: MOV RKVEC,R1
014366 MOV #3$,(R1)+ ;SET UP VECTOR ADRES FOR RK11 INTERUPT
014372 MOV #340,(R1) ;SET UP PSW ON INTERRUPT
014376 BIS #40,@RKDA ;ADRES CYLINDER #1
014404 MOV #111,@R0 ;SEEK, GO WITH IDE SET
014410 WAT.INT ,300 ;WAIT FOR THE DRIVE TO
;INTERRUPT AFTER ADRES WAS RECVD
;WAITING TIME= 1.4 MS FOR 11/20
;280 US FOR 11/45
;ERROR, IF INTERUPT DID NOT OCCUR
;BY NOW
014414 MOV #BADINT,@RKVEC ;RESTORE UNEXPECTED RK11 INTERRUPT
014422 MOV @R0,$REG0 ;GET RKCS
014426 ERROR 75 ;INTERRUPT DID NOT OCCUR AFTER
;SEEK WAS INITIATED WITH IDE SET
014430 BR 3$+4
014432 3$: CMP (SP)+,(SP)+ ;OK, IF RK11 INTERRUPTED TO THIS
;RESTORE STACK POINTER (FROM RK11 INTERRUPT)
014434 CMP (SP)+,(SP)+ ;RESTORE STACK POINTER (FROM
;WAT.INT)
014436 MOV #5$,@RKVEC ;SET UP NEW VECTOR ADRES FOR RK11
014444 BIT #20000,@R0 ;IS SCP CLEAR
014450 BEQ 4$ ;YES, BRANCH
014452 MOV @R0,$REG0 ;GET RKCS
014456 ERROR 76 ;SCP SET BEFORE SEEK TO LAST
;CYLINDER WAS DONE
014460 4$: WAT.INT ,56700 ;WAIT FOR DRIVE TO INTERRUPT
;AFTER SEEK WAS COMPLETED
;WAITING TIME=180 MS FOR 11/20
;36 MS FOR 11/45
014464 MOV #BADINT,@RKVEC :IT'S AN ERROR IF BY THIS TIME
;INTERRUPT HAS NOT OCCURERED
014472 JSR PC,GT3RG ;GO GET RKCS, ER, DS
014476 ERROR 77 ;RK11 DID NOT INTERRUPT AFTER SEEK (TO
;LAST CYLINDER) WAS DONE WITH IDE SET
014500 BR 5$+2
014502 5$: CMP (SP)+,(SP)+ ;OK, IF RK11 INTERUPTED TO THIS AFTER
;SEEK WAS COMPLETED. RESTORE
;STACK POINTER (FROM RK11 INTERRUPT)
014504 CMP (SP)+,(SP)+ ;RESTORE STACK POINTER (FROM
;WAT.INT)
014506 MOV #BADINT,@RKVEC ;RESTORE RK11 INTERRUPT VECTOR ADRES
;FOR UNEXPECTED INTERUTS
014514 BIT #20000,@R0 ;DID SCP BIT SET?
014520 BNE 6$ ;YES, BRANCH
014522 MOV @R0,$REG0 ;GET RKCS
014526 ERROR 53 ;SCP DID NOT SET AFTER RK11 INTERRUPTED
;INDICATING SEEK WAS
|
So, based on the fact that we don't hit error 75 from 14426 (and the fact that the previous test, #36, in this diagnostic is passing) unlike some previous issues the RK11 here is able to generate interrupts and the 11/45 CPU is fielding them. The issue seems related to the seek completion polling circuitry on the RK11.
This circuitry is described in section 3.3.2 of the RK11-C manual, and is detailed in engineering drawings D-BS-RK11-C-12, sheets 1 and 2. When a seek or reset is in progress for any drive and the IDE bit in the controller is set, the controller will poll all drives for completion when it is otherwise idle. When polling is active, a pulse train which drives a count through the polled drives should be visible at B27F2:
A quick look with the 'scope shows no joy here. This clock is initiated by signal POLL; which doesn't seem to be being asserted. Checking the origin of that signal takes us to B26 and A26:
Hmmm, one of these gates (the inverter at A26) is one that had failed and that I had repaired sometime last year... Reseated the socketed replacement on A26, reloaded the diagnostic, but still no go. Well at least it wasn't my repair job! Went ahead and pulled A26 and B26 and bench tested the gates. The 8-input nand that outputs to B26J2 does look fishy. Pulled and socketed the piece, and put a replacement and some spares on order at Jameco where I can pick them up on my way in to work tomorrow. All for now!