I'm restoring a pdp11/34a, and I've got a weird problem I'm hoping
someone out there can help me with.
The system seems to be working just fine, with one exception: bus reset
just doesn't work.
The unibus is configured correctly; I've verified that NPR is
jumpered/granted correctly through the bus. This is a BA11-L box with
the DD11-PK 9-slot backplane, configured like this:
Slot 1 - CPU
Slot 2 - CPU
Slot 3 - A/B M9312 bootstrap/terminator, C/D/E/F M7859 front panel
programmers interface controller
Slot 4 - 128 Kword MOS memory
Slot 5 - RL11 controller
Slot 6 - C/D/E/F M7856 DL11-W console RS232 interface
Slot 7 - C/D G7273 dual bus grant
Slot 8 - C/D G7273 dual bus grant
Slot 9 - A/B M9302 terminator, C/D/E/F M7856 DL11-W configured at 776500
for TU58 tape drive
The symptoms:
At poweron, the system starts the M9312 console emulator software; I
can use it to deposit a small program into memory with no problem.
Attempting to use the "S" command to actually start the program causes
the RUN light to come on; the program never executes, hitting CTRL/HALT
on the front panel causes BUS ERR, the RUN light stays on, and the only
way to regain control of the system is power off. For example:
@ L 1000
@ D 000777
@ L 1000
@ S
fails as described above.
If I use the front panel to install a program and run it, when I push
CTRL/START, the system halts at the start address. For example:
CTRL/HALT
CLR
1000
LAD
CLR
777
DEP
CLR
1000
LAD
CTRL/START
the system halts, run light out, at address 1000. If I then press
CTRL/CONT, the RUN and SR DISP lights come on, and everything works
perfectly. I can CTRL/HALT the system, continue, etc. With no problem.
Furthermore, if the program I enter contains a RESET instruction
(000005), the behavior of the system as it executes that instruction is
pretty much exactly like using the "S" command from the console emulator
- RUN stays on, CTRL/HALT causes BUS ERR, and the system is
non-responsive until I power the beast off.
For example:
CTRL/HALT
CLR
1000
LAD
CLR
240
DEP
DEP
DEP
CLR
5
DEP
CLR
1000
LAD
CTRL/START (the system halts at address 1000)
CTRL/HALT (single steps to location 1002 correctly)
CTRL/HALT (single steps to location 1004 correctly)
CTRL/HALT (single steps to location 1006 correctly)
CTRL/HALT (RUN light comes on, CTRL/HALT causes BUS ERR with RUN, system
is no longer responsive, have to poweroff)
My reading of the unibus tech manuals indicates that there is no ACK of
any kind for BUS INIT, so I am somewhat confused.
There _is_ a bit of noise on the +5V power supply; a voltmeter shows
5.02 volts stable, but there is a .5V spike at 50 kHz showing on the
scope; I can't see this causing problems _only_ with BUS RESET.
I have also verified that CTRL/INIT from the front panel seems to do
nothing at all. For example, if I halt the system and press a key on
the console emulator, examining 777560 shows 000200 (which is correct,
there is a character available at 777562). Pressing CTRL/INIT on the
front panel does _not_ clear 777560 - it still shows 000200.
All the 11/34a M9312 diagnostics run fine. This problem seems to be
limited to attempts at RESET. A disassembly of the M9312 console
emulator code shows that two things happen when you use the "S" command:
000005 (RESET)
followed by a
JMP (R5)
It's the RESET that drives the machine nuts.
Anyone have an idea?
--
<http://www.liveblockauctions.com>
Roe Peterson / Director of Research & Development
O. 306.523.4005 / C. 306.501.6802
*Help Desk: 1.877.694.6100 / 306.694.6100*
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