1 Sep
2014
1 Sep
'14
8:03 p.m.
-----Original Message-----
On 01/09/2014 19:45, Charles wrote:
I'm having really bizarre hardware problems with
my 8/A (M8315/6/7, 32K,
programmer?s panel, limited function panel, two RL02?s, OS/8). Most of the
time it works perfectly, including last night, but today it began refusing
to boot OS/8 again.
If I toggle in a short program (say, to read the keyboard and echo to the
printer) it works. So far so good.
BUT if I hit the boot key (which doesn't boot correctly, although it does
access both RL02's briefly), then halt the system, then run the test
program, I can see while single-stepping that after two instructions it
jumps to location 0 instead and thereafter into space!!
Even more oddly, halting the system again, then running the test program
again, works normally without the unwanted jump to location 0... every
time. Until I hit BOOT, halt, and try and run it, then it jumps away
again...
The only thing that occurs to me right away is that power-fail & restart
has an option to jump to address zero. But I've got that option turned
off. Time to get out the logic analyer and scope... unless this weirdness
is something you've heard of before? Any hints appreciated!
thanks
Charles
I have a PDP-8/A which I've been slowly repairing for some time now. I
got to the stage where I could run simple programs from the console or
from virtual paper tape but trying to boot from floppy
or running OS/8
would result in symptoms like you describe. The last time I powered
it
on however, the console was totally unresponsive. After doing various
checks I found that there was excessive ripple from the power supply and
the -5V in particular was a mess. I haven't got round to investigating
this problem yet. I would check the power supply first with the scope
and go from there. It's probably best to check it under load too, like
when you press the boot key.
Matt
-------------------------------------
Thanks, but that's the first thing I always check... PS voltages are fine.
I learned some more today... interrupts make a JMS 0 and somewhere in the
boot process, apparently OS/8 is enabling interrupts. Both the RL8A
controller card and the SLU (console TTY port) can interrupt (and in fact
the SLU was creating the interrupt) . Explicitly disabling interrupts with
an IOF before running the echo program eliminates the jump to 0.
Does anyone know if OS/8 uses the interrupt facility of the RL8A controller?
Not sure if this is a problem or not with the interrupts also coming from
the console and holding the int request asserted...
However.
More possible clues: at one point I was applying finger-tip pressure to the
RL8A card, hit the boot switch and OS/8 came up! I started listing a
directory, which worked fine until I let go the pressure on the board - the
system crashed immediately. Can't get it to boot again except it will print
"V" on the tty instead of the "." prompt and it won't do anything
else
(endless loop in zero page but ultimately waiting for drive at a
skip-on-done instruction).
Ran some diagnostics. DJKKBA (CPU exerciser) ran 8 passes without a failure.
DHKMAD (memory checkerboard) also ran until I got tired of waiting. AJRLHA
(RL seek/fctn) works on both drives. But - AJRLIA (read/write) on a scratch
pack fails immediately. Then I unloaded the drives and tried AJRLAC (RL8A
diskless diagnostic) a.k.a. controller card test - MANY failures all
involving the middle 4 bits of the 12 bit data starting with DAR - silo. If
this were an interrupt problem it'd be all bits, not the middle nibble!
So I believe that a 4-bit TTL device (buffer, RAM, ?) on the card has
failed. Hoping for a bad solder joint which would explain how flexing the
card temporarily fixed but then made it worse. More likely a defective IC
(pin or bond wire inside the plastic DIP). E13, an 8234, is the first
suspect the next time I feel like wrestling with this.
Anyone got an RL8A (M8433) card I can buy or borrow?
thanks
Charles