11 Jun
2020
11 Jun
'20
12:29 a.m.
torsdag 11 juni 2020 skrev Charles via cctalk <cctalk at classiccmp.org>:
On 6/10/20 4:31 PM, Jon Elson wrote:
Now that you know the T11 is good I think it a good idea to attach a logic
analyzer on the bus.
I would then disassemble the ROM code and match that with the logic
analyzer execution trace. Then it should be possible to find out what is
going on. If one can rely on the fault code on the keyboard it is able to
pass tests 0 to 4 successfully. Of course I have no idea what these test
really do but assuming they do some more than advanced things I doubt that
they would work if there are severe bus contention.
If that would be the case I think the system would fail quite soon rather
than on test 5. A guess is that this is a memory problem.
Good luck!
/Mattis
On 06/10/2020 12:48 PM, Charles via cctalk
wrote:
After much Googling, I discovered/remembered that the RQDX3 M7555 floppy
controller card in my PDP-11/23+ system has a T11 CPU on board!
So I pulled the card and popped the T11 into the VT240. Guess what - the
terminal still doesn't work!! Craptastic. At least it's not the most
expensive and rarest part on the board... but now I'm really stumped. This
isn't my first rodeo - in fact back in the 80's I used to design
microprocessor systems for a living, and have continued to keep my hand in
repairing my video arcade games and a PDP-8 system, among other projects.
Meanwhile... the T11 DAL lines are only connected to a few parts that can
drive onto that local bus. Time to have a look at the glue logic for the
DRAM selects. Although the ROM chip selects seem to work, maybe the DRAM or
something else actually IS conflicting despite the mixed signals (pun
intended) ;)
Time to break out the logic analyzer, and start burning pairs of 27256
EPROMs with test programs. Maybe initially just fill them with NOP's
(000240 octal) with a jump to zero at the end!
That leaves the unlikely possibility that one of the octal TTL devices,
or ROMs. has developed a weird internal pathway that only interferes with
DAL3 & 1 on some bit patterns, but not all the time. Seems like a zebra
rather than a horse. The only part that drives multiple low-order DAL lines
at once besides the E19-22 ROMs is the E55 LS245.
Quite possible that this could happen when a specific device is driving
the bus --
or that NOBODY is driving the bus in that state. When it is
stuck at the ~1V level, try a resistor of about 1 K to ground on one of
those lines. If it moves several hundred mV lower, it is a TTL open
circuit. If it doesn't change at all, it is a bus contention (TWO drivers
driving at once).
Jon