10 Jun
2020
10 Jun
'20
8:08 p.m.
On 6/10/20 4:31 PM, Jon Elson wrote:
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