Malfunctioning VT240 - help please

[Doug Jackson]

I remember testing memory chips with a thumb.  The one that took yoir
fingerprint ofd was always faulty.

Hopefully it's not whisker regrowth.  That would be really frustrating.

On Fri, 12 Jun. 2020, 12:15 am Charles via cctalk, <cctalk at classiccmp.org>
wrote:

>
> On 6/11/20 2:29 AM, Mattis Lind wrote:
> >
> >
> > torsdag 11 juni 2020 skrev Charles via cctalk <cctalk at classiccmp.org
> > <mailto:cctalk at classiccmp.org>>:
> >
> >
> >     On 6/10/20 4:31 PM, Jon Elson wrote:
> >
> >         On 06/10/2020 12:48 PM, Charles via cctalk wrote:
> >
> >
> >             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
> >
> >
> >     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!
> >
> >
> >
> > 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
>
> =========
>
> Thanks for the tip. I didn't see in the manuals that the keyboard light
> pattern was actually a binary code, but that makes sense! I would have
> expected an error message on the screen, but as I previously noted, the
> video system itself does not seem to be working properly.
>
> Unfortunately my logic analyzer is an ancient Tek 7D01, the equivalent
> of stone tools rather than metal ;) It's not really suited for doing
> this kind of work, but it's what I have... I wonder if anyone has
> already disassembled the code?
>
> The 4116's are soldered to the board, too. Since the memory map is shown
> in the tech manual I could write a simple memory test and burn an EPROM.
>
> My fear is that one of the PALs has altered itself from tin-whisker
> migration (fuse regrowth) :(
>
>