Malfunctioning VT240 - help please

Mattis Lind mattislind at
Thu Jun 11 10:31:54 CDT 2020

> 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.
The VT100 also makes use of a binary code for the very early errors like
ROM and RAM faults so assuming the same behaviour here is not that far
fetched I think.

> 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?

Yes. The 7D01 was older than I expected. I thought maybe a HP1630 or
possibly 1615 which is old...
I guess that the memory depth of the 7D01 is not that much.

Assuming that the CPU does a HALT when it stops it should stop reference
memory so if you let your logic analyzer just store all addresses until it
stops you might be able to find the last (whatever memory depth you have)
instructions. Use the memory strobe to clock in the address into the logic
analyzer. Then you can do hand disassembly of this part. Or load it into
Ersatz-11 and SimH and do the disassembly.

But maybe it is a good idea to find a slightly more modern LA? Maybe a HP
166x (There is one 1661 on Ebay at $70) which is quite portable and easy to
use. Or HP 167x which has much better memory depth.

> 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.
Yes. That could be an alternative. Maybe you can figure out how to
communicate over the serial port. Perhaps you can write something
simplistic that outputs something to the serial port?

> My fear is that one of the PALs has altered itself from tin-whisker
> migration (fuse regrowth) :(
That could probably happen. But I have seen more cases with failed memory
chips than PALs that have self-altered.


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