Malfunctioning VT240 - help please

[Mattis Lind]

> The T11 is not halted - it's looping endlessly in the first ROM. There is
> a brief burst of DRAM select activity on the scope just before it hangs in
> the loop.
>
 That burst of DRAM activity might indicate a DRAM problem. One thing I
have tried in the past is to put a known good DRAM on top of one DRAM is
the array. So to say in parallel. In the cases I have tried I managed to
make systems pass the memory check. Then test each and every one in the
DRAM array in the same way. Might be worth a try.

> All the glue logic and memory map adders/multiplexers seem to be grossly
> working with outputs that change state. I was hoping to find a bad or
> immovable line on one of them... Now this makes me even more suspicious
> that there is a bad address (or block of addresses) in RAM and that's where
> the test is hanging.
>
> My 7D01 (16 channels) is hopelessly outclassed here. I looked at that HP
> 1661 but it does not appear to come with the probes, which are so often
> discarded by surplus or scrappers. (A similar aggravation with our classic
> computers, of course).
>
Ahh. Missed that it lacked probes. That is a common problem unfortunately.


> Unfortunately I only have one 27256 in the drawer, so have to order some
> more before making memory test PROMs...and I also have to figure out a
> simple way of outputting the RAM failure address!
>


The 2681 https://www.nxp.com/docs/en/data-sheet/SCC2681.pdf shouldn't be to
difficult to configure and is located on the same bus as the keyboard so
you should be able to send something  on the UART to either the printer/aux
or the host.

>