AT&T 5620 questions

[Doug Gwyn]

In article <19704 at ea.ecn.purdue.edu> wieland at ea.ecn.purdue.edu (Jeffrey J Wieland) writes:
>I have a couple of questions about the AT&T 5620 terminal:
>1.  I tried to upgrade the memory in mine by replacing the 64K ram
>    chips the 256K ram chips.
>    It seems that memory errors are ocurring.  I've tried replacing
>    the 256K chips -- it doesn't seem to make any difference.  The 
>    256K drams are just standard 150 nanosecond parts.
>    Do the dip switches on the processor board need to be changed?
>    What are they for, anyway?

Somebody ran off with my 5620 shop manual and Teletype upgrade
instruction sheets, so I can't be as precise as I would like to be in
my response.  However, as I recall you also need to replace a timing
chip (maybe a 74LS125?) in the corner of the logic board with a faster
version.  I don't know whether 150ns DRAM is sufficiently fast but
from your symptoms it sounds like it must be.

I don't think the DIP switches are related to RAM configuration, just ROM.
If you upgrade from 8;7;3 to 8;7;5 firmware, the new ROMs have twice as
much data and the DIP switches have to be changed for the terminal to work.

>2.  What is the parallel i/o port for?  Did anything actually ever
>    use it?

I've never heard of any use for it.