6 Jan
2005
6 Jan
'05
12:19 a.m.
Bob Shannon wrote:
After 3 years of part-time debugging, I've gotten
my Imlac PDS-1 booting
original software!
Wow! Superb job! Congrats! :-)
<snip>
The Display Processor was another story. Any attempts
to draw short
vectors with a -y displacement
produced +y axis vectors. This was chased down to an original Imlac
option board that expands the
display processors address space from 4K to 8K. With that option pulled
(as the machine could only address 4K anyway) I finally got the machine
to reliably display the HELLO test pattern.
Did you manage to debug that option board also?
But the basic stability problem soon returned. One recent evening I got
a little lucky and found
that the linear regulator for the +5 volt supply was dropping out of
regulation due to a high frequency
oscillation on one of the pins of the regulator IC. This was repaired,
and I once again had a solid
machine to work on.
How did you repair this? Did you replace the 7805 (I presume) or did you
put a bypass cap at the input?
<snip>
Imlac's hardware engineers also decided that the
transistors driving the
console lamps did not need
base current limiting resistors, so they pull a TTL output to ground
when lit. This nailed the new
address bit low as far as the rest of the machine was concerned. A set
of TTL buffers restored
the new address bit with an absolute minimum of modifications.
So the TTL buffers were borked?
Dallas NV SRAMS replaced the srams, so the 'appearance' of core memory
returned. At this point
I thought I had a solid 8K machine, except for the display processor
anyway.
A plug-in replacement for the stock Imlac serial I/O board (110 baud!)
was designed and slipped
into the correct slot, with the ability to load Imlac software
Wow. You design this board yourself or did you copy from the original
schematics?
All in all, very impressive debugging/detective work. Congrats again!
/wai-sun