Imlac PDS-1 success!

From: Wai-Sun Chia <waisun.chia_at_hp.com>
Date: Thu Jan 6 08:19:12 2005

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
Received on Thu Jan 06 2005 - 08:19:12 GMT