Imlac PDS-1 success!

From: Wai-Sun Chia <>
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! :-)


> 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?

> 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

All in all, very impressive debugging/detective work. Congrats again!

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

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