Imlac assembler almost ready...

From: Bob Shannon <bshannon_at_tiac.net>
Date: Sat Sep 14 12:47:00 2002

Ben Franchuk wrote:

<snip>

> It is not that they did not know better, that is all they had to work
> with.


Clearly you haven't looked at the Imlac schematics! Engineers designed
lots of hardware
of the same era without resorting to the kinds of tricks used in the
Imlac.

Did you know that each Imlac had to have RC networks that control its
clocks and timing signals
tuned by hand, for each unit? The components used were garden variety
ceramic caps with very
loose tolerances, not poly or other higher quality components. The issue
here is the quality of hardware
engineering. HP machines of the same period are vastly better
engineered, as are many other machines
from the late '60's and early 70's.

To suggest that nothing better was available to the engineering team at
Imlac is laughable at best.

>
> Mind you cost cutting often did not help any computer product.The real
> problem
> is the Imlac is VECTOR display. Finding a new CRT would be a problem
> with the
> original design. A raster scan display design could be used but then you
> need to buffer the display correctly to have unrefreshed data fade off
> the screen.


The vector display has nothing to do with the design quality whatsoever!

What makes you say something like this Ben? For what the Imlac did,
when it did it, VECTOR was
FAR SUPERIOR to raster displays.

Please note, the Imlac had a 1024 by 1024 addressable display, prior to
1970. This greatly exceeds what
was possible with raster graphics at the time, and the Imlac was
designed for calligraphic applications where
its short vectors made for a mugh higher quality display than a
pixellated raster display of the same resolution
would have. There is also the fact that manipulating raster display is
far more computationally intensive than
manipulating a vector display list. The Imlac CPU would not be well
suited for raster graphics at all, but its more
than sufficient for its intended use.

The problems with the Imlac are issues of engineering quality, like the
total lack of decoupling capacitors, poor
grounding, and poor logic design. This is also reflected in the
manufacturing of early units in the failue to wash the
etchant off the boards (many Imlac boards now have fuzzy green etches,
or no remaining etches at all) and poor metal preperation prior to
painting, and the fact that the design was very quickly repackaged as
the PDS-1D's.

The CRT used in the Imlac was common enough in its day, and that same
tube was also used in much higher quality products as well.

The last comment about the display refesh is hard to understand. The
Imlac does not use a storage tube, and it
must keep the display refreshed in the same way as any raster graphics
display does. I'm not at all sure I understand your point here, can you
expand on this point?

If your suggesting re-implementing the Imlac with a raster display, this
is totally impractical. My ReImlac project will use a real vector
display, as that is the only way to duplicate the capabilities of the
original machine. Remember, the Imlac does not have jagged vectors even
when drawing at any angle. To try to do this on a raster display would
reqire a resolution far far greater than 1024 by 1024. The anti-alising
along would take more logic than the full original machine does.

On the other hand, a Wells Gardner vector monitor is more than able to
display the vector video from an Imlac exactly the way a real Imlac
does. So will most small oscilloscopes, or even a modified TV monitor.
 Small vector display monitors are fairly common on eBay at very
affordable prices. So whats the problem with a vector display?

To be true to the original, I'm sticking with a true vector display.
 After all, I'm quite addicted to vector (and point plot) displays, and
this was my main attraction to the Imlac. If this were replaced by a
raster display, you might as well run a software emulator and not bother
re-implementing the Imlac in hardware at all.

The vector display of the Imlac is a thing of beauty and is a huge part
of what makes an Imlac so unique. To call this "...the real problem..."
is heracy!
Received on Sat Sep 14 2002 - 12:47:00 BST