CBM900 to SVGA monitor

Hello!

As a part of restoring the CBM900 machine to life, I will need to
provide adequate display for it. Since it has the hi-res card, a trivial
MDA monitor will not work.

Here's the info I gathered so far about the card:

* The output has TTL levels for sync, but ECL levels for video data.
This was presumably typical of high-resolution workstations of that era
(Sun, Apollo etc). Actually the whole card is built with TTL chips, and
TTL-to-ECL converter is placed just before the video signal goes to the
connector.

* Vertical refresh is 60 Hz (negative polarity), and horizontal refresh
is 53.7 kHz (positive polarity). This means 895 scanlines, which makes
the advertised resolution of 1024x800 very plausible.

* The above parameters and resolution mean that the video clock would be
somewhere around 64 MHz. A strange thing is that the card does not
contain such oscillator (actually no oscillator at all) and I have no
idea how that frequency would be derived from the system clock of 12 MHz
(some kind of PLL?).

* Surprisingly, there is no 8563 chip in the card. The whole card is
built around TTL chips: http://imgur.com/kIgNsHi

There are several 16V8 chips on sockets, and the big chip is a 82S105
PLA. There are no ASIC chip at all. Presumably this means that the card
is simply a dumb framebuffer.

* There is 128 kB of RAM on the card, which corresponds to the presumed
1024x800 mono resolution. The connector pinout in the documentation
describes an "ECL+ Intensity" pin, but this is clearly an error as there
is no corresponding "ECL- Intensity" pin, and additional intensity
attribute would not fit in the supplied RAM.

My idea is to build a simple converter to connect a multisync SVGA
monitor. The obstacle I am currently facing is how to convert the video
to VGA level. I would probably not bother in converting ECL back to TTL
for that purpose, instead I will route the TTL vide to an unused pin in
the connector.

However I need a way to properly convert TTL levels of 0V/5V to VGA
levels of 0V/1V. I am afraid that a simple resistor voltage reducer
would be too slow to work in the 64 MHz range. This is an area where I
have no expertise so I would appreciate some suggestions.

Regards,
Michau.




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