How to use an old ECL monochrome monitor (i.g. Moniterm Viking)
with a standard VGA adapter.

Copyright timo pelkonen 1999.

std disclaimer: use this at your own risk.

I will discuss Moniterm Viking as it is the display I have.
YMMV.

The picture will not be as crisp as it is with the original 
adapter.  Using an ECL comparator is a better solution but
it is also more expensive and I have not tried it.


Both VGA and Viking use TTL sync signals.  You can wire them straight
thru. If you have problems when trying to make the monitor sync
try changing the sync polarity.

VGA video signal is video level (0 to 0.7 V).

Viking video signal is ECL level -- i.e. -5 to 0 V.  It is also
complementary (video signal has separate plus and minus signals.
The ECL input acts as an comparator.  The result is on or off 
depending on which signal has higher potential.

Both plus-video and minus-video have their on ground wires.

As ECL input is a comparator we can fool it to accept video signal.

First we'll force a fixed DC level to one of the inputs.  I did it
with a diode (0) and a trimmer potentiometer.  Take the GND of 
the input, put the anode of the diode to the GND and the trimmer 
in series after it to the signal input.  The input will sink a 
small current and force the level below GND.  The trimmer is 
used to adjust the level.

Next task is to force the video signal (use Green for monochrome) 
to levels below GND.
I used a trimmer and then a diode (1) in series to GND.  This keep
the level in the 0 to 0.7 V range.

Another diode (2) is used in parallel with the first diode to drop 
the level below GND for the ECL
input. Now the level is from -0.7 to 0 V.  Again a trimmer is placed
between the diode and ECL input.  

A third diode (3) is placed from GND to
the diode side of the trimmer.  This diode will prevent the ECL input 
from pulling the VGA output below GND potential.

I also added a resistor from Green to GND to match the impedances.

The easiest way to obtain a cable is to take it from a defective VGA
monitor.

An ASCII figure:
                               1          3
Green from VGA  -----XXXXXX-+->|--- GND-->|-+
                            |               |
                            +-->|-----------+--XXXXXX--- ECL "+" in
                                2
                  0
ECL "-" GND ----->|---XXXXXX--- ECL "-" in

>| is a Si diode
XXXXX is a 100 ohm trimmer

Carefully adjusting the trimmers can produce shades of gray.
Don't count on it.

Keep the wires short.  Use ferrite rings on the cables.  Put the
gadget into a metal box. I suggest that you use fixed resistors
after finding the values that work.