>1) Ive been getting an older workstation monitor to
>work on my mac by tying the sync lines from the mac into green,
>since the monitor works on sync on green. For 832x624 I just tye
>them together, for 1024x768 I use a 1kOhm resistor, I dont
>understand why these things work, but they do.
>Do you know if Im likely to damage any of the equipment
>involved using this scheme, or do I need to be doing something
>more complicated?

Ours would work regardles of polarity and is more involved. Using just a
resistor is OK on some monitors particularily a few of the older Sony and
Hitachi models. I would drop that value a little and go with -/- polarity.
Also I would always use a resistor as otherwise you are seriously
overdriving the green video amplifier. It is expecting .7 volts video - 1.0
volt with sync on green. With no resistor you are directly inputing 5.0
volts TTL level signals plus your video signal. The Sony video amplifiers
are prone to failure even when you are not overdriving them. Also do NOT
connect the H (15) & V (12) sync directly. This may work at first but
should eventually cause a failure in your video circuitry. You never
directly connect the outputs of CMOS gates unless they are designed for it
(tristated) and these are not.

I think something like a 330 ohm resistor from 3 (composite sync) to 5
(green) should work but without putting it on a scope it would be difficult
to say. Different model monitors would place different loads. If you are
going to run this a long time check it with a scope to make sure you are
not way over the 1.0 volt composite signal. If you are going to use a
simple solution like this probably going with a voltage divider is a better
solution so you do not have to worry about the load. You would want to
divide down the 5 volt sync signal signal to .3 volts. In this type of
arrangement, you would also normally add a capacitor inline on the green
video signal to "bias up" the circuit but I believe this may cause some
degredation in your video quality.


>Interestingly you need -/- if you just want to tie them together, with
>or without a resistor. This is true even to a newer multisync nanao.

When you are tying them directly together this makes sense. A positive sync
signal on either H or V would pull both signal low and not allow either
sync signal to pass. Positive means that the sync signal is low (ground)
except for the short duration of the sync pulse - when the other signal
would be low. Thus you would essentially always be low and there would be
no pulses at all. Further it would ground your green video so even if you
got a picture it would look pretty bad. By the way the Nanao does not
require sync on green to operate.

+               ___
_______|       |________

-______         ________
                |___|

>So you think putting a low resistor in the circuit is better than
>a straight connection, even though I see no difference in video?

I think connecting the sync directly (or indirectly via a resistor) is a
very bad idea period. But if you are going to do it you should definately
use some resistance or eventually you may destroy the card or the monitor
or both.

>I understand that the video signal is 0 to 0.7 volts.
>Does the sync signal sit at 5volts and then drop to -something on sync?

Sync is a standard TTL level negative going signal. It is either +5 or 0
(ground).

>>are prone to failure even when you are not overdriving them. Also do NOT
>>connect the H (15) & V (12) sync directly. This may work at first but
>>should eventually cause a failure in your video circuitry. You never
>>directly connect the outputs of CMOS gates unless they are designed for it

>You mean failure on the monitor or on my HPV card?

In this case the video card. You may get away with this but if you fry it
you cannot just replace a $0.50 part. I know for a fact that it will
eventually fry the sync outputs on a number of Mac models. We never got
around to killing an HPV card but they use similar circuitry.

>What resistance would you guess between H and V if I want to tap
>them together? I guess this is also a general question about CMOS circuits.

None. You would need a logic gate (IC) to "and" the signals. There is no
simple way to do what you are trying to do with discrete components like
resistors. Use the composite sync on pin 3 with a resistor or buy an
adapter. Some PCs also output composite sync on H. It is usually software
selectable.