Re: Problem Using a Commodore 1084s Monitor on a IIgs

["Michael J. Mahon" <mjmahon@aol.com>]

Alex Freed wrote:
> Polymorph wrote:
>
> > I'm just surprised that I have had to drop the sync level to such a 
> > low value though when others have not. I mean its gone from ~3.87V to 
> > ~0.56V, and it seems that it is still too high! This seems like a 
> > pretty big difference. I'm still amazed that later revisions of the 
> > 1084 can handle the IIgs' comp sync signal unmodified. It would be 
> > interesting to know what they changed to allow for this to be possible 
> > (as I'm sure they didn't have the IIgs in mind when making the 1084 
> > monitors ;-).
>
> Again, it was not as much a matter of the LEVEL as a pull-up.
> Doesn't matter how high the positive peak is as long as the LOW level
> is low enough. For some reason the sync input (at least on my) Amiga 
> monitor has a strong pull-up. And the output of the GS seems to be 
> designed to SOURCE rather than SINK a signal. So the resistor is needed 
> not to attenuate the pulse, but to pull it's bottom low enough - below 
> the threshold of the circuit.
> If nothing is connected to the GS's output you will measure (with a 
> scope, not a voltmeter!) a pulse going say from 0 to 3.5 volts.
> The open input of the monitor will have 5 volts.
>
> Take a resistor, say 75 ohms and connect between the sync INPUT and the 
> ground. You can measure, and this time you CAN use a voltmeter as the 
> level is constant, some voltage. Let's say it will be 1 volt. That may 
> be too high for the input circuit and not seen as a LOW level.
>
> If you connect the GS to the monitor directly with no extra resistors, 
> you will see a pulse that may be exactly right P-P, but not going low 
> enough.

Using a strong pull-up on a sync input is quite unusual--more common
for a "dedicated" monitor than a general-purpose monitor.

Since Commodore was nothing if not concerned with cost-reduction,
they may have initially adopted general-purpose monitors with only
minor customization from the OEMs design.

It was apparently common for dedicated monitors to use TTL levels for
H and V drive signals, or even for composite sync.

But I'd expect that a dedicated design would have to be produced in
large quantities before the non-recurring costs of customization were
recouped.  The confidence that this will be a win usually comes later
in a product cycle.  ;-)

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
tool--and it's seriously underused."