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

To: Alex Freed
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.

I *think* I understood that. The 1084 monitor is pulling the sync levels a little too high, so the resistor is used to pull them back to an acceptable level. Is that correct?

Sorry, I'm only a pretend hardware guy - I leave the advanced stuff to you guys (and if this isn't advanced - I rest my case!). :-)

thanks,
Mike
--- Synchronet 3.14a-Win32 NewsLink 1.85
A2Central.com - Your total source for Apple II computing.
.

Relevant Pages

  • Re: Problem Using a Commodore 1084s Monitor on a IIgs
    ... For some reason the sync input Amiga monitor has a strong pull-up. ... If nothing is connected to the GS's output you will measure a pulse going say from 0 to 3.5 volts. ... That may be too high for the input circuit and not seen as a LOW level. ...
    (comp.sys.apple2)
  • Re: Problem Using a Commodore 1084s Monitor on a IIgs
    ... For some reason the sync input Amiga monitor has a strong pull-up. ... That may be too high for the input circuit and not seen as a LOW level. ... minor customization from the OEMs design. ...
    (comp.sys.apple2)
  • Re: Problem Using a Commodore 1084s Monitor on a IIgs
    ... For some reason the sync input Amiga monitor has a strong pull-up. ... If nothing is connected to the GS's output you will measure a pulse going say from 0 to 3.5 volts. ...
    (comp.sys.apple2)
  • Re: Dell P922 monitor
    ... Yes, it's a Sony monitor. ... Most of the problems I have seen are for Excessively bright picture. ... R459 is the best to Mod and Have found that 5.6M ohms to 7.5M ohms to be the ... The best solution is to change a resistor - R459 on the ...
    (sci.electronics.repair)
  • Re: What is a Hex buffer?
    ... which means it outputs a low level. ... I'm still wrestling a bit with the semantics of a "pull-up" resistor. ... likely a reason that these open-collector drivers are ... that we live our lives in chains ...
    (comp.arch.embedded)