Re: Mac to VGA Adapter Question

[dempson@actrix.gen.nz (David Empson)]

Bryan Parkoff <BParkoff@satx.rr.com> wrote:

> Mac to VGA Adapter Question
> 
>     Altex, Inc in Texas sells Mac to VGA Adapter.  I believe that Mac video
> port has the same as Apple IIgs video port.

Mostly, yes, but they are not identical.

The IIgs video port supports the IIgs RGB monitor, and any similar
analog RGB monitor which is able to support scan rates compatible with
NTSC (15.75 kHz) and has compatible synchronization signals.

The Mac video connector uses several pins to identify the monitor which
is connected.  Early Macs used three pins with combinations of diodes in
the plug to identify the monitor.  More recent models support the same
identification scheme as recent VGA monitors, which involves an
I-squared-C protocol communication between the computer and a small chip
in the monitor cable (or inside the monitor).

The actual video pins on the IIgs and Mac monitor connectors are the
same, but Mac monitors (and VGA monitors) have much higher horizontal
scan rates (at least 30 kHz), and they aren't able to support the low
scan rate of the IIgs.

> Do you think that Apple IIgs will be compatible to Mac to VGA Adapter so
> it will allow me to view SVGA monitor to Apple IIgs video port through Mac
> to VGA Adapter.

It won't work by itself.  You would also need a devise which doubles the
scan frequency, so that the monitor is able to lock onto the video
signal.

-- 
David Empson
dempson@actrix.gen.nz