IIgs RGB to VGA experiment


Following up to an older post re what monitors can be used with the
IIgs, I recently ordered an RGB to VGA converter from this place in
Australia:

http://www.converters.tv/products/converters/rgb_converters/41.html

This item performs the conversion from 15.75khz / 60hz to 31.5khz / 60
hz, as well as the RGB to RGBHV.

It has arrived and I'll report back here how it works. I'm awaiting a
DB-15 connector and 5v power supply from Jameco. Testing will be done
with an early IBM VGA 31khz CRT monitor 6112.

~ J

=============
older post:

Ok, did some research on this IIgs / monitor synching question. No

offense to anyone by repeating the obvious here, just want to get the
specs clearly out.


The IIgs is apparently putting out a 15.75khz RGB w/composite
negative sync signal (RGBS?). VGA, by contrast is at least 31khz
RGBHV. So to get from one to the other, you not only have to double
the IIgs sync rate, but you have to generate the H+V sync signals from
the composite sync. So far, I've found one device that will go from
the the IIgs type RGBS to the VGA RGBHV, but it doesn't sync double.

I did confirm that, by the specs at least, the only NEC Multisyncs
that can handle the IIgs format directly are the Multisync II
(JC-1402HMA) and the Multisync 3DS (JC-1404HMA-2). A few have noted
that the 3D (JC-1404HMA) works, but the specs do not show that.

I also found that the IIgs video out connector is in fact the same as

the early Macintosh video output connector, pin signal wise.
However, the Macs have more signals including the H + V signals needed
for VGA.. which is why it's so easy to adapt a VGA screen to the Mac
using one of those little adaptors:


Mac:
1 RED ground
2 RED video
3 Composite sync
4 Monitor ID bit 0
5 Green video
6 Green ground
7 Monitor ID bit 1
8 not used
9 Blue video
10 Monitor ID bit 2
11 Vert sync / comp sync ground
12 Vertical sync
13 Blue ground
14 Horiz sync ground
15 Horiz sync
(shell) Frame ground


IIgs: same as above except pins 4, 7, 8, 10, 11, 12, 14, 15 are either
unused or have different uses.


So, it's perfectly feasible to connect better and more modern VGA
CRTs to the IIgs.. just have to find a sync doubling RGB->VGA (or DVI)
convertor. Probably not cheap.


~ J

.

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