Lack of 8-bit threads (was Re: Linux question)
ard at p850ug1.demon.co.uk
Mon Apr 23 17:13:48 CDT 2007
> > A European PC with a CGA card will output 60Hz NTSC video....
> Whilst this is perfectly true, IIRC the IBM CGA monitor takes the "RGBI"
> signal produced by the card, not the composite one.
Indeed it does. And on an European PC, that RGBI signal is the same spec
as in the US version, that is, with 60Hz vertical sync frequency, etc.
It is possible to re-program the 6845 CRTC chip on the CGA card to get
50Hz vertical. I have a PAL encoder box that plugs into the DE9 connector
on a CGA card and which came with a little bit of software to do just
that. But no IBM machine did that as standard in the 'European' version.
> I believe that the composite output was intended primarily for driving a
> modulator to allow the use of an ordinary TV with the system.
The composite signal is output on 2 connectors on the IBM CGA card.
Firstly on the RCA phono socket on the back, indented to link to US TVs,
etc. And secondly on a 4 pin BERG header on the card. This was used to
connect the monitor in the 5155 PortablePC (the monitor in that is an
amber-screen Zneith composite mono monitor), and also to connect an RF
modulator (There's a 12V power output on another pin of that connector.
> Almost none of the "clone" CGA cards I've come across over the years support
> the composite video output.
Odd. Just about all the ones I've seen did, although not necessarily on
an RCA phono conenctor on the bracket.
Some had that connector -- I have a clone CGA card which is pretty much a
copy of the IBM one apart from using different DRAM chips (I can't
rememebr if it's that they're 8 off 16K*1 5V-only ones, or a pair of
I saw a clone CGA card _years_ ago which was one big ASIC and a couple of
RAMs. It had a 2-pin header plug on the PCB that carried composite NTSC
video. I never _owned_ that card though.
But I do have something called a 'Colorgraph+' It's a CGA card which can
take 48K of RAM (6 off 16K*4 chips), and thus has a 640*200 in 8 colours
(separate R,G,B bitplanes) mode, along with a 320*200 in 16 colours mode,
etc. This only has a DE9 connector, but pin 7, normally unused on a CGA
card, carries the composite NTSC signal.
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