8" DSDD disk
cctalk at randy482.com
Mon Jun 6 21:16:09 CDT 2005
From: "Ethan Dicks" <ethan.dicks at gmail.com>
Sent: Monday, June 06, 2005 6:57 PM
> On 6/6/05, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
>> The original IBM CGA card (and many clones of it) used the 14.3... MHz
>> clock from the motherboard. This was divided by 4 on the CGA card to give
>> the NTSC colour subcarrier anr by 3 on the motherboard to give the
>> processor clock.
>> On the AT motherboard (both the 6 and 8 MHz versions) there's a separate
>> 14.3... MHx oscillator to provide this clock signal...
> Because it's part of the ISA definition, the GG2 Bus+ has a 14.3.. MHz
> oscillator just to provide that one signal... it's the second most
> expensive single part on the board (behind the 22V10 GAL). If I ever
> do another build, I may just make it an option, since I've never seen
> an I/O board that uses it either, certainly not the IDE, Ethernet, or
> parallel/serial boards that are the options supported under AmigaDOS.
It was common to just change the crystal to 24Mhz and ignore the 14.3.
I used a few CGA cards all had their own clocks. I believe the one I used
the most was an MCT 1/3 length card that included composite video on the
monochrome pin. It came with a 9 pin to RCA plug.
Another I used a lot was an MCT full length card that included 2 serial, 1
lpt, clock, and floppy. It had jumpers to allow PAL on the composite out.
I would use this and a hard disk controller for a full system with only two
It's been a while but on every system I over-clocked I never had an issue
with the CGA but I can't guarantee what cards I used. I used a good variety
of video cards but which ones on systems with changed crystals and which
ones used a separate oscillator (switchable) I can't swear to. I did use
CGA cards on systems without the 14.3Mhz line being valid. I started by
changing the crystals but all of the last hacked systems were switchable and
left the 14.3Mhz crystal in.
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