8" DSDD disk

[Randy McLaughlin]

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.
>
> -ethan

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 
cards.

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.


Randy
www.s100-manuals.com