Re: 6845 versus 6545

From: Richard Atkinson (
Date: 2001-03-23 02:25:34

On Wed, 21 Mar 2001, William Levak wrote:

> > I heard that the 6545 is not compatible with the 6845. But a long time ago 
> > I exchanged a 6845 with an faulty 6545 and the cbm worked again. Later 
> > swaps dibn't work. As I haven't this board anymore, I don't know for sure 
> > what 6845 I used but I thought an old original Motorola one.
> I have had the same experience.  I have even seen some CBM's come from
> Commodore with a 6845 in them.  The 6845 has had a long run.  There have
> been many variations through the years, only some are compatible enough to
> work in the CBM.  The version that was current when the 80 column CBM
> first came out seems to be compatible.  I have tried earlier and later
> versions and they are not compatible.  I don't know the exact range of
> versions that are compatible.

6845 versions are something the Amstrad CPC community are very familiar
with. Ask on comp.sys.amstrad.8bit or maybe see the FAQ.

Briefly, CPC users talk of 3 different varieties of 6845 CRC, all of which
work in their CPCs but with varying levels of compatibility with certain
programs. There seem to have been some changes with R/W# vs seperate RD#
and WR# lines which affects whether an Amstrad with a particular 6845 can
read its lightpen registers (read only registers). Also, I think there
were some small changes in CRC timing which became important when Amstrad
coders started playing with the registers for effects such as overscan.


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