CRTC

[pepax]

Hi!
I've just got hold of  a cheap HD68A45SP. Is it the same CRTC type as the HD6845S?
Which type is the most common and are certain CRTC types likely to be found in certain CPC models?
Which type is supported by most demos? Does the type matter in games at all?
Is there any switchable dual CRTC hardware mod?

I'm sorry for so many questions... 

BTW: I've also found a source of cheap 765/8272 FDC controllers and 4164 RAM.  Is it worth buying a few "just in case"? What is the most perishable IC in the CPCs? 

[Bryce]

Hi,
     A dual CRTC mod should be pretty simple. all you should have to do is piggy-back the two chips on top of each other and disconnect either the 5V pin (20) from both chips and install a switch to select which one gets the juice, or disconnect the Chip Select pin (23) with a similar selection switch. Haven't looked at the chip in great detail, so their may be other small modifications needed, but nothing major.

Are the differences between the S and SP version so great as to warrant the mod? I doubt it.

Bryce.

[pepax]

Hi!

Thanks for the advice.

What I had in mind was switching between CRTC type 0 and 1 of course.  Not sure what type I have in my CPCs now but I'll find out soon. Is there any short Basic program for testing the CRTC type?

[pepax]

So I got an answer to some of my questions here http://factor6.ic.cz/cpc/crtc.htm. (Thanks F6!)
Is the type 2 really virtually useless?

[Longshot]

According to the splitting technic you want to use...
With R4=R9=0, you can change the offset for each line on other crtc.
A crtc type 2 cannot split at each line with this technic.