6847 frame sync output question

discussion of games, software, hardware & emulators relating to the Acorn Atom
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decle
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6847 frame sync output question

Postby decle » Thu Apr 27, 2017 3:36 pm

Hi everyone,

Is disconnecting the 6847 frame sync line (pin 37) from pin 10 of the 8255 PIA a standard modification, perhaps for when using a colour card? I ask because both the Atoms I have had access to seem to be missing this link. The 6847 is linked to pin 5 of the PL4 header but not to the 8255 which leads to a failure to boot when reverting to a bare bones configuration.

Thanks

decle

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roland
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Re: 6847 frame sync output question

Postby roland » Thu Apr 27, 2017 6:22 pm

That is a very unusual mod. If PC7 (pin 10 of the 8255) is not fed with a block wave signal the Atom will probably hang. The FS is used for waiting for the video flyback of the CRT. At that moment the cpu can write to the video memory without causing "snow" on the screen. This slows down displaying text or drawing lines.

Some people made a mod by replacing this input from FS (60Hz) to PC4 (pin 13 of the 8255) which is a 2400 square wave. That makes printing and drawing a lot faster but will cause a distortion (snow) on the screen.
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decle
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Re: 6847 frame sync output question

Postby decle » Thu Apr 27, 2017 8:38 pm

Interesting. As you say both machines were stalled in the MOS flyback wait loop at $FE6B and did not boot. I'm not certain the change is a mod. I could not see a track break, although in this instance the track runs under a solid bottomed DIP socket. Hence my question. However, it was not a socket failure and a patch wire between PL4 and the 8255 fixed things. It just seemed a little weird for both machines to have an identical fault to me.


Cheers

decle


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