Re: VBL on an Apple //c

[apple2pd@ground.ecn.uiowa.edu (ground.ecn AppleII Librarian)]

In article <PBlokland-2012991120340001@lon-c45-013-vty108.as.wcom.net>,
Pim Blokland <PBlokland@compuserve.com> wrote:
>In one of my BASIC programs, I use the VBL signal (PEEK(49177) for timing
>purposes. This works on //es and IIgses, but lately I found out that it
>does not work on a //c.
>Apparently, on a //c you cannot see the VBL signal directly, but you have
>to let the computer do it for you.
>

I haven't seen any replies to this message yet, and I am a bit surprised.
Maybe I missed the responses.  Sorry, I don't have any code to do what you
ask for.  Also, I never had a //c until a couple years ago and have never
tried to master any of its pecularities.  However, I have buried in my
mindset the notion that the VBL signal is present on a //c, but with
reversed logic.  That is, its logic state is opposite what we expect on a
IIe or IIGS.  If we wait for a 0 to 1 transition in the memory bit on a
IIe, then on a //c you need to look for a 1 to 0 transition.  Please try
to reverse your code logic and try it on a //c.  I haven't looked at what
memory location is the VBL signal and if it is the same location on the
//c.  Please let us know if my aging memory completely failed on this
pecularity of the //c.  I don't think I have any //c specific manual to
check either.

Good luck!

  --Steve

-- 

 --Steve  (apple2pd@ground.ecn.uiowa.edu)