Path: utzoo!attcan!uunet!lll-winken!decwrl!ucbvax!ADMIN.HumberC.ON.CA!GRAY From: GRAY@ADMIN.HumberC.ON.CA (Kelly Gray) Newsgroups: comp.sys.apple2 Subject: Re: Proposal: Apple II+ 60Hz Timing Signal Message-ID: <90Nov2.100834est.57385@ugw.utcs.utoronto.ca> Date: 2 Nov 90 14:00:29 GMT References: Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 51 Morgan Davis writes: > This is a note to fellow Apple II software developers. Recently, I've been > studying the possibility of building a small circuit that gives the Apple > II+ a 60Hz signal by which software can poll to synchronize timing loops. > This is more important now than ever since many II+'s are becoming equipped > with faster CPUs (Zip Chips, Transwarps, etc.). > [lots of stuff deleted] > > Unless there is strong objection, I'd like to propose this modification > as the standard by which 8-bit applications synchronize all timing > loops on the Apple II+. > > --Morgan Davis I think that this isn't a bad idea. There is another advantage to this modification. The signal being used for the 60 Hz source is the high bit of the vertical line counter. This bit goes high when the video circuitry starts to scan line 16 of the text screen, and falls at the end of line 23. (rows 136 to 128 of hires graphics) This means that a program can synchronize with the vertical blanking period by waiting for the signal to go from a 1 to a 0. If this modification is adopted as a standard for //+ timing loops, it will also supply a standard equivalent to the VBL signal found in the //e, //c and //gs As has been pointed out though, there are a couple of minor drawbacks to the modification. It does tie up the cassette input address, making it unusable for cassette input. Since this is rarely used, it is not much of a problem. Unfortunately for cassette users, there are only eight addresses on a stock //+ that can be used to read a one bit input. Four of these addresses are used by the paddle inputs on the game port, and three more are used by the button inputs. The last one, and the one least likely to interfere with anything else, is the cassette input. The other drawback is that the timing is different in Eurapples. Apple //+ computers modified for use with PAL monitors will produce a 50Hz signal on pin 11 of D11. There is no easy way around this, so programmers will have to either live with the fact that the timing will be about 15% slow on some Apples, or else incorporate some sort of installation dependant code to allow the user to adapt the program. _________________________ ________________________________________ / \ / \ | Kelly Gray | The opinions expressed in the preceding | | | message are not guaranteed to represent | | GRAY@ADMIN.HumberC.ON.CA | any form of rational thought whatsoever | \_________________________/ \_________________________________________/