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From: mdavis@pro-sol.cts.com (Morgan Davis)
Newsgroups: comp.sys.apple2
Subject: Proposal: Apple II+ 60Hz Timing Signal
Message-ID: <5391@crash.cts.com>
Date: 1 Nov 90 02:36:05 GMT
Sender: root@crash.cts.com
Lines: 57

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.).

After learning about a 60Hz signal source on the motherboard, I scrapped the
idea of building a circuit using a 555 timer IC.  The source is located on pin
#11 of the 74LS161 at location D-11 (the chip just right of the resistor pack).

I am proposing to the developer community that since we have access to this
timing source that we standardize on how it is used.  My proposal is that this
signal can be safely tied to pin #4 of the 74LS251 at location H-14 (the chip
immediately south of the 16-pin game I/O socket).  This maps to location $C060
in the computer, or the cassette tape input register.

Wiring Diagram:
                          _______
                        [|   7   |]
                        [|   4   |]
                        [|   L   |]
                  D-11  [|   S   |]
                        [|   2   |]----------+ 4
                        [|   5   |]          |
                        [|   1   |]          |
                        [|___n___|]          |
                                             |
   +-------------------//--------------------+
   |      _______
   |    [|   7   |]
   |    [|   4   |]
11 +----[|   L   |]
        [|   S   |]  H-14
        [|   1   |]
        [|   6   |]
        [|   1   |]
        [|___n___|]


This modification is desirable in that it requires only a single wire.  It
is inexpensive and easy to install.  It only involves connecting pins of ICs
instead of hacking into the game I/O socket inputs which would interfere
with a joystick.  Since it uses the cassette tape location (one that is
practically useless today) it doesn't sacrafice a joystick button input.
(Coincidentally, $C060 is known as "butn3" on the IIGS).  This wiring is
compatible with the popular Shift-Key modification and is unobtrusive to
most applications that do not access the cassette tape port.

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

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