From: jbye@acorn.co.uk (James)
Date: 14 Jul 92 07:33:50 GMT


SWI Interface for Joystick Devices
----------------------------------

The Joystick module provides a SWI interface for reading the state of a
joystick.  When the module  initialises, it tests for the existence of
built-in joystick hardware and if it does not find any then it will not 
initialise.  Third parties can replace this module to provide support for
different hardware.  It is  recommended that any such modules have version
numbers greater than 2.00 so that Acorn can upgrade its  own module without
preventing its replacement.       


SWI  Joystick_Read (SWI &43F40)
-------------------------------

Returns the state of a joystick.

On entry:               R0 = joystick number

On exit:                R0 = joystick state

Interrupts:             Interrupt status is not altered

Processor mode:         Processor is in SVC mode

Re-entrancy:            Not defined

Use:                    This SWI is used to obtain the state of the requested
                        joystick.  The state is returned in the following
                        format:

                        Byte 0:         signed Y value (range -127 to 127)
                                        -64, 0 or 64 for single switch joystick
                                        (corresponds to Down, Rest, Up).

                        Byte 1:         signed X value (range -127 to 127)
                                        -64, 0 or 64 for single switch joystick
                                        (corresponds to Left, Rest, Right).

                        Byte 2:         Switches (e.g. fire buttons) starting in
                                        bit 0, unimplemented switches return 0.

                        Byte 3:  Reserved.

Note that this format allows both digital and analogue devices to be
supported.  Applications which are only  interested in state (up, down,
left, right) should not simply test the bytes for positive, negative or
zero.  It is  recommended that the 'at rest' state should span a middle
range, say from -32 to 32 since analogue joysticks  cannot be relied upon to
produce 0 when at rest.



From: s001532@kowande.bu.oz.au (Jeremy Lee)
Subject: $10 DIY Joystick interface
Date: 9 Sep 91 03:40:38 GMT

I had this idea a while ago. The Arc lacks a joystick port and nobody
wants to fork out #100 for a joystick podule, so how about this alternative.
It is based on the belief that you don't print while you play games.


             Joystick 1           Joystick 2

         com F1 2 3 U D L R   com F1 2 3 U D L R
          o   o o o o o o o    o   o o o o o o o
          |   | | | | | | +----+---+-+-+-+-+-+-*
          |   | | | | | +------+---+-+-+-+-+-* |
          |   | | | | +--------+---+-+-+-+-* | |
          |   | | | +----------+---+-+-+-* | | |
          |   | | +------------+---+-+-* | | | |
          |   | +--------------+---+-* | | | | |
          |   +----------------+---* | | | | | |
          |      NOT           |   | | | | | | |
          | +----o<|---*-----*-+-+ | | | | | | |
          | |                | | | | | | | | | |
          ---                --- | | | | | | | |
       AND| |             AND| | | | | | | | | |
          \ /                \ / | | | | | | | |
           T                  T  | | | | | | | |
           +------------------*  | | | | | | | |
                              |  | | | | | | | |    * Join
                              |  | | | | | | | |
                              |  | | | | | | | |    + Cross
                          +---+  | | | | | | | |
                          |      | | | | | | | |
                          o      o o o o o o o o
                        *rdy    D7 6 5 4 3 2 1 0

                             Printer Port

Or, for the REAL pennypincher :

                      Joystick

                 com F1 2 3 U D L R
                  o   o o o o o o o
                  |   | | | | | | |
                  |   | | | | | | |
                  o   o o o o o o o
                *rdy D6 5 4 3 2 1 0

                    Printer Port

Both are compatible (except that it you try to use the single joystick
version with software that wants two, then the computer will think that both
joysticks are doing the same thang!


Software works like this. D7 chooses which joystick through the gates.
The program then strobes D0-D6 and looks at the *rdy line. If it is "on" then
the signal got through and the switch obviously is closed. (It of course, has
to strobe each switch seperatly, and for each joystick.)

The design here allows for a standard eight-direction joystick and up to three
fire buttons. A little clever sofware and you're away (anyone want to write it,
or build it for that matter?)

The only restriction is that it has to use bog-standard switches as the
computer sends the signals back to other way. Rapid-fire joysticks
probably won't work (in rapid fire mode) nor will any devices that directly
drive the pins. (trackballs & extra mice are right out. So are other DIY
sensor projects that you see in a lot of Mags.)

But, It's cheap and easy to just zap them space invaders.


***********************************************************************
*   /|  Jeremy Lee s001532@sand.sics.bu.oz.au      I'm Baaaack!...    *
*  /_|             "Then again, I might just be Insane"               *
* /  | "I'm an academic kleptomaniac. I just keep picking things up!" *
***********************************************************************