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From: bill@hpcvlx.cv.hp.com (Bill Frolik) Newsgroups: comp.sys.ibm.pc Subject: Re: How to read game port Date: 6 Apr 90 17:47:16 GMT

1) Is there anyone that can tell me the specifications for the game port
connector?

15-pin D-shell connector:

PIN	SIGNAL		PIN	SIGNAL
---	------		---	------
 1	+5V		 8	+5V		
 2	button 1	 9	+5V
 3	stick 1		10	button 3
 4	GND		11	stick 3
 5	GND		12	GND
 6	stick 2		13	stick 4
 7	button 2	14	button 4
			15	+5V

Game card has a female connector;
Joystick cable has male connector.
Joystick wiring:

	+5	________________
	                        |
	stick1	________	|
			| 100K  |
		    _/\/\/\_____|
	stick2	________	|
			| 100K  |
		    _/\/\/\_____|
	stick3	________	|
			| 100K  |
		    _/\/\/\_____|
	stick4	________ 	|
			| 100K  |
		    _/\/\/\_____|
		      ___
	button1	______o o_______ 
		      ___       |
	button2	______o o_______|
		      ___       |
	button3	______o o_______|
		      ___       |
	button4	______o o_______|
				|
	GND	________________|

2) As near as I can figure, I have to use the inpw() function (I'm using
C) to read the ports. Unfortunately, I don't know the port number to
give the function to read the port. If this isn't the right/best way,
please let me know.

You need to do byte I/O to and from port 201h.
To read the joysticks (or your slide pot positions), you
must first write a byte to port 201h.  It doesn't matter
what value you send, as long as you perform an I/O write.
This triggers the 558 timer on the game adapter.

Game port 201h byte:
 _______________________________________________________
|   7  |   6  |   5  |   4  |   3  |   2  |   1  |   0  |
| but4 | but3 | but2 | but1 | stk4 | stk3 | stk2 | stk1 |
|______|______|______|______|______|______|______|______|

The most machine-independent way to sample the game port
is to use a timer.  Note the time (e.g., read the countdown
register in Timer 0, you need pretty fine resolution and this
timer performs 65535 counts every 55 ms) just before you
trigger	the 558.  After triggering, sit in a loop reading
port 201h and examining bits 0-3.  For those bits that have
a joystick pot attached, you'll see them sit for a while at 0,
then become 1.  As each bit flips back to 1, note the time
again.  When all bit 0-3 have flipped back to 1, you're almost
done.  Compute elapsed time for each bit, and you end up with
a value that is proportional to pot position.

If you're lazy and don't care about machine independence,
you can just trigger the card, then sit in a loop and count
up from zero.  As each bit flips back to 1, save the count
for that bit.  I've seen a few games that probably use
this method.  Problem is that if you then run the same code
on a faster/slower machine, your calibration can get very
easily messed up.

Buttons can be read at any time just by reading port 201h
and looking at bits 4-7.  No triggering is required.
Button bits are normally 1; while a button is depressed,
its bit will flip to 0.

___

Bill Frolik Hewlett-Packard Co. hp-pcd!bill Corvallis, Oregon bill@hpcvlx.cv.hp.com (503)750-4082