Re: Game port

[Rubywand <rubywand@swbell.net>]

Frank Carney writes ...

>>
Questions:
[1] How do I tell if a button has been pressed?
[2] Is the port grounded by a switch when it is pressed?
[3] Can a TTL signal be accurately read from this port?
Thanks in advance.

P.S. Please do not respond to ask what I am doing.  It is a surprise and
will be announced later.  Thankyou.
<<

and

> 
> Made a mistake that may have misslead people.  I am not talking about
> AN0-AN3.  I am talking about PB0-PB2.  Sorry for the mistake.
 ....


>[1] How do I tell if a button has been pressed?

     For buttons 0-2 (sometimes labeled 1-3), you can look at
$C061-$C063 respectively-- e.g. AD 61 C0 in machine code. If bit 7 is a
"1", the corresponding button is pressed*.

     In BASIC, you can do something like* ...

200 If PEEK(49249) > 127 then PRINT "BUTTON 0 IS PRESSED!"
210 If PEEK(49250) > 127 then PRINT "BUTTON 1 IS PRESSED!"
220 If PEEK(49251) > 127 then PRINT "BUTTON 2 IS PRESSED!"

*Note: These examples assume that a joystick, etc. is connected. With no
connection to a switch input, the default condition is a "1" at bit 7.



[2] Is the port grounded by a switch when it is pressed?

     No. It is connected to +5V.

     On the Apple II stick (see below), each button switch goes to +5V.
The other end goes to GND through a resistor (one resistor for each
button).  A button's Output is from the junction of the switch and its
resistor. When the button switch is not closed, its Output is near 0V
(=logic 0).  Pressing a button sends +5V to the output (= logic 1).

+5V
|
|
X Button Switch
|
|_____Button output to Apple (Press => "1")
|
Z
Z 680 Ohm resistor
Z
|
GND


     Again, the resistor for each button circuit (680 Ohms or so) is
inside the joystick.

     Inside the Apple II you have just a wire going from the Game I/O
socket to a TTL or LSTTL IC's input (unless there is some "Shift Key
mod", etc. wired up). A TTL IC normally sees an open input as a "1".

  

[3] Can a TTL signal be accurately read from this port?

     As long as some other circuit is not connected to a button input, a
TTL signal should be readable at a button input.




Rubywand