Often I get the question if SRM supports external call buttons. Well ofcourse it does! :-)

SRM supports call buttons, external and internal. You can use the spacebar as a call button, but you can also connect external call buttons.
 
The external call buttons can be any "short circuit" button (like a doorbell button or a button of an old joystick). Connect this button to a GROUND pin and one of the DATA IN pins (10, 11, 12, 13 or 15) of the parallel port using a resistor of at least 100 ohm in line to prevent a short circuit.
 
So in other words:

GROUND PIN ------- BUTTON ---- 100ohm RESISTOR ------- DATA PIN

Instead of one button, you can also use multiple buttons, all in a parallel circuit to each other.