This application allows you to control an LED matrix, like the one available here: http://www.futurlec.com/LED/LEDM88OG.shtml

It specifically uses 24 digital IO pins to control the LED matrix.

It includes a main loop() that runs a demo on the matrix, and also a loop2() that is not run by default unless you change it’s name to loop() that allows you to experiment with which pins control which rows and columns on the LED matrix – by sending the row and column over the serial connection to the computer.