You may want to check out their page for the pictures. Just a direct connection. NO messy interrupts to deal with. NO extra pins needed. Has functions for inputting numbers as well as strings Works in graphics mode as well as the text mode.
Supports shift and caps lock, and automatic scrolling if the input gets too big.
Includes 2 demo ROMs, the include file, and a connection diagram. Jeff Frohwein Allows 4 GBs to connect together for multi-player games that support it. Diode is needed by some PC ports. Here's a schematic for connecting your GameBoy to an IBM-PC Parallel Port for exchanging data. Here's assembly code, a timing diagram, and a circuit for controlling external projects with a GameBoy.