Home of the LEDmePlay®. Since 2013.
LEDmePlay® X-Link is a hardware extension which allows to connect two LEDmePlays via a cross cable in order to play games against each other. Technically, it is an ansynchronous serial connection using the UART (=Universal Asynchronous Receiver Transmitter) protocol. It uses the additional Serial1 interface of the Arduino Mega to avoid interferences with the standard serial connection used if the LEDmePlay® is connected with a computer. Currently, the racing game Grand Tour and Tetromino make use of LEDmePlay® X-Link.
It is easy to a add a link interface to an existing LEDmePlay® (of course it is easier if you directly plan to integrate it). You need a 3.5mm stereo jack plug which needs to be built into the housing of the LEDmePlay® (typically, on the rear side beside the other connectors). Furthermore, you need a 1KOhm resistor that you have to connect to the RX1 pin (pin 19). After that, you can connect the 3.5mm plug in the following way:
Tip 1KOhm resistor connected to RX1 (digital 19)
Ring TX1 (digital 18)
With regard to the stereo jack plug you need a cable with 3.5mm stereo jacks to connect two LEDmePlays. However, it is necessary that the cable is a cross cable so that the TX1 pin of one LEDmePlay® is connected to the RX1 pin of the other LEDmePlay® and vice versa. The grounds need to be connected directly. Since it is not possible to buy such a cable, you have to solder it yourself. A good choice is to push the three lines through a heat shrink tube to let the final cable look a bit more professional (it is also more robust).