Sega Master System / Mark III / Game Gear
There is a forum topic for Gear to Gear Cable
It's the cable used to connect two GG consoles for two-player games.
|Signal (Function)||One end||Other end|
There are two modes: "Parallel" and "Serial". Both are only available in "GG mode" on the Game Gear - in SMS mode, the console acts as a Master System.
|Sent as bit||Received as bit|
The disadvantage of this mode is that there is no flow control or handshaking, and you have to poll the port. It is only known to be used by the "Jon's Squinky Tennis" minigame in some Codemasters games.
This offers full handshaking and flow control. It works by raising an NMI whenever there is a byte to be received or the connected GG is powered off (or just disconnected?). Inside the NMI, you should read the status port and act accordingly.
|0||1 if send buffer is occupied (do not send while occupied)|
|1||1 if receive buffer is occupied (do not receive while not occupied)|
|2||1 if connected console is not powered on|
|3-5||Must be 1 to enable Gear-to-Gear operation|
Note that the "GG powered off" signal is apparently raised repeatedly, so you should disable the connection (by resetting bits 3-5) when that happens, and only enable it temporarily when attempting to detect a connected system.
Sends a complete byte to the connected console. You should check the status port bit 0 before sending.
Receives a complete byte from the connected console. Should only be read when the status port indicates there is data.
In SMS mode, the Gear-to-Gear port can be used for a second controller port via an adaptor. It is unknown whether in this mode software may use any of the pins as outputs to enable two-way communication; there are no 2-player SMS-GG games.
Serial mode operates as a 5V TTL UART device transmitting on pin 6 (PC4) and receiving on pin 9 (PC5). The protocol uses 8 data bits with the least significant bit first, no parity bit, and 1 stop bit. Logical 1 is represented by +5v and logical 0 by 0v.