4-Player Adapter

The 4-Player Adapter (DMG-07) is an accessory that allows 4 Game Boys to connect for multiplayer via serial data transfer. The device is primarily designed for DMG consoles, with later models requiring Link Cable adapters.

Communication

The DMG-07 protocol can be divided into 2 sections, the “ping” phase, and the “transmission” phase. The initial ping phase involves sending packets back and forth between connected Game Boys probing for their current connection status. Afterwards, the DMG-07 enters into transmission mode where the Game Boys exchange data across the network.

A very important thing to note is that all Game Boys transfer data across the DMG-07 via an external clock source. Apparently, the clock source is provided by the DMG-07 itself. Trying to send data via an internal clock results in garbage data.

Ping Phase

When a “master” Game Boy (Player 1) is first connected to the adapter, setting Bit 7 of SC to 1 and setting Bit 0 of SC to 0 causes the accessory to send out “ping” packets periodically. All connected Game Boys will receive 4 bytes as part of the ping packet at a rate of about 2048 bits per second, or about 256 bytes per second. Essentially, the ping seems to run 1/4 as fast as the clock used for normal serial transfers on the DMG (1KB/s). The ping data looks like this:

3 “STAT” bytes are sent indicating the current connection status of the other Game Boys. Each byte is usually the same, however, sometimes the status can change midway through a ping, typically on STAT2 or STAT3. Each STAT byte looks like such:

The Player ID’s value is determined by whichever port a Game Boy is connected to. As more Game Boys connect, the upper bits of the STAT bytes are turned on.

When talking about Game Boys and the “connection”, this refers to a Game Boy properly responding to STAT1 and STAT2 bytes when receiving a ping packet from the DMG-07. In this way, the Game Boy broadcasts across the Link Cable network that it is an active participant in communications. It also acts as a sort of acknowledgement signal, where software can drop a Game Boy if the DMG-07 detects an improper response during a ping, or a Game Boy simply quits the network. The proper response is to send $88 after receiving the ID Byte and STAT1, in which case the upper-half of STAT1, STAT2, and STAT3 are updated to show that a Game Boy is “connected”. If for whatever reason, the acknowledgement codes are not sent, the above bits are unset.

Some examples of ping packets are shown below:

It’s possible to have situations where some players are connected but others are not; the gaps don’t matter. For example, Player 1 and Player 4 can be connected, while Player 2 and Player 3 can be disconnected (or non-existent, same thing); most games do not care so long as Player 1 is active, as that Game Boy acts as master and orchestrates the multiplayer session from a software point of view. Because of the way the DMG-07 hardcodes player IDs based on which port a Game Boy is physically connected to, in the above situation Player 4 wouldn’t suddenly become Player 2.

During the ping phase, the master Game Boy is capable of setting up two parameters that will be used during the transmission phase. The clock rate for the transmission phase can be adjusted, as well as the packet size each Game Boy will use. The master Game Boy needs to respond with one byte for STAT2 and STAT3 respectively. The chart below illustrates how a master Game Boy should respond to all bytes in a ping packet:

----------------------------
DMG-07		Game Boy
----------------------------
\$FE	<-->	(ACK1) = \$88
STAT1	<-->	(ACK2) = \$88	
STAT2	<-->	(RATE) = Link Cable Speed 
STAT3	<-->	(SIZE) = Packet Size

The new clock rate is only applied when entering the transmission phase; the ping phase runs at a constant 2048 bits-per-second. The formula for the new clock rate is as follows:

DMG-07 Bits-Per-Second --> 4194304 / ((6 * RATE) + 512)

The lowest setting (RATE = 0) runs the DMG-07 at the normal speed DMGs usually transfer data (1KB/s), while setting it to $FF runs it close to the slowest speed (2042 bits-per-second).

SIZE sets the length of packets exchanged between all Game Boys. Nothing fancy, just the number of bytes in each packet. It probably shouldn’t be set to zero.

Transmission Phase

When the master Game Boy (Player 1) is ready, it should send 4 bytes (AA AA AA AA, if those are actually required should be investigated further). This alerts the DMG-07 to start the transmission phase. The RATE and SIZE parameters are applied at this point. The protocol is simple: Each Game Boy sends a packet to the DMG-07 simultaneously, then the DMG-07 outputs each packet to all connected Game Boys. All data is buffered, so there is a 4 packet delay after each Game Boy submits their data (the delay is still 4 packets long even if some Game Boys are not connected). For example, say the packet size is 4 bytes; the flow of data would look like this when sending:

And when receiving, the flow of data would look like this:

Again, due to buffering, data output to the DMG-07 is actually delayed by several transfers according to the size of the packets. All connected Game Boys should send their data into the buffer during the first few transfers. Here, the packet size is 4 bytes, so each Game Boy should submit their data during the first 4 transfers. The other 12 transfers don’t care what the Game Boys send; it won’t enter into the buffer. The next 16 transfers return the packets each Game Boy previously sent (if no Game Boy exists for player, that slot is filled with zeroes).

With the buffering system, Game Boys would normally be reading data from previous packets during transfers 0-15, in addition to sending new packets. Likewise, during transfers 16-19 each Game Boy is sending new packets. In effect, while receiving old data, Game Boys are supposed to pump new data into the network.

When the DMG-07 enters the transmission phase, the buffer is initially filled with garbage data that is based on output the master Game Boy had sent during the ping phase. At this time, it is recommended to ignore the earliest packets received, however, it is safe to start putting new, relevant data into the buffer.

Restarting Ping Phase

It’s possible to restart the ping phase while operating in the transmission phase. To do so, the master Game Boy should send 4 or more bytes (FF FF FF FF, it’s possible fewer $FF bytes need to be sent, but this has not been extensively investigated yet). The bytes alert the DMG-07 that the ping phase should begin again, after which it sends ping packets after a brief delay. During this delay, the transmission protocol is still working as intended until the switch happens.