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.

Power

The DMG-07 will not power on until its Player 1 cable is plugged into a Game Boy link port to supply power. The Player 1 cable is the only one permanently attached to the device and has the power pin connected, unlike typical link port cables.

Clock Control

Serial clock for all transfers is provided by the DMG-07, with connected Game Boys operating in external clock mode. Trying to send data via internal clock mode results in garbage data and should not be used.

Communication Phases

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 is switched to the transmission phase where the Game Boys exchange data across the link cable network. Later, connected Game Boys may switch back to ping phase by sending the “ping restart sequence”.

Ping Phase

When the DMG-07 is powered up it will begin operation by automatically sending out ping packets periodically on every port. In order to start receiving these ping packets a connected Game Boy should use external clock mode (bit 0 of SC set to 0) and request a transfer (bit 7 of SC set to 1).

All connected Game Boys will receive 4 bytes for each ping packet. Transfer of the 4 bytes is not spread evenly over the packet time period; instead, transfer is clustered at the start, followed by a much longer delay.

The power-up timing for ping packets is as follows:

• Serial Clock period: 15.95 microseconds (62.66 KHz)
• Transfer time per byte: 128 microseconds
• Delay between bytes: 1.42 milliseconds
• Transfer time for all bytes: 4.71 milliseconds
• Delay between packets: 12.29 milliseconds
• Total packet and delay time: 17 milliseconds

This means one ping packet with 4 bytes and its subsequent delay takes a little more time than a single Game Boy video frame.

Ping Packet Fields

The ping data received by each Game Boy looks like this:

The chart below illustrates how Game Boys should respond to bytes in a ping packet:

Note: Bytes in the Received column should have the matching byte in the Reply column loaded into the SB register to be sent during the next serial transfer.

ACK Replies

The ACK1 and ACK2 bytes use a fixed value ($88) and are sent in reply to the PING HEADER and STAT1 bytes.

RATE Setting

The RATE setting configures packet timing and works differently in the ping and transmission phases. It is sent in reply to the STAT2 byte.

Note: In both phases the value $00 for RATE has special behavior where it does not change the speed, so it should not be used.

RATE in Ping Phase

In ping phase RATE only adjusts the delay between packets (byte timing is unchanged) and takes effect immediately upon the next packet. The timing is calculated as follows:

Delay between packets = (12.2 milliseconds) + ((RATE & 0x0F) * 1 millisecond)

Where:

• Transfer time for all bytes: 4.71 milliseconds (always)
• Delay between packets: 12.2 to 27.21 milliseconds

This yields a range of 12.20 to 27.21 milliseconds for the total packet and delay time.

RATE in Transmission Phase

In transmission phase the Clock Rate setting is determined by the last RATE value transmitted before exiting ping phase. The timing is more complex and is calculated as described below.

First determine the delay between packet bytes:

Delay between bytes = ((RATE >> 4) x .106 milliseconds) + 0.887 milliseconds

Then the total packet and delay time will be whichever of the following is larger:

((RATE & 0x0F) x 1 milliseconds) + 17 milliseconds
or
(Transfer time per byte + Delay between bytes) x Byte Count) + (between .36 to 2.15 milliseconds)

Where:

• Transfer time per byte: ~0.128 microseconds
• Byte count: 4, 8, 12 or 16 depending on the setting for SIZE
• The formula for calculating the .36 to 2.15 milliseconds add-on is not yet understood.

This yields a range of 17.0 to 41.6 milliseconds for the total packet and delay time.

SIZE Packet Size setting

SIZE sets the number of usable data bytes sent by each Game Boy during a packet in transmission phase. It is sent in reply to the STAT3 byte.

The total number of bytes broadcasted by the DMG-07 in a packet will be SIZE×4. For example if SIZE is 3 then the total packet size will be 12 bytes (3×4).

The range of values which work without issue is 1 to 4.

Ping Connection Status

The 3 STAT bytes sent by the DMG-07 indicate the current connection status of all the 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 has the following fields:

The Player ID values are determined by whichever port a Game Boy is connected to. As more Game Boys connect and properly reply to pings, the upper bits of the STAT bytes are turned on.

In this way, each Game Boy broadcasts its presence across the link cable network. 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 upper-half of STAT1, STAT2, and STAT3 are updated based on the ping responses to show when Game Boys are “connected”. If for whatever reason, the ping responses are not sent, the status bits are unset.

Some examples of ping packets sent by the DMG-07 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.

Transmission Phase

Entering Transmission phase

In ping phase, when the connected Game Boys are ready, then one of them (typically Player 1) should send the begin transmission sequence, which is 4 bytes of $AA in a row (AA AA AA AA). The transmission should be aligned such that the first byte is sent in reply to the PING HEADER field. This causes the DMG-07 to switch to the transmission phase.

At the same time the DMG-07 is receiving the fourth consecutive $AA byte it will be sending the first of four $CC bytes in a row (CC CC CC CC). This is the indicator that connected Game Boys should use for switching to transmission phase.

After the DMG-07 finishes sending the indicator packet of $CC bytes it will immediately begin sending data packets and the transmission phase RATE and SIZE settings take effect.

The following chart is an example of switching from ping to transmission phase, with SIZE of 1 (so total packet size is 4 bytes) and only the Player 1 Game Boy connected.

Note: Bytes in the Received column should have the matching byte in the Reply column loaded into the SB register to be sent during the next serial transfer.

Transmission Protocol

The protocol is simple: At the same time the DMG-07 is sending data it received during the previous packet, the Game Boys are replying with their new data. In effect, while receiving old data, Game Boys are supposed to pump new data into the network.

Data received by the DMG-07 is buffered until it is broadcast during the next packet. This means there is a one-packet delay between when the Game Boys send data and when they receive the packet which combines all their data together.

The following chart shows data for two consecutive packets with SIZE of 2 (so total packet size is 8 bytes). The label for a player byte is P[player num].[packet num], so P3.1 is Player 3, Packet 1.

Note: Bytes in the Received column should have the matching byte in the Reply column loaded into the SB register to be sent during the next serial transfer.

All connected Game Boys should send their data into the buffer during the first few serial transfers. Here, the packet size is 2 bytes, so each Game Boy should submit their data during the first 2 transfers. The other 6 transfers don’t care what the Game Boys send; it won’t enter into the buffer. The next 8 transfers return the data each Game Boy previously sent (if no Game Boy exists for a player, that slot is filled with zeroes).

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 first packet received, however, it is safe to start putting new, relevant data into the buffer immediately.

Restarting Ping Phase

It’s possible to restart the ping phase while operating in the transmission phase. To do so, any connected Game Boy can send the ping restart sequence, which is 3 or more $FF bytes in a row (FF FF FF FF). The transmission should be aligned such that the first byte is sent in reply to the first serial transfer of a packet. This causes the DMG-07 to switch back to the ping phase.

After the third consecutive $FF byte is received by the DMG-07 and transmission of the current packet is completed then the DMG-07 will begin transmitting a packet where all the bytes are set to $FF. This is the indicator that connected Game Boys should use for when to switch back to ping phase.

To avoid false positives the Game Boys should only perform a switch to ping after receving an entire packet of consecutive $FF bytes. For example, if the SIZE setting is 3 then the total packet size is 12 bytes (3×4), which will be the number of consecutive $FF bytes to require from the DMG-07 to indicate a switch.

After the DMG-07 finishes sending the indicator packet of $FF bytes it will immediately begin transmitting ping phase packets.

The following chart is an example of switching from transmission back to ping phase, with SIZE of 1 (so total packet size is 4 bytes) and the other 3 Game Boys sending $A5 for their data byte.

Note: Bytes in the Received column should have the matching byte in the Reply column loaded into the SB register to be sent during the next serial transfer.