Nintendo GAME BOY Programming Manual page 39

2.6.3.5 Transmitted Data
When data is transmitted and received, it is transmitted in packets. Each packet
comprises the 4 parts shown below, and each part is sandwiched between synchronous
pulses. For more information, see Section 2.6.3.7, Details of Data Transmission and
Reception.
The data that comprises a packet is transmitted 1 bit at a time beginning from the MSB.
Transmission Packet
Connector
Connector:
Signal that implements an infrared communication connection between two Game Boy
Color machines. This is always required in the initial packet. When the receiver receives
the connector and recognizes it as a connecting pulse, the receiver returns the same pulse
to the sender. The sender then determines whether this signal is a normal connecting
pulse. If it is not recognized as a normal pulse, transmission is interrupted at this stage.
Header:
Data indicating the type of data being sent and the total number of bytes.
Byte 1: Communication command
5Ah: transmission of raw data
At present, any value other than 5Ah causes an error.
(To be used for by other devices in future)
Byte 2: Total number of data in data portion of the packet
01h-FFh: Number of data
00h: Indicates completion of communication to receiver.
Data:
The transmitted data itself. Maximum of 255 bytes.
There are no data if completion of communication is indicated to the receiver.
(The data portion of the packet consists only of a synchronous pulse.)
Checksum:
2 bytes of data consisting of the sum of the header and all data in the data portion of the
packet. Following this, the communication status is returned from receiver to sender.
2.6.3.6 Flow of Data Transmission and Reception
When data is transmitted and received, both Game Boy Color units are first placed in
receive status. The one with the send indicator is then designated as the sender, and the
other one is designated as the receiver. The flow of data transmission is shown below.
Connector
Sender
Header Data
Header Data Checksum
39
Chapter 1: System
Checksum
Header