Nintendo GAME BOY Programming Manual page 43

2.6.3.9 Communication Error Processing
If an error described above in Communication Status is generated, the following error
codes are returned by subroutine.
Error Code
04h
10h
20h
Error Description
Checksum error (same for sender and receiver):
The results of the checksum calculated by the receiver
and the checksum sent by the sender do not agree.
Pulse width error:
Generated by the receiver when the width of the pulse of
the signal sent by the sender is too wide or narrow.
Generated by the sender when the width of the pulse of
the signal sent by the receiver is too wide or narrow.
Communication error:
Communication prevented by other causes.
The subroutine provided by Nintendo treats as an error
the case when the data value of the second byte of the
received header exceeds the number of data items to be
received, as determined beforehand by the receiver.
The routine also generates an error if the communication
command value of byte 1 of the header is not 5A.
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Chapter 1: System