Fujitsu F2MCTM-16LX Hardware Manual page 503

Register Function
Sets transmitted message data (any of 0 to 8 bytes).
Data is transmitted in the order of BYTE0, BYTE1, ..., BYTE7, starting with the MSB.
Stores received message data.
Data is stored in the order of BYTE0, BYTE1, ..., BYTE7, starting with the MSB.
Even if the received message data is less than 8 bytes, the remaining bytes of the data register (DTRx), to
which data are stored, are undefined.
Note:
A write operation to this register should be performed in words. A write operation in bytes causes
undefined data to be written to the upper byte at writing to the lower byte. Writing to the upper byte is
ignored.
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