Rockwell Automation 2706-F11J_F11JC_F21J_F21JC DL50 INSTALLATION MANUAL - Manual (Page 63)

Chapter 5
Serial Communications
5–13
Field 6: Message Attributes (continued)
Third Byte Message Attributes- The third byte of the Message Attributes
field is reserved for future enhancements. Always set the eighth bit to 1 so
that the field is not misinterpreted as a control character:
Bit
7
Bit
6
Bit
5
Bit
4
Bit
3
Bit
2
Bit
1
Bit
0
Always Set
To 1 Reserved
Field 7: Checksum
This is a three byte field that contains:
Checksum
Dummy Byte
Byte 1
1 Byte
Checksum MSB
Byte 2
1 Byte
Byte 3
1 Byte
Checksum LSB
If either the Checksum Most Significant Byte (MSB) or Checksum Least
Significant Byte (LSB) contain a value equivalent to a CR (decimal 13), the
content of the dummy byte is adjusted to alter the value of the Checksum
bytes. Refer to Appendix F for a description of how the checksum bytes
are calculated.
Important: To disable the checksum, set the DL50 DIP switch S2 position 8
to Off (refer to Figure 3.2). With the checksum disabled, your host duplex
messages must still contain 3 bytes in Field 7, but the checksum value is
ignored by the DL50.
Duplex Protocol (continued)