Rockwell Automation 2755-DM9, -DM9E High Speed Decoder - Manual (Page 127)

Chapter 8
Host Commands Using the RS-485 Interface
8–15
FUNCTION CODE (Byte 4)- The function code specifies the diagnostic
command:
00 = Diagnostic Loop
01 = Read Diagnostic Counters
03 = Read Diagnostic Status
07 = Reset Diagnostic Counters.
Diagnostic Link Reply Format
The decoder’s reply to a diagnostic command has the following structure:
LINK DIAGNOSTIC REPLY
STATUS 01000110
TRANSACTION WORD
HI Byte LOW Byte
DATA
The following are explanations of each byte in the reply message:
REPLY (Byte 0) - The reply byte has the following structure:
0 1 0 0 0 1 1 0
LSB
0110 = Link
Diagnostics
Command
Bit #6 Set
For Reply
Always Set to 0
MSB
STATUS (Byte 1) - Refer to Table 8.D for an explanation of the status codes.
TRANSACTION WORD (Bytes 2 &3) - The transaction word is returned
exactly as it was sent in the command.