Chapter 6. Serial I/O, SNP & RTU Protocols
GFK-2222AD
April 2018
243
Message (08): Loopback/Maintenance (General)
Format:
Address
Diagnostic
c
Code
Data
Error Check
Func
08
Query
0, 1, or 4
DATA 1
DATA 1
Address
Diagnostic
c
Code
Data
Error Check
Func
08
Normal Response
0, 1, or 4
DATA 1
DATA 1
Figure 59: RTU Loopback/Maintenance Message Format
Query:
▪
The Function code is equal to 8.
▪
The Diagnostic Code is two bytes in length. The high order byte of the Diagnostic Code is the first
byte sent in the Diagnostic Code field. The low order byte is the second byte sent. The
loopback/maintenance command is defined only for Diagnostic Codes equal to 0, 1, or 4. All other
Diagnostic Codes are reserved.
▪
The Data field is two bytes in length. The contents of the two Data bytes are defined by the value of
the Diagnostic Code.
