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C
C-3GFK–0804B Appendix C RTU Protocol
Message Fields
A typical message has the fields shown below:
Information
FRAME
Station
Address
Function
Code
Error
Check
Station Address
The station address is the address of the slave selected for the data transfer. It is one
byte in length and has a value from 0 to 247 inclusive. An address of 0 selects all slave
stations, and indicates that this is a broadcast message. An address from 1 to 247 selects
a slave station with that station address. The station address for a Micro PLC is one of its
software Setup parameters.
Function Code
The function code identifies the command being sent. This field is one byte in length
and may have a value from 0 to 255:
Function
Code
Description Function
Code
Description
0 Illegal Function 8 Loopback Maintenance
1 Read Output Table 9–14 Unsupported Function
2 Read Input Table 15 Force multiple outputs
3
4
Read Registers
Read Analog Input
16 Reset multiple registers
3
4
5
Read Registers
Read Analog Input
Force Single Output
17 Report device type
4
5
Read Analog Input
Force Single Output
18–127 Unsupported function
6
7
Preset Single Register
Read Exception Status
128–255 Reserved for Exception
Responses
Information Field
The information field contains all of the other information required to further specify or
respond to a requested function. Detailed specification of the contents of the information
field for each function code is found in the Message Descriptions that start on page C-9.
Error Check Field
The error check field is two bytes in length. It contains a cyclic redundancy check
(CRC–16) code. Its content depends on the station address, function code, and
information field. For information about generating the CRC– 16 code, see Cyclic
Redundancy Check (CRC) beginning on page C-5. Note that the information field is variable
in length. To properly generate the CRC– 16 code, the length of frame must be determined.
See RTU Message Length (page C-8) to calculate the length of a frame for each of the
defined function codes.