Modbus Rtu; Structure Of The Job Message Frame; Table A- 4 Structure Of The Message Frame; A.2 Modbus Rtu - Siemens SENTRON PAC3100 Manual

A.2

Modbus RTU

A.2.1

Structure of the job message frame

Structure
Data traffic between the master and the slave and between the slave and the master begins
with the address of the slave. The job message frame consists of the following elements:
1. Address of the MODBUS slave
2. Function code
3. Data of the message frame
4. Checksum of the message frame (CRC)
The structure of the data field depends on the function code used.
Table A- 4
Address
Byte
Cyclic Redundancy Check (CRC)
The Cyclic Redundancy Check checks the data flow. The CRC consists of 2 Bytes:
● one LSB
● One MSB
The transmitting device calculates the CRC and appends it to the message. The receiving
device calculates the CRC again and compares the newly calculated value with the received
CRC. If the two values do not agree, an error has occurred.
End of a message frame
If no characters are transferred for the space of 3.5 bytes this is taken as the end of the
message frame. A check is made to determine the validity of the message frame.
Validity of the message frame
Gaps in the message frame are filled with 0xFFFFFFFF. FFFFFFFF means the message
frame contains no measured values. This means it is invalid. If the message frame has a
different content it is in principle valid.
See also
Function codes (Page 127)
SENTRON PAC3100
Manual, 07/2009, A5E02385159B-01
Structure of the message frame
Function code
Byte
data
n byte
Appendix

A.2 Modbus RTU

CRC
2 bytes
125