Parameter Assignment/Addressing; Modbus Rtu; Structure Of The Job Message Frame; Table 5-1 Structure Of The Message Frame - Siemens SENTRON PAC RS485 Manual

Parameter assignment/Addressing

5.1

MODBUS RTU

5.1.1

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 5-1
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.
PAC RS485
Manual, 02/2008, A5E02091800B-01
Structure of the message frame
Function code
Byte
data
n byte
5
CRC
2 bytes
29