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Request from master
The function code in the request informs the addressed slave which action is to be carried
out. The address and data bytes contain all additional information that the slave requires in
order to carry out the action.
Example: The master uses function code 03 to request the slave to read the process output
data and send its content to the master. The data and address bytes need to include the fol-
lowing information: from which register reading should start and how many registers should
be read. Using the CRC check value, the slave is able to detect whether the complete tele-
gram has been received.
Response from slave
If the response from a slave is valid, the function code will match the request from the mas-
ter. The address and data field contains the data recorded by the slave (e.g., register val-
ues).
The function code is modified in the event of an error. The address and data field then con-
tains a code that describes the error. By using the CRC check value, the master is able to
determine whether the telegram content is valid or not.
The Modbus/RTU telegrams are separated by telegram pauses known as end-of-frame
times. The end-of-frame time must be at least 3.5 x as long as the time required for one
transmitted character. The end-of-frame time cannot be changed.
Table 9-2 Modbus protocol: structure of telegrams (frames)
9.3 Addressing registers
Function code 04
You must enter 0000 (hex0000) as the start address in order to read register 30001. The
address area 3xxxx is already defined by the function code field.
Function codes 03 and 16
In order to read/write registers 40032 ... 40039, you must enter 0031 (hex001F) as the start
address. The address area 4xxxx is already defined by the function code field.
Frame Description Size
Slave address Slave address, valid area 1 ... 247 8 bits
Function code Definition whether the parameter is to be read or writ-
ten
8 bits
Address Register address 16 bits
Data – E.g., from the master: Which parameters are
requested?
– E.g., from the slave: Content of the requested
parameters
N x 16 bits
CRC (Cyclic
Redundancy Check)
Test value for the cyclic redundancy check in order
to detect errors in data transmission
16 bits