Modbus Server Supplement to User Manual - ENGLISH
52 SU_ModbusV2-8, 2015-03-02
3 Transmission of Data
The Modbus protocol is based on a master/slave architecture.
Communication is initiated by the master, i.e., the slaves can only send after receiving a request.
The transmitter (slave) supports the operating mode Modbus RTU (Remote Terminal Unit).
The information is transmitted via telegrams.
3.1 Structure of the Telegram
Besides the device address and the actual data, the function code and the CRC checksum are transmitted.
At the beginning and at the end of each telegram, the transmission is interrupted for the duration of 3.5 characters.
3.2 Function Code
Indication of the function to be carried out by the slave. For the function codes of the transmitter see Tab. 3.1.
3.3 CRC Checksum
The CRC checksum is formed by all bytes of a telegram. It is used for detecting transmission errors in the telegram.
start device address function code data CRC checksum end
after t 3.5 characters 8 bit 8 bit n
.
8 bit 16 bit after t 3.5 characters
Tab. 3.1: Function codes of the transmitter
function code function purpose example
3 (0x03) Read Holding Registers reading of readable Modbus registers measured value
state of a physical quantity
4 (0x04) Read Input Registers
6 (0x06) Write Single Register writing of writable Modbus registers ID of the unit of measurement
resetting the totalizers
16 (0x10) Write Multiple Registers
17 (0x11) Report Slave ID reading of device-specific information firmware version
device address
function code
CRC checksum
t t
start end
data data
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