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Modbus Communications and Connectivity
24
the OpenComms NIC module primarily uses Function Code 3 (Read
Holding Registers) and Function Code 6 (Preset Single Register).
Data Field(s)
The data field varies in length depending on whether the message is a
request or a response to a packet. This field typically contains infor-
mation required by the slave device to perform the command specified
or to pass back data to the master device.
Error Check Field
The Error Check Field consists of a 16-bit (2 byte) Cyclical Redun-
dancy Check (CRC16). It allows the receiving device to detect a packet
that has been corrupted with transmission errors.
5.5 RTU Framing Examples
5.5.1 Read Registers (Function Code 0x03 & 0x04)
Query: for reading holding /input registers (requested by a client)
Response: for reading holding/input registers (response by the
server)
5.5.2 Write single holding register (Function Code 0x06)
Query: for writing a single holding register (requested by a client)
Response: for writing a holding registers (response by the server)
Server
Address
Function Code
(0x3/0x4)
Index of
starting register
Number of
registers to
be read
16 bit
CRC
1 byte 1 byte 2 bytes 2 bytes 2 bytes
Server
Address
Function Code
(0x3/0x4)
Byte
Count
Data from
registers
16 bit
CRC
1 byte 1 byte 1 byte 2*m bytes (= Byte Count) 2 bytes
Server
Address
Function Code
(0x06)
Index of
the register
16 bit value
to be written
16 bit
CRC
1 byte 1 byte 2 bytes 2 bytes 2 bytes
Server
Address
Function Code
(0x06)
Index of
the register
16 bit value
been written
16 bit
CRC
1 byte 1 byte 2 bytes 2 bytes 2 bytes