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ET-2200 Series Ethernet I/O Modules
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Modbus RTU Data Structure
Byte 06
Byte 07
Bytes 08 - 09
Bytes 10 - 11
Net ID
(Station Number)
Function Code
Data Field
Reference Number
(Address Mapping)
Number of Points
Specifies the address of the receiver (i.e., the Modbus/TCP slave).
Function Code:
Specifies the message type.
Data Field:
The data block.
Net ID (Station Number)
The first byte in the frame structure of a Modbus RTU query is the address of the receiver. A valid
address is in the range from 0 to 247. Address 0 is used for general broadcast purposes, while
addresses 1 to 247 are assigned to individual Modbus devices.
Function Code
The second byte in the message structure of a Modbus RTU query is the function code, which
describes what the slave device is required to do. Valid function codes range between 1 and 255. To
answer the query, the slave device uses the same function code as contained in the request. The
highest bit of the function code will only be set to ‘1’ if an error occurs in the system. In this way,
the master device will know whether or not the message has been correctly transmitted.
Code
Function
Reference
(Address)
Read the Status of the Coils (Read DO Readback values)
0xxxx
Read the Status of the Input (Read DI values)
1xxxx
Read the Holding Registers (Read AO Readback values)
4xxxx
Read the Input Registers (Read AI values)
3xxxx
Force a Single Coil (Write DO value)
0xxxx
Set a Single Register (Write AO value)
4xxxx
Force Multiple Coils (Write DO values)
0xxxx
Set Multiple Registers (Write AO values)
4xxxx