Siemens SIMATIC S7 - Modbus Read and Write Functions

Libraries
9.2 Overview of Modbus communication
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
475
9.2.2
Modbus read and write functions
The Modbus RTU master instructions utilize the Modbus functions shown below to read or
write a specific Modbus address. The Modbus RTU slave device must support the
appropriate Modbus function to read or write a particular Modbus address.
Table 9- 2 Required Modbus slave function support
Modbus address
Read or write
Modbus slave function required
00001 – 09999 discrete outputs
Read
Function 1
Write Function 5 for a single output point
Function 15 for multiple output points
10001 – 19999 discrete inputs
Read
Function 2
Write not possible
30001 – 39999 input registers
Read
Function 4
Write
not possible
40001 – 49999 holding registers
400001 - 465535
Read
Function 3
Write Function 6 for a single register
Function 16 for multiple registers
Modbus message length
The S7-200 SMART CPU supports Modbus messages with up to 240 bytes (1920 bits or
120 registers) of data per message. Some slave devices might support fewer than 240 bytes
of data