Modbus Read And Write Functions - Siemens SIMATIC S7 System Manual

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
Modbus address
00001 – 09999 discrete outputs
10001 – 19999 discrete inputs
30001 – 39999 input registers
40001 – 49999 holding registers
400001 - 465535
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
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Required Modbus slave function support
Read or write
Read
Write
Read
Write
Read
Write
Read
Write
9.2 Overview of Modbus communication
Modbus slave function required
Function 1
Function 5 for a single output point
Function 15 for multiple output points
Function 2
not possible
Function 4
not possible
Function 3
Function 6 for a single register
Function 16 for multiple registers
Libraries
475