Modbus Rtu Examples - Siemens SIMATIC S7-1200 System Manual

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Setting the maximum record length for PROFIBUS communication

When using a CM1243-5 PROFIBUS Master module to control an ET 200SP or ET 200MP

PROFIBUS device that uses an RS232, RS422, or RS485 point-to-point module, you need

to explicitly set the "max_record_len" data block tag to 240 as defined below:

Set "max_record_len" in the Send_P2P section of the instance DB (for example,

"Modbus_Slave_DB".Send_P2P.max_record_len) to 240 after running Modbus_Comm_Load.

Explicitly assigning max_record_len is only necessary with PROFIBUS communication;

PROFINET communication already uses a valid max_record_len value.

Modbus RTU examples

Example: Modbus RTU master program

Modbus_Comm_Load is initialized during start-up by using the first scan flag. Execution of

Modbus_Comm_Load in this manner should only be done when the serial port configuration

will not change at runtime.

Network 1: Configure/initialize the RS485 module communications port only once during the

One Modbus_Master instruction is used in the program cycle OB to communicate with a

single slave. Additional Modbus_Master instructions can be used in the program cycle OB to

communicate with other slaves, or one Modbus_Master FB could be re-used to communicate

with additional slaves.

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK

Communication processor and Modbus TCP

13.5 Modbus communication