6.8 Modbus - Example communication
The example establishes a communication between a master and a slave via Modbus.
The following combination options are shown:
n
CPU 31xS as Modbus RTU master
n
CPU 21xSER-1 as Modbus RTU slave
n
Siemens SIMATIC Manager and possibilities for the project transfer
n
Modbus cable connection
1.
Assemble a Modbus system consisting of a CPU 31xS as Modbus master and a
CPU 21xSER-1 as Modbus slave and Modbus cable.
2.
Execute the project engineering of the master! For this you create a PLC user appli-
cation with the following structure:
n
OB 100:
Call SFC 216 (configuration as Modbus RTU master) with timeout setting and
error evaluation.
n
OB 1:
Call SFC 217 (SER_SND) where the data is send with error evaluation. Here
you have to build up the telegram according to the Modbus rules. Call SFC 218
(SER_RECV) where the data is received with error evaluation.
3.
Execute the project engineering of the slave! The PLC user application at the slave
has the following structure:
n
OB 100:
Call SFC 216 (configuration as Modbus RTU slave) with timeout setting and
Modbus address in the DB and error evaluation.
n
OB 1:
Call SFC 217 (SER_SND) for data transport from the slave CPU to the output
buffer. Call SFC 218 (SER_RECV) for the data transport from the input buffer to
the CPU. Allow an according error evaluation for both directions.
Structure for the according PLC programs for master and slave:
Overview
Approach
System 300S
+
Deployment PtP communication
Modbus - Example communication
HB140 | CPU | 315-4PN23 | en | 18-02
88
