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The port configuration of the RS485 interface in the STEP 7 (TIA Portal) device view is irrelevant here. The communication parameters BAUD (transmission rate) and PARITY (parity) must be identical for all nodes. After inserting the communication module in the hardware configuration you can select the symbolic name of the communication module at the PORT parameter. The "Modbus_Comm_Load" block must be called in the first program cycle (by activating the "Initial Call" of a cyclic OB, for example, or as a call in Startup OB 100). Figure 1-3 The "Modbus_Comm_Load" block is used to select the communication module, set the communication parameters and parameterize the connection with the master or slave parameters.
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Figure 1-1 MODBUS-Master MODBUS-Slave CPU 1211C CSM 12C STEP 7 (TIA Portal) Entry ID:, 2.0, 02/2017 3ĥ 1.2.1 Modbus_Comm_Load The configuration block "Modbus_Comm_Load" is called on both sides (master and slave) for MODBUS communication. By the same principle, the address of the slave to be addressed can be changed between two actions (here writing and reading), thus making it possible to exchange data with multiple slaves. Since the RS485 has a two-wire interface (half-duplex), writing and reading must be conducted consecutively.
CM 1241 MODBUS RTU CODE
There is alternate writing to the holding register of the slave (function code 16) and reading from the holding register of the slave (function code 03). We will take a sample project to describe the configuration procedure in STEP 7 (TIA Portal) V14 for communication with multiple slaves using the CM 1241 RS Instructions We have taken the example of MODBUS communication between a master and a slave. With up to 32 slaves one after the other using the RS485 (via CM or CB 1241 RS485). With up to 10 slaves one after the other using the RS422 (via CM 1241 RS422/RS485). If you select MODBUS Master, you can communicate With only one slave using the CM 1241 RS232 (physical limitation). Using the MODBUS library integrated in STEP 7 (TIA Portal) you can define each communication module as a MODBUS master or slave. You can extend each S controller with up to three communication modules (+ one RS485 communications board). 9 Entry ID:, 2.0, 02/2017 2ģ CM 1241 (RS422/485) CM 1241 (RS422/485) Siemens AG 2017 All rights reserved The SIMATIC S enables point-to-point communication via the CPU extension with the interfaces RS485 und RS232. To stay informed about product updates, subscribe to the Siemens Industrial Security RSS Feed under Contents Instructions Application Library "MODBUS (RTU)" Modbus_Comm_Load Modbus_Master Modbus_Slave Sample Project. Use of product versions that are no longer supported, and failure to apply latest updates may increase the customer s exposure to cyber threats.
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Siemens strongly recommends to apply product updates as soon as available and to always use the latest product versions. For more information about industrial security, please visit Siemens products and solutions undergo continuous development to make them more secure. Additionally, Siemens guidance on appropriate security measures should be taken into account. use of firewalls and network segmentation) in place. Systems, machines and components should only be connected to the enterprise network or the internet if and to the extent necessary and with appropriate security measures (e.g. Customers are responsible to prevent unauthorized access to their plants, systems, machines and networks. Siemens products and solutions only form one element of such a concept. In order to protect plants, systems, machines and networks against cyber threats, it is necessary to implement and continuously maintain a holistic, state-of-the-art industrial security concept. The conditions of use specified there apply ( Security Informati on Siemens provides products and solutions with industrial security functions that support the secure operation of plants, systems, machines and networks. 1 FAQ 02/2017 How do you establish MODBUS-RTU communication? SIMATIC S FW V4.2 / STEP 7 V14 (TIA Portal)Ģ This entry originates from Siemens Industry Online Support.