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Cp341 Modbus Without Dongle Crack

How to use CP341 Modbus without dongle

Modbus is a serial communication protocol that is widely used in industrial automation and control systems. It allows devices to exchange data over different types of media, such as RS-232, RS-485, Ethernet, etc. Modbus can be implemented as a master-slave or a client-server architecture, depending on the application requirements.

CP341 is a communication processor module for SIMATIC S7-300 PLCs that supports various serial interfaces and protocols, including Modbus RTU. CP341 can be configured as a Modbus master or a Modbus slave, depending on the role of the PLC in the network. However, to use CP341 as a Modbus slave, a special hardware dongle is required to be inserted into the module. This dongle contains a license key that enables the Modbus slave functionality of CP341.


But what if you want to use CP341 as a Modbus slave without buying the dongle? Is there a way to do that? The answer is yes, but it requires some programming skills and knowledge of the Modbus protocol. In this article, we will show you how to implement a Modbus RTU slave protocol on CP341 using the ASCII driver of the module and S7 code.


  • A SIMATIC S7-300 PLC with a CP341 RS-232 or RS-485 communication module.

  • A Modbus master device that can communicate with CP341 over the serial interface.

  • A PC with SIMATIC Manager software and STEP 7 programming software.

  • A programming cable to connect the PC to the PLC.

  • A serial cable to connect the CP341 module to the Modbus master device.


  • Create a new project in SIMATIC Manager and add the S7-300 PLC and the CP341 module to the hardware configuration.

Configure the CP341 module as follows:

  • Select "ASCII" as the protocol type.

  • Set the baud rate, parity, data bits, and stop bits according to the Modbus master device settings.

  • Enable "Freeport mode" and "Interrupt-driven operation".

  • Assign an interrupt number for the module (e.g., 10).

  • Assign an address area for the module (e.g., DB100).

  • Download the hardware configuration to the PLC.

Create a new S7 program block (e.g., FC1) and write the code for the Modbus RTU slave protocol. You can use the example code provided by TIBI68 as a reference, or you can write your own code based on the Modbus RTU specification. The code should perform the following tasks:

  • Read incoming data from the CP341 module using SFC14 (RDREC) and store it in a buffer (e.g., DB101).

  • Check if the data is a valid Modbus RTU frame by verifying the address, function code, data length, and CRC checksum.

  • If the frame is valid, parse the function code and data and execute the corresponding Modbus function (e.g., read coils, write registers, etc.).

  • Prepare a response frame with the appropriate data and status code and write it to the CP341 module using SFC15 (WRREC).

  • Create an organization block (e.g., OB35) and call FC1 from there using an interrupt instruction (e.g., INT 10).

  • Download the program blocks to the PLC.

  • Connect the CP341 module to the Modbus master device using a serial cable.

  • Start the communication between the devices and test if the CP341 module responds correctly to the Modbus requests from the master device.


In this article, we have shown you how to use CP341 as a Modbus slave without dongle by implementing a custom protocol driver using S7 code. This method can save you some money and time if you don't want to buy the dongle or wait for its delivery. However, it also requires some programming skills and knowledge of the Modbus protocol, so it may not be suitable for everyone. If you prefer a more convenient and reliable solution, you can always buy the dongle and use the official Modbus RTU slave driver from Siemens.


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