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Serial communication in Modbus environments

Integrating serial devices with the W&T Com-Server

W&T Com-Server in speech bubble

Modbus is a communication protocol introduced by Modicon towards the end of the 1970s for the exchange of data between controllers, SCADA systems, data loggers, etc. To this day, it is a widely accepted standard in the industry and is also widely used in the networking of PV and wind power components (inverters, data loggers, storage, energy meters etc.).

A large number of detailed descriptions and articles on Modbus and the specification itself are freely available on the Internet. At this point we will provide only a brief overview of the most important features to show the possible uses of the Com-Server in different environments. You will find various application options for the Com-Server in Modbus environments further below on the page.

Modbus in general

Modbus is a strict Master/slave procedure. This means: Modbus slaves may only respond to requests addressed to them, the so-called Requests. Communication is based on the reading or writing of registers by the master. Each request contains:

Length Name Description
1 byte Address Slave address
1 Byte Function Code Type or width of the register and read or write access
2 bytes Start address Access start address (High Byte first)
2 bytes Register Count Number of registers to be read/written

An addressed slave may reply with a so-called Response. A successful response contains:

Length Name Description
1 byte Address Slave address
1 byte Function Code Identical to the request
1 byte Length Number of following bytes with the content of the requested registers

Two possible error cases are possible:

  1. The slave understands the request syntactically, but does not recognize the requested registers and responds with an exception (Function Code Response = Function Code Request + 0x80)
  2. The slave does not respond at all.

Modbus/RTU

Modbus/RTU is the common standard for Modbus communication via serial interfaces (RS232/422/485). The Modbus requests and responses have appended a checksum 2 bytes in length. The end of the datagram is recognized by the master and slave by means of a transmission pause of at least 3.5 character lengths.

Modbus/TCP

Modbus/TCP is used for communication in TCP/IP networks. The requests and responses are preceded by a Modbus/TCP header. Thanks to the checksums already contained in the network protocols, the additional checksums of the Modbus datagram can be dispensed with.

Length Name Description
2 bytes Transaction-ID Consecutive number
1 byte Function Code Identical to the request
1 byte Length Number of following bytes with the content of the requested registers

On the wire: Modbus/RTU vs. Modbus/TCP

Protocol structure

Modbus/ASCII and Modbus/TCP Secure Protocol

Both are further Modbus protocol variants that are not widely used in practice. Neither Modbus/ASCII nor Modbus/TCP Secure Protocol are supported by W&T devices.

Application examples

  • Application

    Schematic representation of the application
    Box-to-Box:

    Connection of remote RTU slaves to an RTU master via network

  • Application

    Schematic representation of the application
    Modbus/TCP gateway:

    Connecting serial Modbus/RTU slaves to a Modbus/TCP master

  • Application

    Schematic representation of the application
    Windows COM Port Redirector:

    Operation of a Modbus/RTU master in virtual environments

Products

  • #58665

    Product photo: Com-Server PoE 3x Isolated
    Com-Server ++

    1x RS232/422/485

  • #58662

    Product photo: Com-Server ++
    Com-Server ++

    1x RS232/422/485

  • Support

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    Could you use some assistance?
    We’re here for you!

    You can reach our applications engineers at +49 202-2680-110

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