The Relationship between Modbus and TCP Protocol

Modbus and TCP protocols are different, but they are also closely connected. The combine of the two protocols is based on Modbus/TCP/IP protocol remote network communication, which prompts the usage of Modbus in the equivalents applications.

Modbus protocol is mainly used in electrical automation and process control, which adopts RS232 and RS485 interfaces. The disadvantages of Modbus protocol are short distance and slow transmission speed, because of that, a lot of applications were restricted. TCP/IP protocol is mainly used in the Application of the Internet, the advantages of TCP/IP are long transmission distance and fast transmission speed, the applications of TCP/IP protocol are wider.

1、Modbus protocol

Modbus is a serial communication protocol developed by Modicon published by Modicon® in 1979 for use with its programmable logic controllers (PLCs). In simple terms, it is a method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves. In a standard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The Master can also write information to the Slaves.

Modbus protocol is a universal language that used in electrical automation electronic controllers. This protocol allows controllers as well as other network devices to communicate with each other, and it became an industry-standard protocol. Modbus protocol has good interactivity and open standards and has been widely used in the industrial field.

Modbus has two communication modes: ASCII and RTU, Modbus-RTU(Modbus Remote Terminal Unit) and Modbus-ASCII(American Standard Code for Information Interchange). Although there are two communication modes, only one communication mode can be selected when using, and mixed-use is not supported.

Modbus is an open protocol, meaning that it's free for manufacturers to build into their equipment without having to pay royalties. It has become a standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices. It is used widely by many manufacturers throughout many industries.Modbus is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus ASCII) and for Ethernet (Modbus TCP).

 2. TCP/IP Protocol

TCP/IP Protocol is short for Transmission Control Protocol/Internet Protocol，which is a complete set of network protocols over the physical network. The role of TCP is to provide the transport layer services, and the role of IP is to provide network layer services. The core part of the TCP / IP protocol is the transport layer protocol (TCP / UDP), the network layer protocol (IP), and the physical interface layer.

Protocols are sets of rules for message formats and procedures that allow machines and application programs to exchange information. These rules must be followed by each machine involved in the communication in order for the receiving host to be able to understand the message. The TCP/IP suite of protocols can be understood in terms of layers (or levels).

TCP/IP carefully defines how information moves from sender to receiver. First, application programs send messages or streams of data to one of the Internet Transport Layer Protocols, either the User Datagram Protocol (UDP) or the Transmission Control Protocol (TCP). These protocols receive the data from the application, divide it into smaller pieces called packets, add a destination address, and then pass the packets along to the next protocol layer, the Internet Network layer.

The Internet Network layer encloses the packet in an Internet Protocol (IP) datagram, puts in the datagram header and trailer, decides where to send the datagram (either directly to a destination or else to a gateway), and passes the datagram on to the Network Interface layer.

The Network Interface layer accepts IP datagrams and transmits them as frames over a specific network hardware, such as Ethernet or Token-Ring networks.

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