ModBus Protocol in Industrial Routers

1. Introduction
Modbus is a request-response protocol based on a master/slave or client/server architecture. In a master-slave relationship, communication occurs in pairs. A device must initiate a request and then wait for a response. The initiating device (called the master) is responsible for initiating each interaction. Typically, the master device is an HMI or SCADA system and the slave devices are sensors, programmable logic controllers (PLCs), or programmable automation controllers (PACs). In a standard Modbus network, there is one master device and up to 247 slave devices. Each slave device has a unique slave device address from 1 to 247. 2. Modbus Protocol in Detail
Currently, the modbus protocol is mainly used for serial and Ethernet. For serial connections, it is subdivided into modbus RTU and mobus ASCII, which differ slightly in data and protocol details. For Ethernet connections, modbus TCP is used.
2.1 Modbus Data Transfer Modes
1) Modbus RTU is used for serial communication and uses binary data for protocol communication.The RTU format uses cyclic redundancy checksums as an error checking mechanism to ensure the reliability of the data.Modbus RTU is the most common implementation of Modbus. A Modbus RTU message must be transmitted continuously one at a time without inter-character pauses.Modbus messages are separated by idle (silent) periods.
2) Modbus ASCII is also used for serial communication, using ASCII characters for protocol communication. the ASCII format uses a longitudinal redundancy checksum. modbus ASCII messages are identified by a colon (‘:’) and a trailing line feed (CR/LF) as a header and a footer.
3) Modbus TCP is used for TCP/IP network communication on Modbus, connecting on port 502. It does not need to calculate checksums, as TCP/IP already provides checksum protection in the underlying layer.
2.2 Differences between Modbus ASCII, RTU, TCP
Modbus RTU and Modbus ASCII use the same protocol. The only difference is that the transmitted bytes are presented as binary in RTU and as readable text in ASCII. Note that RTU messages do not have a start-of-text flag, and the receiver of the communication uses a ‘quiet’ time to determine the start of a new message, whereas ASCII has a start-of-text flag. Binary messages are shorter than ASCII, so theoretically RTU is faster to transmit/receive than ASCII.
Modbus TCP can basically be understood as the Modbus RTU protocol transmitted over Ethernet TCP/IP. Modbus data structures are defined using the application layer of the TCP/IP protocol. the TCP protocol is used to ensure that the data is received and sent correctly, whereas the IP protocol contains address and routing information. the Modbus TCP commands are transmitted over the TCP/IP protocol. Modbus RTU commands packaged for transmission. the advantage of Modbus TCP is the ability to use generic Ethernet devices.
3. ModBus Applications in Jincan Network Devices
Jincan Network DTU, RTU and industrial router devices can support the ModBus protocol to collect sensor or device data, uploaded to the remote server through 5G/4G communication network, support for passthrough and protocol conversion.