A Modbus gateway is a device that converts the Modubs TCP gateway protocol to the Modbus RTU gateway protocol. It can be understood as an industrial router that supports the modbus protocol. I believe that if you call it this way, you will understand it better. Modbus is widely used in the field of meters and sensors, and the data of the meters and sensors can be obtained.
1. Modbus RTU gateway
The modbus gateway is a network hub communication protocol conversion device of the industrial automation system. It is connected to the intelligent instrument at the bottom, collects the data of the lower instrument to the modbus gateway, and is connected to the computer network on the top, and communicates with the network computer according to the Modbus TCP gateway protocol.
DTU, RTU, and intelligent gateway are such "gateways". DTU is a device for mutual conversion between serial data and IP data. RTU is based on DTU and has control functions, which can collect the input and output of analog and switch values.
2. Modbus TCP gateway
Because the traditional RS485-based Modbus RTU gateway (or ASCII) has poor speed and scalability, Modbus TCP gateway, a network-based protocol, emerged later. Compared with the traditional modbus RTU protocol, the Modbus TCP gateway protocol is faster and more scalable!
Therefore, the converter of Modbus gateway came into being. It supports more protocols and more interfaces. Modbus gateways on the market now generally support 8 working modes: TCP client, TCP server, UDP client, UDP server, RTU_Master--TCP_Slaver, TCP_Master--RTU_Slaver, ASCII_Master--TCP_Slaver, TCP_Master--ASCII_Slaver.
Note: The mobus protocol over TCP transport is completely different from the modbus TCP protocol.
3. The difference between Modbus RTU gateway and Modbus TCP gateway:
3.1 Different concepts: MODBUS is a standard industrial control data exchange protocol, which can exchange protocol data through RTU and ASCII. RTU transmits data directly through binary data, while TCP transmits data in the form of TCP codes by converting each byte of binary data into a fixed two-digit hexadecimal string and then concatenating them in sequence. RTU is usually used the most.
3.2 Different communication modes: Ethernet, the corresponding communication mode is MODBUS TCP gateway. Asynchronous serial transmission (various media, such as wired RS-232-/422/485/; optical fiber, wireless, etc.), the corresponding communication mode is MODBUS RTU gateway or MODBUSASCII. High-speed token transmission network, the corresponding communication mode is ModbusPLUS.
4. Several common Modbus gateways for IoT wireless data communication
The Modbus gateway mentioned below refers to Modbus RTU gateway to Modbus TCP gateway, and does not involve the processing of Modbus ASCII data frames. Modbus ASCII only supports transparent transmission.
4.1 Simple protocol conversion
The most common and common Modbus gateway function, simple protocol conversion is the most efficient Modbus gateway mode for data processing. It only extracts key fields in the data, and then encodes the extracted data in another protocol (Modbus RTU gateway and Modbus TCP gateway) and sends it.
4.2 Multi-host gateway
The multi-host gateway can only work in the TCP server mode, and can process multiple Modbus TCP gateway host requests at the same time. The serial server receives other host requests when one host request is not completed. At this time, the serial server will perform on the RS485 bus. Arbitration output (in layman's terms, it blocks subsequent instructions).
4.3 Storage Gateway
The first thing to understand is that the communication rate of the serial port is much lower than that of the network port. The host usually only processes a few registers when requesting the RTU device. The "storage gateway" is used in this working environment.
4.4 Take the initiative to report
Active reporting is a mode created by the TCP host that does not support the Modbus protocol and still wants to query the RTU device. In this mode, the gateway directly acts as the host and automatically initiates a request to the connected RTU device, and sends the RTU device to the RTU device. The returned results are directly uploaded to the server. The server only needs to parse the returned data. Active reporting only supports client mode. Active reporting supports the return of data frames in two formats (Modbus RTU gateway and Modbus TCP gateway formats). In this mode, only Modbus data frames are supported and non-Modbus data frames are not processed.