Collecting Datawith Modbus

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. (Definition provided bySchneider Electric)

否定是最古老的工业自动化communication protocol and is offered acrossmany types and brands of devices. It’s a common language used for connecting automation devices and equipment, enabling them to communicate within the connected factory or network. It’s often used for process automation and systems like SCADA.

Having been originally developed to work over Serial networks, Modbus messages are now able to be sent over Ethernet and TCP/IP.

Image Source:Virtual Serial Port

Modbus was initially designed as a serial communication protocol. It allows data transmission via serial bus lines for communication between connected devices. It’s an open-source protocol, so developers can create devices to fit into its ecosystem.

Modbus transmits data from devices to a controller or data collection system such as a machine data platform. This transmission may include temperature, humidity, fill levels, deviation alarms, setpoint, sensor failure, and more. It’s highly used in SCADA systems, enabling managers to deploy solutions based on insights generated by an MES or machine data platform.

Because it’s the oldest communication standard used in industrial automation, Modbus has the distinction of more or less being its own standard. It’s managed by the Modbus organization and has grown to become a free open-source tool for providing devices with an automation protocol for industrial use.

Modbus offers many options for use as a transport layer and was designed for use over serial bus lines. It can also be used with EtherNet and the Internet Protocol (IP) suite.

The primary use of Modbus is over serial lines. It works on both RS-232 and RS-485 serial communication standards; it can also run with Serial RS-422 andEtherNet. When operating on RS-485, Modbus can accept longer cable distances and higher speeds. It also allows multiple devices to be deployed across a single multi-drop network.

Modbus uses a traditional master-slave or server/client scheme to operate. The master operates as the server unit, and slave units supply data to the master unit. Slave units may be I/O ports, network drives, and devices that measure machine conditions.

Image Source:Control Global

As the server unit, the master may send instructions to one, all, or a select group of slaves. In RS-485 systems, there can be as many as 247 slaves per master unit. Data is transmitted from the machine and collected by the slave units. The data is then sent in binary code to the master. The RS-485 protocol also uses a two-wire connection with differential voltage, meaning the system can realize two-way data transfer.

Modbus uses hexadecimal to group data together, making troubleshooting the system easier. Slave units hold the data in one of four tables. When transmitted, the slave device address is sent in the header. Master units send a function code, so slave units know the correct table to access and whether they are to read or write data to the table.

Modbus has several versions, including Modbus RTU, Modbus ASCII, and Modbus TCP. The distinction with Modbus ASCII is that it uses an ASCII table instead of hexadecimal tables for data storage within slave devices.

Modbus is a free, open-source protocol.

Modbus has many benefits for users. They include:

1. Reliability: Modbus is the oldest industrial automation communication protocol around. It’s simple to use and program, so learning curves are low.
2. Open-Source: Modbus is an open-source protocol, so any vendor can use it for a broad range of connectivity andsmart sensor devices.
3. Legacy Infrastructure: Many manufacturers invested significantly in early automation. Modbus is highly friendly to configurations, DLR, nodes, drops, and other infrastructure that could be obsoleted with a new or more advanced protocol.
4. Rapid Deployment: Modbus is easily and immediately integrated into SCADA and other control systems.
5. Flexibility: Modbus has adapted to new and emerging technology. For example, Modbus TCP can be converted with adaptors to integrate with LAN and remote-control systems. It can also utilize web-based and cloud-based platforms.
6. Simplicity: Because communication is simple, it can easily be extended to new technologies. For example, Modbus TCP/IP is fast-deployed because of the simple instruction set. It can also work with EtherNet without added chips or boards.

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