In this article, we will go beyond the basic concept and take a closer look at how Modbus RTU works, its typical real world applications, and the correct wiring method. We will also explore why Modbus RTU over RS485 has remained one of the most widely trusted communication standards in industrial automation since 1979.

WHAT ARE MODBUS RTU AND MODBUS RTU RS485?

Modbus RTU is used to exchange data between devices in automation systems, including PLCs, sensors, measurement instruments, variable frequency drives, controllers, HMIs, and BMS or SCADA systems.

Modbus was developed for industrial control systems. In essence, it is an application layer communication protocol that can operate over different physical transmission methods. In serial communication, RS485 is the physical layer most commonly used with Modbus RTU.

Note: Modbus RTU and RS485 are not the same thing. Modbus RTU defines how data is organized and exchanged, while RS485 defines how the electrical signal is transmitted over the cable → "Modbus RTU RS485" refers to a system in which the Modbus RTU protocol operates over the RS485 physical layer.

A simple way to visualize this is:

• Modbus RTU → communication language
• RS485 → signal transmission path

Modbus RTU and RS485 are an excellent choice for systems that need to collect data from multiple devices while reducing signal cabling, simplifying control architecture, and allowing the system to be expanded easily in the future.

One important factor that system designers should remember is the baud rate of Modbus RTU over RS485. It typically ranges from 1200 to 115200 bps, with 9600 and 19200 bps being the most common values. All devices connected to the same bus must be configured with the same baud rate and data format.

HOW DOES THE MODBUS RTU PROTOCOL WORK?

The Modbus RTU protocol operates using a master and slave architecture. In this architecture:

One master device, typically a PLC, central controller, or HMI, initiates all communication on the network.

Slave devices, each assigned a unique address, can include up to 247 devices on a single bus and respond when the master sends a command or request to their specific address.

The entire communication process follows this sequence: Master sends a command → slave receives the correct address → slave processes the request and sends a response.

The master and slave operating principle of Modbus RTU helps prevent signal collisions because only one device responds at a given time.

MODBUS RTU DATA FRAME STRUCTURE

A Modbus RTU data frame consists of four main sections arranged in a fixed sequence:

Slave device address: 1 byte, with values from 1 to 247. Address 0 is reserved for broadcast commands sent simultaneously to all slave devices.

Function code: 1 byte that specifies the operation to be performed, such as reading registers, writing values, or reading input and output states.

Data field: Variable length depending on the function code.

CRC 16 error checking code: 2 bytes calculated based on the preceding contents of the data frame. The receiving device calculates the CRC again and compares the result. If the two values do not match, the frame is identified as corrupted and discarded. This helps prevent electrical interference or data corruption from causing incorrect commands. Between two consecutive frames, there must be a minimum silent interval equivalent to the transmission time of 3.5 characters, which indicates the end of one frame and the beginning of another.

WHY IS MODBUS RTU WIDELY USED IN INDUSTRIAL AUTOMATION?

Imagine that every field sensor requires two dedicated wires back to the PLC. Two sensors would require four wires, three sensors would require six wires, four sensors would require eight wires, and so on. As the number of devices increases, the amount of wiring increases linearly, quickly making the system more complicated.

Modbus RTU solves this problem by allowing multiple sensors and measurement devices to communicate over the same network. With a single pair of communication wires, the protocol can connect dozens or even hundreds of measurement points, depending on the network design and device limitations. In particular, a Modbus RTU sensor can provide not only measurement values but also device status, alarms, configuration parameters, and other information depending on the model.

In other words, Modbus RTU allows users to establish a digital communication channel between field devices and the control system. This is particularly useful for large industrial automation systems with many distributed measurement points, multiple sources of electromagnetic interference, and frequent requirements for future expansion.

MODBUS RTU APPLICATIONS IN INDUSTRIAL SYSTEMS

The most common applications of the Modbus RTU protocol in industrial systems include:

• Multiple sensors connected to a single PLC.
• Multiple energy meters installed in one electrical control panel.
• Variable frequency drives connected to a central controller.
• HVAC equipment connected to a BMS.
• Distributed controllers and I/O modules.

Specifically:

In HVAC systems: AHU and FCU control panels are often equipped with Modbus RTU communication to transmit temperature, humidity, damper actuator status, alarm signals, and other operating data to the BMS. This allows operators to monitor the entire facility from a single interface without manual intervention.

In industrial sensors: Temperature sensors, humidity sensors, force sensors, gas concentration sensors, and other measurement devices can be equipped with Modbus RTU outputs to transmit real time measurement data to PLC or SCADA systems. This data can be used for early warning, data logging, and continuous automated operating decisions.

In energy monitoring systems, motor variable frequency drives, modern lighting systems, and environmental monitoring stations, where stable communication over long distances is required but high bandwidth is not necessary, Modbus RTU is also widely used.

HOW TO WIRE MODBUS RTU RS485

In general, devices on an RS485 bus are connected in a daisy chain configuration. In a practical network, the wiring arrangement of devices in a Modbus RTU RS485 network can be visualized as follows:

PLC/Controller → Device 1 → Device 2 → Device 3 → Device 4 → Device 5

Rather than:

PLC → Device 1
PLC → Device 2
PLC → Device 3
PLC → Device 4
PLC → Device 5

This means:

• The A wire of one device is connected to the A wire of the next device, while the B wire is connected to the B wire. The same principle is followed throughout the network.
• A common GND signal wire can be routed along the entire network and connected to the GND or COM terminal of each device when required by the manufacturer's wiring specification.

Note: Termination resistors with a typical value of 120 ohms should be installed at the two farthest ends of the RS485 bus to absorb signal reflections and reduce communication interference. This becomes particularly important for long bus segments or high communication speeds. In addition, each slave device must be assigned a unique address. Duplicate addresses must be avoided to prevent communication conflicts on the network.

In summary, Modbus RTU combined with RS485 is one of the most important and widely adopted communication methods in industrial automation today. It not only significantly reduces system cabling costs but also improves data acquisition and centralized system management.

For detailed information about the Modbus RTU protocol, Modbus RTU RS485, or sensors and measurement equipment with Modbus RTU communication, please contact Sensors Vietnam directly via hotline 0911 660 660 or visit the websites https://cambien.com.vn/ | https://www.sensors.vn/ for professional and prompt technical consultation.