MODBUS Serial

The Language of Industrial Communication: What is Modbus Serial (RTU/ASCII)?

Developed by Modicon in 1979, Modbus Serial is the oldest, simplest, and most widely used communication protocol in the industrial automation world. Today, it is still possible to find a Modbus port on almost every sensor, variable frequency drive (VFD), and PLC.

1. How Does Modbus Serial Work?

Modbus Serial is built on a Master-Slave architecture. On a single network, there can be only one Master (the one that sends requests) and up to 247 Slaves (the devices that respond).

Working Principle:

Request: The Master sends a command to a specific Slave address (e.g., Device 5).
Response: Only the Slave with the matching address responds. Other devices remain silent.

2. Two Different Modes: RTU vs. ASCII

Modbus Serial can be used in two different formats:

Modbus RTU (Remote Terminal Unit): Data is sent in binary format. This is the most common form because it is faster and consumes less bandwidth.
Modbus ASCII: Data is sent as readable characters (0-9, A-F). It is easier for debugging but is approximately twice as slow as RTU.

3. Physical Layer: RS-485 and RS-232

The Modbus Serial protocol typically runs over two different physical connections:

RS-485: The industrial standard. It can provide communication up to 1200 meters over just two wires (+ and -). It allows multiple devices (multi-drop) to be connected to the same line.
RS-232: Generally used for point-to-point (PC to device) short-distance connections.

4. Modbus Data Types (Register Structure)

In the Modbus protocol, data is stored in four primary tables:

Data Type	Read/Write	Description
Coils	Read/Write	1-bit digital outputs (e.g., Turn relay ON/OFF).
Discrete Inputs	Read Only	1-bit digital inputs (e.g., Sensor status).
Input Registers	Read Only	16-bit analog data (e.g., Temperature measurement).
Holding Registers	Read/Write	16-bit general-purpose settings (e.g., Setpoint).

5. Modbus Serial Applications with Hubbox

The Hubbox Connect series, with its built-in RS-485/232 ports, carries legacy field devices into the modern IIoT world.

Data Conversion: Hubbox reads Modbus RTU data from the field and converts it to Modbus TCP or MQTT protocols to be transferred to ERP/SCADA systems.
Node-RED Integration: You can visualize complex Modbus register maps in seconds and filter out faulty readings (CRC errors, etc.).
Remote Access: Without going to the field, you can update device parameters over the serial port through a secure tunnel created by Hubbox.

6. Summary: Why Still Modbus Serial?

Cost: Hardware and software implementation is extremely inexpensive.
Simplicity: It does not require complex libraries.
Universality: It acts as a "universal language" where devices from every brand can talk to each other.