Comprehensive Explanation of the Modbus Communication Protocol

The Modbus protocol is a universal language used in electronic controllers, facilitating communication between controllers, networking via Ethernet, and interaction with other devices. It has become a widely adopted industrial standard, enabling control devices from different manufacturers to connect into industrial networks for centralized monitoring.

Transmission on Modbus Networks

The standard Modbus interface uses an RS-232C-compatible serial interface, defining pin connections, cables, signal levels, baud rates, and parity settings. Controllers can connect directly or through modems.

Communication in Modbus networks uses master-slave technology, where only one device (the master) can initiate transmissions (queries). Other devices (slaves) respond to data requests made by the master. Typical master devices include host computers and programmable instruments, while typical slave devices include programmable controllers.

A master device can communicate individually with slave devices or broadcast messages to all slaves. In individual communication, a response message is expected from the slave. In broadcast queries, no response is anticipated. The Modbus protocol defines the format for master device queries, including the device (or broadcast) address, function code, data to be sent, and error-checking field.

Slave devices respond with messages also structured by the Modbus protocol. These responses include an acknowledgment of the action to be taken, any data to be returned, and an error-checking field. If an error occurs during message reception or if a slave cannot execute the command, an error message is generated and sent as a response.

Transmission on Other Network Types

On other networks, controllers use peer-to-peer technology for communication, allowing any controller to initiate communication with other controllers. In such cases, controllers can act as either master or slave devices during individual communication processes. Multiple internal channels are provided to facilitate simultaneous transmission processes.

In terms of message structure, the Modbus protocol still adheres to the master-slave principle, even though network communication methods are peer-to-peer. If one controller sends a message, it is considered the master device and expects a response from the slave device. Likewise, when a controller receives a message, it constructs a slave device response format and returns it to the sending controller.

Query-Response Cycle

(1) Query

The function code in a query message informs the selected slave device of the desired action to perform. The data segment contains any additional information required for the slave device to execute the function. For example, function code 03 requests a slave device to read holding registers and return their contents. The data segment must include information for the slave device, such as which registers to start reading and how many registers to read. The error-checking field provides a means for the slave device to validate the message's content.

(2) Response

If the slave device generates a normal response, the function code in the response message corresponds to the function code in the query message. The data segment includes data collected by the slave device, such as register values or status. If an error occurs, the function code is modified in the response message to indicate that the response is erroneous, and the data segment includes a code describing the error. The error-checking field allows the master device to confirm the message's validity.

Two Transmission Modes

Controllers can be set to either of the two transmission modes (ASCII or RTU) in standard Modbus network communication. Users choose their preferred mode, including serial communication parameters (baud rate, parity, etc.). When configuring each controller, all devices on a Modbus network must select the same transmission mode and serial parameters.

(1) ASCII Mode

When a controller is configured to communicate in ASCII (American Standard Code for Information Interchange) mode on a Modbus network, each 8-bit byte in the message is transmitted as two ASCII characters. The primary advantage of this mode is that it allows a one-second character spacing without errors.

Code System:

Hexadecimal, ASCII characters 0...9, A...F

Each ASCII character in the message is represented as a hexadecimal character.

Bits per Byte:

1 start bit

7 data bits, with the least significant bit sent first

1 parity bit (optional)

1 stop bit (with parity) or 2 bits (without parity)

Error Checking Field:

LRC (Longitudinal Redundancy Check)

(2) RTU Mode

When a controller is configured to communicate in RTU (Remote Terminal Unit) mode on a Modbus network, each 8-bit byte contains two 4-bit hexadecimal characters. The main advantage of this mode is that it can transmit more data compared to ASCII mode at the same baud rate.

Code System:

8-bit binary, hexadecimal numbers 0...9, A...F

Each 8-bit field in the message consists of two hexadecimal characters.

Bits per Byte:

1 start bit

8 data bits, with the least significant bit sent first

1 parity bit (optional)

1 stop bit (with parity) or 2 bits (without parity)

Error Checking Field:

CRC (Cyclic Redundancy Check)

Wireless Modbus Implementation

By adhering to the Modbus communication protocol, it is possible to transform devices with RS-485/RS-232 interfaces into wireless Modbus communication devices. In other words, if wired Modbus network communication is working correctly, you only need to interface with hardware parameters compatible with the Modbus communication protocol through wireless modules (DTD433M/DTD434M/DTD435M/DTD418M/DTD419M) to achieve wireless Modbus network communication for multiple devices.