How Does Modbus RTU Exchange Data?

• Master
• Slaves

What you will learn

• Describe the roles of master and slave.
• Name the parts of a Modbus RTU message.
• Explain why the silent gap matters.

Before you start

It helps to know what UART and differential signals are.

How does Modbus ask and answer?

Modbus is a request-response protocol implemented using a master-slave relationship. The initiating device, the master, is responsible for initiating every interaction. A conversation is always started by a master in the network.

For unicast, the address identifies the receiving slave; the other slaves ignore that request. A slave normally replies to a valid unicast request sent to its address. Address 0 is the broadcast address: slaves should process a valid write request sent to it, and none should reply.

• Master
• Slave with the matching address
• Request
• Response

What parts does a Modbus RTU message have?

With Modbus RTU the data is exchanged in a binary format, where each byte of information is coded in one communication byte. A message holds an address, a function code, data and an error check.

Modbus masters must request access to the slave's data through various function codes. Not all Modbus devices recognize the same set of function codes.

• Address
• Function code
• Data
• Error check

When answering, a slave normally uses the same function code as in the request. In an exception response, the most significant bit of the original function code is set. An error check is appended so the receiver can check whether a communication error occurred.

How does a receiver know a message has ended?

Modbus RTU uses time gaps of silence on the communication line for framing. So each message has to be sent as a continuous stream; gaps between characters must stay below the framing threshold.

A device configured for Modbus ASCII cannot understand Modbus RTU messages and vice versa, so both sides must use the same mode.

• Silent gap
• One continuous message

Real-world example

With an adult, think of an electricity meter in a building. The monitoring system asks the meter for a value, and only the meter with the right address answers.

Common misunderstandings

A slave can send data whenever it wants. A conversation is always started by a master.

Every Modbus device understands every function code. Not all devices recognize the same set of function codes.

Frequently asked questions

How do master and slave differ?

The master initiates a request; a slave normally replies to a valid unicast request sent to its address. No slave should reply to broadcast address 0.

What is in a Modbus RTU message?

An address, a function code, data and an error check.

What does a function code with its highest bit on mean?

In an exception response, the most significant bit of the original function code is set.

How does Modbus RTU mark frames?

With time gaps of silence on the communication line.

Summary

• Modbus is request-response: the master starts; a slave normally answers valid unicast, while slaves should not reply to broadcasts.
• A Modbus RTU message holds an address, a function code, data and an error check.
• Silent gaps mark frames, so each message must be sent continuously.