What is Modbus Heartbeat?

Modbus is one of the most used fixed gas detectors. In this post, I will explain what a Modbus heartbeat is and how it is used.

What is Modbus Heartbeat?

A Modbus heartbeat is either a bit or a holding register that changes the state for external devices to tell that the controller (mostly the PLC) is running. 

What is the main function of the Modbus Heartbeat?

The main function of the heartbeat is to facilitate the detection of communication problems in programming environments where the transport layer communication error information is unavailable.

This increments approximately every 5 to 10 seconds. The system integrator is responsible for notifying plant personnel if a Modbus master ( the PLC or DCS system) fails to communicate with the transmitter. This register can facilitate this notification.

How is Modbus Heartbeat implemented?

The implementation of a heartbeat depends upon the master and slave in question. For most master applications, a heartbeat can be any simple message sent out to each slave to ensure they are online and in communication, even if they have nothing new to report.

The master’s timer register will expect the information every 5 seconds (some manufacturers have this time of 10 seconds).

The external device communicating with this device would compare over time to see if the register changes. If it does not, the external device will declare either a master failure or a comm fail.

In most applications, there is no need to implement the heartbeat. This is because most Modbus slave units do not respond to Modbus commands unless they are operating. In such cases, use a normal Modbus Read function.

What is heartbeat in PLC?

A programmable logic controller (PLC) periodically sends a heartbeat signal to indicate it is still functioning properly. The PLC’s watchdog timer uses the signal to confirm that the PLC is running correctly.


The Modbus heartbeat is like a watchdog that ensures communication between the master and slave in a Modbus communication.

This bit or register is not widely used in industrial communication, as most slave devices will not respond to a master’s queries if they are not in communication.

Leave a Reply