thinQmaster Virtual testing partner Adaptive practice exams that find your weak spots and drill them. Every answer explained. From $69
Enroll

thinQmaster

Find your exam

Five questions free, before you decide.

Step 1 · choose a vendor

In one of our camps? Your practice exams come with the tuition — they are on your study page already.

Application & OT Security · Foundational

Modbus

A simple master/slave (client/server) industrial communication protocol created by Modicon in 1979, widely used to read and write PLC registers.

Definition

Modbus RTU runs over serial links and Modbus/TCP over TCP port 502. Classic Modbus has no authentication or encryption, so any device that can reach port 502 can read or write coils and registers. Defenses include segmentation, allowlisting of Modbus clients and function codes in industrial firewalls, monitoring, and Modbus/TCP Security (TLS on port 802) where supported.

Example

An industrial firewall allows only the HMI's IP to send Modbus write function codes to the PLC.

Exam tip

Modbus/TCP = port 502, no native authentication.

Related terms