Industrial Protocols: Modbus and OPC UA Study Guide

This set of vocabulary flashcards covers the historical evolution, architectural differences, data models, and security profiles of the Modbus and OPC UA industrial protocols.

Modbus

An application layer messaging protocol developed by Modicon in $1979$ for early Programmable Logic Controllers (PLCs), designed for simplicity and strict request-response interaction.

Master (Client)

The device in a Modbus communication model that initiates all interactions with one or more destination devices.

Slave (Server)

A device in a Modbus network that responds to the requests initiated by a Master (Client) device.

Modbus RTU

A serial version of the Modbus protocol using $RS-232/RS-485$ for point-to-point or daisy chain connection, typically operating at low speeds (kbps).

Modbus TCP

A version of Modbus that runs on standard Ethernet using the $TCP/IP$ stack on Port $502$, supporting high speeds from $10\text{Mbps}$ to $1\text{Gbps}$.

Discrete Inputs

A Modbus data type representing read-only single-bit values, such as limit switch status, often identified with the prefix $1xxxx$.

Coils

Read/write single-bit data in Modbus used for relay or valve control, identified with the prefix $0xxxx$.

Input Registers

Read-only $16$-bit unsigned word data in Modbus, such as unscaled ADC temperature values, identified with the prefix $3xxxx$.

Holding Registers

Read/write $16$-bit unsigned word data in Modbus used for configuration settings or setpoints, identified with the prefix $4xxxx$.

MBAP Header

A $7$-byte header in a Modbus TCP packet containing the Transaction ID, Protocol ID, Length, and Unit ID.

Function Code

A $1$-byte instruction in the Protocol Data Unit (PDU) that tells the server what action to perform, such as $03$ for Read Holding Registers.

Modbus Security

A variant introduced in $2018$ that blends traditional Modbus with Transport Layer Security (TLS) and $X.509v3$ certificates for encryption and authentication.

OPC Classic

The first solution (also known as OLE for Process Control) created in $1995$ as a universal translator, but restricted by its dependency on Microsoft's COM/DCOM technology.

OPC Unified Architecture (UA)

A platform-independent, service-oriented architecture (SOA) launched in $2006$ that integrates all classic sub-specifications into one extensible protocol.

Information Modeling

A feature of OPC UA where data tags include rich metadata defining complex relationships rather than just referencing raw memory addresses.

OPC Server

The central hub in an OPC UA architecture that converts proprietary plant protocols (like Modbus or S7) into structured OPC tags.

OPC Client

The consumer of data in an OPC UA system (such as SCADA or HMI) that queries the server to retrieve tag values without needing to know the physical device's protocol.

OPC Gateway

A component that bridges distinct network segments to facilitate secure data transfer from the OT (Operational Technology) layer to the IT or Cloud layer.

RBAC (Role-Based Access Control)

An authorization method used in OPC UA to provide granular control where access (reading or writing) is granted based on user roles.

X.509 Digital Certificates

Used by OPC UA to perform strict identity verification and authentication for both applications and users.