Programmable Logic Controllers: Core Components & Types

Understanding Programmable Logic Controllers (PLCs)

A Programmable Logic Controller (PLC) is an electronic device capable of processing analog variables, digital, numeric, and alphanumeric information. PLCs are designed for real-time monitoring and control of automated industrial processes.

External circuits consist of sensors, which send information from the industrial process to the automaton, and actuators, responsible for executing the program's commands.

PLC Memory

The primary function of PLC memory is to store program instructions, data, internal states, and input/output states. PLC memory is typically organized into distinct work areas.

Memory Types

• RAM (Random Access Memory): Read-write memory. In the absence of power, information volatilizes and disappears. If information needs to be retained, a battery backup is required.
• ROM (Read-Only Memory): Memory where data can only be read, meaning data must be recorded beforehand.

Inputs and Outputs

• Digital Inputs: Change state based on voltage changes (an 'all or nothing' state), corresponding to maximum or minimum power supply voltages.
• Analog Inputs: Measure voltage or current values, triggering events when predefined thresholds in the user program are reached.
• Passive Sensors: Have two distinct states (non-activated/activated), with activation typically occurring through a mechanical action.
• Active Sensors: Electronic devices requiring a voltage supply, which changes their logic state.

Classification of PLCs

• Compact PLCs: Contain all essential elements (I/O, CPU, power supply) within a single enclosure.
• Semi-Compact PLCs: Have some elements located outside the main enclosure.
• Modular PLCs: Each element is housed in a different enclosure, installed on a common rack.

Potential for PLC Expansion

As automated processes increase in complexity, the number of necessary inputs and outputs (both analog and digital) also increases proportionally. This necessitates an increase in modules connected to the PLC or, where appropriate, higher processing power. Most PLCs allow interconnection with dedicated expansion modules.

In compact and semi-compact PLCs, expansion capabilities are quite limited; however, in modular systems, they are significantly enhanced.

Industrial Buses

A bus is an interconnection system that allows multiple devices to communicate with each other using a minimal number of wires.

Among the bus systems widely used in industrial automation, Profibus and AS-i bus are prominent.

Profibus

Profibus is a universal, high-performance network developed specifically for industrial automation. It enables electronic devices from different manufacturers to communicate seamlessly.