How Are PLCs Classified? What Are the Key Performance Metrics?

How Are PLCs Classified? What Are the Key Performance Metrics?

PLCs can be categorized in the following ways:

By Structural Composition:

Integral PLC (or Unitary PLC): All components, including the power supply, CPU, and I/O interfaces, are integrated into a single housing.

Modular PLC (or Assembled PLC): Components such as the power supply module, CPU module, and I/O module are independently structured. These can be combined on a fixed rack or track based on specific application requirements to form a complete PLC system.

By I/O Point Capacity:

Compact PLCs: Typically have fewer than 256 I/O points, such as the Siemens S7-200SMART PLC.

Medium-Sized PLCs: Use modular structures and generally have I/O points ranging from 256 to 1024, such as the Siemens S7-300 PLC.

Large PLCs: Usually have over 1024 I/O points, such as the Siemens S7-400 PLC.

Key Performance Metrics of PLCs:

While PLCs from different manufacturers may vary in features, they share several common performance metrics:

Input/Output (I/O) Points: I/O points represent the number of external input and output ports connected to the PLC panel. The more I/O points there are, the more control capabilities the PLC has. This is one of the most critical factors when selecting a PLC.

Scanning Speed: This metric indicates how fast the PLC executes its program. It is typically measured in milliseconds required to execute 1K instructions.

Storage Capacity: Storage capacity is often expressed in terms of kilowords (KW), kilobytes (KB), or kilobits (Kbit), where 1K = 1024. Some PLCs offer expandable storage.

Instruction Set: The instruction set reflects the functional strength of the PLC's software. A richer instruction set means stronger programming capabilities.

Internal Registers (Relays): PLCs contain many registers for storing variables, intermediate results, and data. The configuration of these registers is also an indicator of PLC functionality.

Expansion Capability: This reflects how well the PLC can integrate additional modules for specialized functions, such as A/D, D/A, high-speed counting, or remote communication modules.

Comparison Between PLCs and Relay Control Systems:

Before the advent of PLCs, relay hardwired circuits were the primary means of logical and sequential control. They were simple and cost-effective but lacked flexibility. Since the introduction of PLCs, almost every aspect of their performance has surpassed that of relay control systems.

Development Trends of PLCs:

Moving toward higher performance, faster processing, and greater capacity.

Enhancing networking and communication capabilities.

Becoming smaller, more cost-effective, and easier to use.

Continuously improving programming software functionality.

Developing new modules tailored for PLC applications.

Advancing the miniaturization and software-based evolution of PLCs.