Overview
What are PLC CPUs & Processors?
PLC CPUs (Central Processing Units) are the core hardware components of a Programmable Logic Controller. They are responsible for executing the control program, managing data memory, and coordinating communication between input/outputs (I/O), HMI interfaces, and networked drives. In modular systems, the CPU is typically a standalone module that slides into a chassis or mounts on a DIN rail, whereas in compact PLCs, the processor is integrated into the main unit.
Main Manufacturers
PALM Parts Solution stocks a comprehensive inventory of CPU modules from the industry’s leading automation brands:
- Allen-Bradley: High-performance processors for discrete and process applications.
- Siemens: Industry-standard CPUs for modular and PC-based control.
- Mitsubishi Electric: High-speed processing for complex motion and logic.
- Schneider Electric: Robust processors for infrastructure and industrial machine control.
- GE Fanuc / Emerson: Reliable logic controllers for power and utility sectors.
- Omron, ABB, Delta Electronics, B&R Industrial Automation, and Lenze: Specialized processors for high-speed packaging, robotics, and textile machinery.
Typical Applications
PLC processors are found in virtually every automated industrial environment. Standard applications include:
- Material Handling: Coordinating conveyors, sorters, and Palletizers using ControlLogix or SIMATIC S7-1500 processors.
- Motion Control: Synchronizing multi-axis servo systems with MELSEC iQ-R or Lexium controllers for precision packaging.
- Process Automation: Managing PID loops and analog data in chemical and water treatment plants using ABB AC800M or Schneider Modicon units.
- Machine Safety: Utilizing "GuardLogix" or "F-series" fail-safe processors to manage SIL3/PLe safety functions alongside standard logic.
Selection Guide
Choosing the correct processor requires matching the hardware to the application's complexity:
- I/O Count: Determine the total number of local and remote points. A CompactLogix 5370 may suit a small machine, while a ControlLogix 5580 is required for plant-wide control.
- Memory Requirements: Ensure the CPU has sufficient program and data memory (KB/MB) to handle the logic and data logging requirements.
- Network Topology: Select a CPU with built-in ports for the required protocol (e.g., dual Ethernet ports for DLR or PROFINET MRP redundancy).
- Environment: For extreme temperatures or corrosive atmospheres, source "XT" (Extended Temperature) or conformal-coated versions of Siemens or Allen-Bradley processors.
4 products · ABB
Frequently asked questions
What is the difference between a PLC CPU and an I/O module?
A PLC CPU (Central Processing Unit) acts as the 'brain' of the automation system, executing logic and managing I/O data. An I/O module is a peripheral device that interfaces with physical sensors and actuators. The CPU processes program instructions and then sends commands to the I/O modules to control field equipment.
How do I know if a replacement CPU is compatible with my existing rack?
Compatibility depends on the backplane and chassis architecture. For example, an Allen-Bradley ControlLogix 5580 CPU requires a 1756 chassis, while a Siemens SIMATIC S7-1500 CPU uses a specific U-connector for its modular rail. Always verify the firmware version and backplane power consumption (Current Draw) before upgrading a processor within an existing rack.
What are the primary specifications to consider when selecting a PLC processor?
Key specifications include memory capacity (user memory and retentive data), execution speed (scan time per 1k instructions), communication ports (Ethernet/IP, PROFINET, Modbus TCP), and the maximum number of local and remote I/O points supported. For motion control, ensure the CPU supports integrated CIP Motion or equivalent high-speed synchronization.
Which communication protocols do these CPUs support?
Most modern high-performance processors rely on Industrial Ethernet protocols like EtherNet/IP (Allen-Bradley), PROFINET (Siemens), or EtherCAT (Beckhoff/B&R). For legacy communication, many CPUs still offer RS-232, RS-485, or DH+ via optional plug-in modules or integrated ports.
Can I replace an obsolete PLC CPU with a newer model?
Yes, manufacturers often provide migration paths. For example, moving from a SLC 500 to a CompactLogix 5380, or from a Siemens S7-300 to an S7-1500. While physical wiring and hardware change, software utilities often help convert legacy code to the newer environment, though manual logic adjustments are usually required.