Selecting the Right PLC for a Job

When designing or upgrading an automated control system, selecting the right Programmable Logic Controller (PLC) is a crucial step. PLCs serve as the backbone of many industrial systems automating tasks, increasing accuracy, improving reliability, and reducing human error. However, with a broad range of PLC options available, especially from Allen-Bradley, it’s important to choose a model that aligns with your system’s needs in size, performance, I/O capacity, and environmental durability.

Core Function of a PLC

At its most basic level, a PLC collects input signals from field devices (such as sensors or switches), processes logic-based programs, and activates outputs that control machinery or other equipment. Applications range from simple motor start/stop control to complex, multi-axis motion systems.

Allen-Bradley offers PLCs across a spectrum of use cases:

• MicroLogix series – Small applications

• CompactLogix series – Medium control systems

• ControlLogix series – High-performance, large-scale automation

Key PLC Selection Factors

1. Enclosure and Mounting Requirements

All Allen-Bradley PLCs are DIN-rail or panel mountable, but when integrating into an existing system, it's essential to confirm physical fit and enclosure compatibility. For environments with dust, oil, or moisture, a properly rated NEMA or IP enclosure may be required to protect the hardware.

2. Input and Output (I/O) Needs

Determine the number and type of inputs and outputs:

• Digital I/O: For discrete devices like push buttons, relays, or proximity sensors.

• Analog I/O: For devices such as RTDs, thermocouples, or variable frequency drives.

• Voltage/Current Ratings: Ensure I/O modules support your system's electrical requirements.

Modular PLCs like CompactLogix and ControlLogix allow you to tailor I/O configurations by adding modules. For small jobs, fixed I/O controllers such as the MicroLogix 1000 or 1100 may be sufficient.

3. System Size and Expandability

Allen-Bradley’s PLC families scale as follows:

• MicroLogix: Compact, cost-effective, fixed I/O options

• CompactLogix: Modular I/O and advanced networking

• ControlLogix: Largest memory, I/O support, and performance

Choose a platform that supports current needs and anticipated expansion, avoiding unnecessary complexity or expense.

Feature and Performance Considerations

• Memory & Speed: Applications that require frequent updates, fast scan times, or intensive math (e.g., PID control, motion control, or encoder feedback) will benefit from higher-performance PLCs with faster CPUs and more memory.

• Programming Capabilities: Most Allen-Bradley PLCs support ladder logic; mid- to high-end models also support structured text, function block, and SFC programming via RSLogix 500 or Studio 5000.

• Communication Protocols: Modern automation often requires Ethernet/IP, Modbus TCP, or DeviceNet support. Many CompactLogix and ControlLogix models include built-in Ethernet ports for seamless integration with HMIs, drives, and SCADA systems.

Environmental Conditions

Select a PLC rated for your site’s conditions:

• Temperature Tolerance: Ensure operating temperature is within spec.

• Ingress Protection: Dust, water, or corrosive environments may require IP65+ enclosures.

• Hazardous Locations: Class I, Division II environments require specially rated PLCs and components.

Programming Interface

• PC Interface: Most programming is done via a PC using Allen-Bradley’s software. All current-generation PLCs support this method.

• Handheld Programming: Some MicroLogix models support handheld programming for field convenience.

• Remote Access: Ethernet-enabled PLCs can be programmed and monitored remotely, improving maintenance response time.

Modular vs Fixed Systems

Modular PLCs like the ControlLogix and CompactLogix families offer maximum flexibility, with the ability to expand memory, add I/O, or incorporate advanced communication and motion modules.

Fixed PLCs like those in the MicroLogix family are ideal for simpler, space-constrained applications where minimal expansion is expected.

Final Considerations

When selecting a PLC:

• Start with a detailed assessment of I/O count, voltage types, and communication requirements.

• Match your system size and performance needs to the appropriate PLC tier.

• Consider future scalability and environmental conditions.

• Choose a system with appropriate programming flexibility and network connectivity.

From a compact MicroLogix 1100 for controlling a pump panel to a powerful ControlLogix system managing entire production lines, Allen-Bradley offers a controller to meet every automation challenge.

Need Help Selecting a PLC?
Reach out to PLG Automation for assistance with specifying or sourcing the right Allen-Bradley PLC for your application.
📧 Email: sales@plgautomation.com
📞 Phone: 800-906-9271