How to Choose a PLC for an Automation System

The choice of PLC (programmable logic controller) determines how reliable, flexible, and maintainable the entire automation system will be. A mistake at this stage is costly: a controller sized with no headroom has to be replaced at the very first upgrade, and unsuitable interfaces make it hard to connect SCADA. In this article, Atlas Scada engineers break down the criteria for choosing a PLC and the mistakes to avoid.

Counting inputs and outputs

First, draw up a list of the facility's signals and sort them by type

• Discrete inputs/outputs — on/off signals: pump status, a command to a valve.
• Analog inputs/outputs — continuous values: temperature, pressure, 4–20 mA, 0–10 V.

Count the number of points with a 15–20% reserve for future growth — this is cheaper than replacing the controller when the facility is expanded.

Performance and memory

For simple tasks (a boiler room, a pumping station) a compact controller is enough. For complex logic, a large number of points, and fast processes, you need a more powerful processor and a larger amount of memory. A reserve of resources ensures stable operation and room for upgrades.

Protocols and communication

To transmit data to SCADA and dispatch systems, the controller must support the required interfaces and protocols — Modbus RTU/TCP, OPC UA, Ethernet. If remote monitoring is planned, Ethernet and support for industrial protocols are mandatory.

Operating conditions

Take into account the temperature, humidity, dust, and vibration at the facility. For outdoor and unheated premises, choose controllers with a wide operating temperature range and provide cabinet heating or climate control.

Support and availability

The availability of the controller and components on the Kazakhstan market, the presence of specialists, documentation, and a development environment all matter. An exotic brand without support is a risk of being left without service for a long time.

Common selection mistakes

• Sizing the PLC with no headroom on the number of inputs/outputs — no reserve for growth.
• Ignoring communication protocols — connecting SCADA later becomes impossible.
• Disregarding operating conditions — the controller fails.
• Choosing by price without accounting for support and spare-parts availability.

Popular controllers

In practice, the most commonly used are Siemens (LOGO!, S7-1200, S7-1500), OWEN (PLK110/160/210, PR200), Schneider Electric (Modicon M221/M241/M340), and Delta. The choice depends on the task, the budget, and availability.

Modularity and expandability

It is convenient when the controller is modular: input/output modules are added to the base unit as the facility grows — this is cheaper than replacing the controller as a whole. For small tasks, a monoblock PLC with a fixed set of points will do.

Discrete or analog signals

For simple control (start/stop, open/close) discrete channels are enough. If you need to regulate temperature, pressure, or flow, analog inputs and outputs and PID control support will be required — this is factored into the controller choice right away.

Compatibility with existing equipment

If the facility already has controllers, operator panels, or SCADA, it is better to choose a new PLC from the same ecosystem or with support for the same protocols — this simplifies integration and maintenance, while a single development environment reduces staffing costs.

The key points in brief

The right PLC means a sufficient reserve of inputs/outputs and performance, the required communication protocols, conformance to operating conditions, and accessible support. If you are unsure of your choice — we will help you select a controller for the task and program it right away. For more on what a PLC is and how it works, read the separate article, and for the service itself, see the "PLC programming" page.