How to choose a programmable controller: where to start
PLC selection starts not with a catalog, but with your technical requirements. How many I/O points? Are there analog signals or only discrete? Do you need positioning? What network — Modbus, PROFINET, Ethernet/IP? Answering these questions narrows hundreds of models down to 3-5 specific options.
We select controllers for projects every day — from small panels for pump stations to distributed systems spanning entire workshops. Here is the algorithm we use ourselves.
Step 1: count your I/O points
The first and most important parameter is I/O count. Tally all discrete inputs (buttons, limit switches, relays), discrete outputs (contactors, valves, indicator lights), analog inputs (sensors for pressure, temperature, level), and analog outputs (VFD control, regulating valves).
- Up to 20 I/O: smart relay (Mitsubishi Alpha2, Siemens LOGO!)
- 20-100 I/O: compact PLC (Delta DVP, Mitsubishi FX5U)
- 100-300 I/O: compact PLC with modules (Siemens S7-1200, FX5U-80M)
- 300+ I/O: modular PLC (Siemens S7-1500, Mitsubishi iQ-R)
Add 20-30% reserve for expansion. If you need 40 I/O now, choose a controller that can expand to 60-80. Migrating to a different CPU mid-project is expensive and painful.
Step 2: identify signal types
Discrete control (on/off) is the simplest. Any PLC handles that. But once analog signals appear, the choices narrow.
| Signal type | Example | PLC requirement |
|---|---|---|
| Discrete input 24V | Button, limit switch | Standard DI |
| Discrete output | Contactor, valve | Relay or transistor DO |
| Analog 4-20 mA | Pressure sensor | 12-16 bit AI module |
| Analog 0-10 V | Temperature sensor | AI module |
| Thermocouple / RTD | Pt100, Type K | Specialized AI module |
| Pulse output | Stepper motors | PTO/PWM output (transistor) |
Pay attention to output type: relay outputs (DO Relay) handle higher current (2A) but are slow. Transistor outputs (DO Transistor) are fast and suitable for PWM and pulse outputs but limited to 0.5A. Servo drive control requires transistor outputs specifically.
Step 3: communications and integration
A controller rarely works alone — it communicates with HMI panels, VFDs, sensors, and SCADA systems. Check which protocols your project needs:
- Modbus RTU (RS-485): the most common. Supported by nearly all PLCs. For VFDs, sensors, meters
- Modbus TCP (Ethernet): faster Modbus variant. Requires an Ethernet port on the PLC
- PROFINET: Siemens ecosystem. Only S7-1200/1500
- CC-Link: Mitsubishi ecosystem. Only FX5U/iQ-R
- Ethernet/IP: Allen-Bradley (Rockwell). Rare in Ukraine
- OPC UA: universal protocol for IoT and Industry 4.0
If the project mixes equipment from different manufacturers, Modbus RTU/TCP is the safest choice. Everyone supports it.
Step 4: programming environment
This is often overlooked, which is a mistake. The environment affects development speed, license costs, and availability of qualified engineers.
- Delta ISPSoft: free, simple, LD/FBD/ST. Few specialists in Ukraine
- Mitsubishi GX Works3: powerful, 5 IEC languages, simulation. License from $300
- Siemens TIA Portal: most widespread in Ukraine, Basic version free for S7-1200. Largest pool of specialists
If the facility already has maintenance staff trained on Siemens — choose Siemens. Retraining costs time and money.
Step 5: budget and availability
PLC price is only part of the system cost. Add expansion modules, software license, HMI panel, power supply, and programming time.
| Manufacturer | CPU (basic) | Software | Availability in Ukraine |
|---|---|---|---|
| Delta DVP | $80-300 | Free | In stock, 2-5 days |
| Mitsubishi FX5U | $400-1,500 | $300+ | Made to order, 2-4 weeks |
| Siemens S7-1200 | $200-700 | Free (Basic) | In stock, 1-3 days |
Delta is the cheapest option with the fastest delivery. Siemens is the sweet spot for price and ecosystem. Mitsubishi is for tasks requiring specific capabilities (CC-Link, Motion).
Frequently Asked Questions
Which PLC is best for beginners?
Siemens S7-1200 with TIA Portal Basic. The most learning resources available in Ukrainian, free software, built-in PLCSIM simulator. Delta DVP is a second option: cheaper but less documentation.
Can you mix equipment from different manufacturers?
Yes, via Modbus RTU or TCP. For example, Siemens PLC + Delta VFDs + Autonics sensors — it works. The key is that all devices support a common protocol.
How much does a PLC-based automation system cost?
Basic: $500-1,500 (PLC + HMI + power supply). Mid-range: $2,000-5,000 (PLC + HMI + modules + VFDs). Large: $10,000+ (modular PLC + distributed I/O + SCADA).
Compact or modular PLC — which is better?
Compact — for projects up to 200-300 I/O. Modular — when you need configuration flexibility, hot-swap modules, or Motion Control. For 80% of projects, compact is sufficient.
How do I connect a 4-20 mA sensor to a PLC?
You need an analog input module (AI). The sensor connects with two wires to the module input. In the PLC program, the analog value is converted to engineering units (degrees C, bar, m3/h) using linear interpolation.
Summary
PLC selection algorithm: count I/O → identify signal types → choose communication protocol → consider programming environment → weigh the budget. For typical projects in Ukraine, we recommend Siemens S7-1200 or Delta DVP — depending on budget and customer requirements.
