Programmable Logic Controllers (PLCs)

Programmable Logic Controllers (PLCs): Engineering Guide

A Programmable Logic Controller (PLC) is a rugged industrial computer used to automate machines, process skids, utilities, and discrete manufacturing lines. In practice, a PLC reads field inputs such as pushbuttons, sensors, and analog transmitters; executes control logic in a deterministic scan cycle; and drives outputs such as contactors, valves, drives, and safety interlocks. PLCs are the backbone of industrial automation because they combine reliability, modular I/O, industrial communications, and maintainability in a form suitable for panels and plant environments.

What a PLC is and how it works

A PLC typically contains a processor, power supply, memory, communication interfaces, and local or distributed I/O modules. The classic operating model is the scan cycle:

1. Read inputs into an input image table.
2. Execute the user program, usually ladder logic, function block, structured text, or sequential function charts.
3. Update outputs from the output image table.
4. Perform communications, diagnostics, and housekeeping.

The scan time must be short enough to meet process response requirements. If an interlock requires a maximum reaction time of 100 ms, the PLC scan plus input filtering, output actuation, and device response must remain comfortably below that limit. A common engineering rule is to design for total control response at no more than 50% to 70% of the allowable process time.

For analog control, the PLC samples signals such as 4–20 mA or 0–10 V, scales them to engineering units, and applies control algorithms. For example, a 4–20 mA pressure transmitter representing 0–10 bar is scaled as:

$$P = \frac{I - 4}{16} \times 10$$

where $I$ is the measured current in mA and $P$ is pressure in bar. At 12 mA, the process value is:

$$P = \frac{12 - 4}{16} \times 10 = 5 \text{ bar}$$

Main PLC vendors and product families engineers should know

Engineers should be familiar with the dominant families because availability, software ecosystem, and lifecycle support often influence project risk more than raw hardware specifications.

• Siemens: SIMATIC S7-1200, S7-1500, ET 200SP CPU, and safety variants such as S7-1500F.
• Rockwell Automation: CompactLogix 5380, ControlLogix 5580, Micro800 series, and GuardLogix safety controllers.
• Schneider Electric: Modicon M221, M241, M251, M262, and Modicon M580 for process and Ethernet-based architectures.
• Beckhoff: CX Embedded PCs and TwinCAT PLC runtime, widely used where PC-based control and motion integration are needed.
• Omron: NX1P2, NJ/NX Series, and CP1E/CP2E for compact machine automation.
• Mitsubishi Electric: iQ-F (FX5U/FX5UC), iQ-R, and legacy FX families.
• ABB: AC500 and AC500-eCo.
• WAGO: PFC200 and modular I/O-based PLC systems.
• Phoenix Contact: PLCnext Control family for open automation architectures.

For selection, also consider software licensing, remote I/O strategy, safety integration, motion support, and long-term spare parts availability.

Selection criteria with concrete sizing rules

PLC sizing should not be based only on the number of I/O points. A robust selection considers CPU performance, memory, communications, environmental conditions, and future expansion.

1) I/O count and expansion margin

Count all hardwired points, then add margin for future changes. A practical rule is to add 20% to 30% spare capacity for standard projects and 30% to 50% for brownfield or process plants with expected modifications.

Example: a skid has 96 digital inputs, 64 digital outputs, 12 analog inputs, and 8 analog outputs. With 25% spare capacity:

$$DI = 96 \times 1.25 = 120$$

$$DO = 64 \times 1.25 = 80$$

$$AI = 12 \times 1.25 = 15 \Rightarrow 16$$

$$AO = 8 \times 1.25 = 10$$

Select a PLC platform and I/O architecture that can support at least 120 DI, 80 DO, 16 AI, and 10 AO, preferably with room for networked remote I/O.

2) Scan time and logic complexity

For discrete machine control, aim for a typical scan time under 10 ms if fast interlocks, pulse counting, or coordinated motion are present. For slower utility and process applications, 20 ms to 100 ms may be acceptable. A useful engineering check is that worst-case scan time should be at least 5 times faster than the fastest required control action.

Example: if a safety-related permissive must be evaluated every 50 ms, the non-safety PLC logic should ideally scan at 10 ms or less. For safety functions, however, use certified safety PLCs and safety-rated architecture rather than relying on standard logic.

3) Memory and program size

Estimate program size from function blocks, alarms, recipes, trends, and communications. For small machines, 1 MB to 5 MB is often sufficient. For larger plants with diagnostics and data buffering, consider 10 MB or more. Also verify retentive memory for setpoints and production counters.

4) Communications and cybersecurity

Choose Ethernet-based PLCs with OPC UA, PROFINET, EtherNet/IP, Modbus TCP, or EtherCAT as needed by the project. For EU-connected systems, design with IEC 62443 principles and align remote access, credential management, and segmentation with NIS2-driven cybersecurity expectations. Where applicable, implement role-based access, logging, and secure engineering workstation practices.

5) Worked selection example

A packaging machine requires 140 DI, 92 DO, 18 AI, 6 AO, one HMI, one VFD network, and future safety expansion. Add 25% spare:

$$DI = 140 \times 1.25 = 175$$

$$DO = 92 \times 1.25 = 115$$

$$AI = 18 \times 1.25 = 22.5 \Rightarrow 24$$

$$AO = 6 \times 1.25 = 7.5 \Rightarrow 8$$

A sensible choice is a mid-range modular PLC such as Siemens S7-1500, Rockwell CompactLogix 5380, or Schneider M580, depending on the site’s installed base and network standard. If motion or high-speed counting is extensive, a higher-performance platform or distributed control architecture may be justified.

Where PLCs fit in automation, panels, SCADA, and contracting projects

In a typical architecture, the PLC sits between field devices and higher-level systems. It handles machine logic, interlocks, sequencing, and deterministic control. The SCADA or HMI layer provides visualization, alarms, historian integration, and operator commands. The electrical panel provides power distribution, protection, terminalization, EMC management, and maintainability.

For EPC and contracting teams, the PLC scope must be defined early because it affects control panel footprint, cable schedules, network architecture, FAT/SAT procedures, and software deliverables. In process plants, the PLC may exchange data with MCCs, drives, analyzers, safety systems, and plant historians. In machine automation, it often integrates directly with servo drives, vision systems, and industrial Ethernet networks.

Applicable standards and clauses engineers should know

For European projects, PLCs are not selected in isolation; they are part of a compliant machine or installation.

• IEC 61131-2: electrical equipment requirements for PLCs, including input/output characteristics and environmental considerations.
• IEC 61131-3: PLC programming languages and software organization.
• IEC 60204-1, especially Clause 4 and Clause 9: electrical equipment of machines, including control circuits and wiring practices.
• IEC 61439-1/-2: low-voltage switchgear and controlgear assemblies, relevant when the PLC is installed in a control panel.
• IEC 61000-6-2 and IEC 61000-6-4: EMC immunity and emission for industrial environments.
• IEC 62443: industrial automation and control systems cybersecurity; relevant for segmentation, access control, and secure development.
• NFPA 79, especially Article 9 and related control-circuit provisions, for North American machine installations.
• UL 508A for industrial control panels in North American contexts.

For machine safety, PLCs used in safety functions must be part of a validated safety architecture. Standard PLC logic is not sufficient for safety-related stop functions, guard monitoring, or emergency stop chains unless the system is specifically safety-certified and designed accordingly.

Installation considerations: wiring, EMC, segregation, and thermal design

PLC reliability is often determined by panel design rather than the CPU itself. Separate AC power, DC control power, and sensitive analog or communication cabling. Route motor cables, VFD outputs, and contactor wiring away from analog and network cables. Use shielded cables where required, terminate shields according to the manufacturer’s EMC guidance, and maintain low-impedance bonding to the panel backplate.

Segregate wiring by voltage and function. A practical rule is to keep SELV/PELV control wiring separate from mains and power circuits, and to avoid shared trunking with noisy loads. For analog signals, use twisted pair shielded cable and avoid parallel routing with inverter outputs. For Ethernet, maintain bend radius and separation from high-energy conductors.

Thermal design matters because PLC CPUs, power supplies, and comms modules are sensitive to ambient temperature. Verify enclosure temperature rise and derating. If the panel ambient may exceed the PLC’s rated operating temperature, include forced ventilation or air conditioning. A simple heat estimate is to sum internal dissipation:

$$P_{total} = P_{CPU} + P_{PSU} + P_{I/O} + P_{network}$$

If the PLC system dissipates 35 W inside a sealed enclosure, the panel thermal strategy must reject that heat while maintaining component limits and allowable temperature rise. Always verify manufacturer derating curves rather than assuming nameplate operation at 40°C or 55°C.

Copy-ready PLC specification table

Specification item	Project requirement	Notes
PLC vendor / family		Example: Siemens S7-1500, CompactLogix 5380, Modicon M580
CPU type		Standard / safety / motion-capable / embedded
Digital inputs		Include 25% spare minimum
Digital outputs		Relay, transistor, or triac as required
Analog inputs		4–20 mA, 0–10 V, RTD, TC
Analog outputs		4–20 mA or 0–10 V
Communications		PROFINET, EtherNet/IP, Modbus TCP, OPC UA, EtherCAT
Scan time target		State required typical and worst-case scan
Ambient temperature		Verify derating and enclosure cooling
EMC class / immunity		Align with IEC 61000-6-2 and site conditions
Cybersecurity		IEC 62443-aligned access control, backups, segmentation
Safety integration		Safety PLC or safety I/O if required

In summary, the best PLC is not simply the fastest or cheapest. It is the one that fits the machine or plant architecture, supports the required standards, integrates cleanly with the panel and SCADA environment, and can be maintained for the full lifecycle of the asset.