PLC Main Components Explained 2026: CPU, Memory, I/O, Power and Communication
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PLC Fundamentals · Engineering Notes · 2026 Update
By the KOEED Engineering Desk · 2026-06-30 · 8 min read · Engineering Notes
A modern Programmable Logic Controller (PLC) still rests on the five functional blocks introduced when the category was born in 1968 — CPU, memory, I/O, power supply, and communication — but in 2026 every block has been re-engineered for OPC UA, edge analytics, and IEC 61131-3 4th edition. This Engineering Note walks through every block, the 2024–2026 deltas that matter to maintenance engineers, and the cross-references you need when sourcing spares from KOEED’s nine major brand collections.
AI Summary
- A 2026 PLC is built from five blocks: CPU, memory, input/output, power supply, and communication — the same model defined by IEC 61131-1 since 1968.
- IEC 61131-3 4th edition (published 2025) adds object-oriented FB/Method extensions and stricter static type checking — now standard on 2026 firmware.
- Flagship 2026 CPUs (Allen-Bradley 1756-L89, Siemens S7-1500 G2, Mitsubishi iQ-R, Omron NX502) embed an OPC UA server in firmware and add a hardware security core.
- Channel-level diagnostics on modern I/O modules remove most external signal-conditioning hardware.
- Cybersecurity has moved from footnote to hard requirement: IEC 62443-4-2 SL2 with signed firmware is now the procurement baseline.
A 2026 PLC keeps the five functional blocks defined in 1968, but every block now ships with OPC UA, cybersecurity, and edge analytics — changing how engineers select, source, and maintain spares.
Why the five-block model still matters in 2026
Every Programmable Logic Controller on the market — from a $120 Click PLC to a rack-mounted Allen-Bradley ControlLogix 5580 — is built from the same five functional blocks defined in IEC 61131-1: CPU, memory, input modules, output modules, power supply, and a communication interface. Vendor datasheets do not all use the same physical layout — some integrate the I/O on-board, some split the CPU into a base and an extension, some keep a separate communication module — but the block model holds. Knowing it lets you read any vendor’s catalog, regardless of whether the I/O is on-board, modular, or distributed over EtherNet/IP or PROFINET.
Block 1 — CPU (Central Processing Unit)
The CPU executes the user program, manages the I/O scan, handles communication, and runs diagnostics. In 2026 the CPU is rarely a single chip. It is a System-on-Module with at least three cores — one for the deterministic scan loop, one for the OPC UA / MQTT communication stack, and one for edge analytics. The flagship references in 2026 are:
| Brand | 2026 CPU Reference | Key Change vs 2024 |
|---|---|---|
| Allen-Bradley | 1756-L89 (ControlLogix 5580) | Quad-core SoC, native OPC UA, hardware security core, 64 MB user memory |
| Siemens | 6ES7518-4AP00 (S7-1500 G2 CPU 1518) | OPC UA server in firmware, 10 ns bit execution, isolated security core |
| Mitsubishi | R08ENCPU (MELSEC iQ-R) | C/C++ runtime alongside ladder, CC-Link IE TSN, signed firmware |
| Omron | NX502-1500 (NX/NJ Series) | OPC UA Pub/Sub, Sysmac AI model import, IEC 62443-4-2 SL2 |
> Tip
When the original 1756-L73 or 1756-L75 is on a 2024 EOL watch list, the in-place upgrade target is the 1756-L89. The chassis, power supply, and most 1756 I/O modules carry over — you swap the CPU, re-validate, and the program continues to run on the new OPC UA-aware firmware.
Block 2 — Memory
PLC memory splits into three logical regions. System memory holds the firmware and is non-volatile. Program memory stores the user project — on 2026 controllers this is non-volatile NAND flash with battery-free retention, so there is no coin-cell to replace every three years. Data memory holds the I/O image, retain tags, and the production recipe. In 2024 a typical compact PLC shipped with 256 KB program + 256 KB data. In 2026 the same form factor ships with 8–16 MB program and 4 MB retain, which is what makes object-oriented IEC 61131-3 4th edition code practical on the shop floor. Retain tags are stored in FRAM (10¹⁴ write cycles) and working memory lives in DDR4 with ECC, so a power loss loses only the volatile scan-time data.
Block 3 — I/O modules (input and output)
Input modules condition, isolate, and digitise field signals — 24 VDC, 120 VAC, 4–20 mA, RTD, thermocouple, high-speed counter. Output modules reverse the path: digital transistor, relay, analog 4–20 mA, servo pulse. In 2026 the trend is channel-level diagnostics — each channel reports short-circuit, wire-break, and overload events to the CPU tag database, removing the need for external signal conditioners. For distributed I/O, vendors now ship IP67-rated block I/O that sits on the machine next to the sensor and talks back over PROFINET or EtherNet/IP. Specialty I/O — SSI encoder, load-cell, IO-Link master — round out the 2026 catalog.
Block 4 — Power Supply
The power supply converts incoming AC (120/230 V) or DC (24 V) into the logic-level voltages the CPU and I/O need. A 2026 best practice is a dedicated 24 VDC switch-mode supply with a DC-UPS module on the input, sized to ride through at least one full scan cycle. For multi-rack systems, redundant power supplies with current-sharing (e.g. 1756-PA75R / 1756-PB75R) are now standard. When validating a 2026 retrofit, the most common issue is a power supply that has drifted in output voltage — KOEED tests every power supply on the bench before it leaves the warehouse.
Block 5 — Communication interface
The communication module is the block that has changed most since 2024. The 2024 article listed serial and Ethernet. The 2026 reality is OPC UA server embedded in CPU firmware, TSN (Time-Sensitive Networking) on the backplane, MQTT publish to cloud brokers, and a hardened web server for diagnostics. Fieldbus protocols (Profibus, DeviceNet, CC-Link) are still in service but are no longer the default for greenfield installations. Engineers specifying a 2026 PLC should plan for at least one TSN port and an OPC UA server endpoint — this is the new minimum for IIoT integration and the requirement most vendors will quote against.
Block 6 — HMI and engineering tools (the optional peer block)
Strictly speaking the HMI is external to the PLC, but in a 2026 architecture it is treated as a peer. Most modern HMIs run on the same network as the CPU and consume the OPC UA namespace directly. PanelView 5500, SIMATIC HMI Unified, GOT2000 and NB / NX HMI all read tags straight from the OPC UA server, which removes a layer of driver configuration that used to eat half a day during commissioning.
Note
Use KOEED’s free PLC Error Code Database to look up fault codes on Allen-Bradley, Siemens, Mitsubishi, and Omron controllers — updated weekly.
What changed since 2024
| Block | 2024 Baseline | 2026 Reality |
|---|---|---|
| CPU | Single-core ARM, ladder only | Multicore SoC, ladder + OO IEC 61131-3 4e + edge AI |
| Memory | 256 KB program, battery-backed | 8–16 MB NAND, FRAM retain, DDR4 ECC working |
| I/O | Module-level diagnostics | Channel-level diagnostics + IO-Link |
| Power | Single supply, no redundancy | Redundant supplies + DC-UPS common |
| Communication | EtherNet/IP, PROFINET, Fieldbus | OPC UA + TSN + MQTT native |
| Security | Password, optional firewall | IEC 62443-4-2 SL2 mandatory, signed firmware |
Sourcing implications for maintenance teams
The five-block model also helps when you are sourcing spares. Most EOL notices name a specific module — the CPU, or a single analog input card — not the whole PLC. KOEED keeps a parallel inventory of the legacy block (for example SLC 500 chassis and 1746/1747 modules) and the modern block (ControlLogix 1756) so a maintenance team can run the old line while engineering plans the migration. Send your BOM to Moritta@KOEED.COM for a quote within 24 hours.
Working on a PLC retrofit or EOL spares project?
Send your BOM to Moritta@KOEED.COM. Active stock, EOL stock, and cross-references — all in one quote within 24 hours.
Send My BOM →Related on KOEED
- Allen-Bradley ControlLogix & SLC 500 modules
- Siemens SIMATIC S7-1500 / S7-300 spares
- Mitsubishi MELSEC iQ-R and FX series
- Omron SYSMAC CP / CJ / NX series
- PLC Error Code Database (free tool)
- AI Diagnostic Tool — 24/7 fault support
- Create a Quote — BOM upload form
Frequently asked questions
What are the five main components of a PLC in 2026?
CPU, memory, input modules, output modules, and power supply. Communication is sometimes treated as a sixth block in modern architectures, and the HMI is treated as a peer on the network.
Is the IEC 61131-3 4th edition relevant to maintenance engineers?
Yes. The 4th edition (2025) introduced object-oriented FB/Method blocks and stricter type safety, which is why 2026 firmware on ControlLogix 5580, S7-1500 G2, and iQ-R exposes more OOP interfaces.
Can I still source SLC 500 spare modules in 2026?
Yes. KOEED keeps 1746 and 1747 series modules in stock for in-place repair while teams plan the migration to ControlLogix 1756. Send your BOM to Moritta@KOEED.COM for a quote within 24 hours.
Does a 2026 PLC need OPC UA?
For greenfield projects, yes — OPC UA is the de-facto IIoT standard and ships in firmware from Allen-Bradley, Siemens, Mitsubishi, and Omron. Legacy fieldbus still works but is no longer the default.
How do I get a quote for PLC spare modules?
Email your instruments list or BOM to Moritta@KOEED.COM, or use the Create a Quote form. Quote within 24 hours.
KOEED Engineering Desk
Industrial automation editors at KOEED. We write about PLC sourcing, cross-reference, and legacy system support for engineers working with Allen-Bradley, Siemens, Mitsubishi, Omron, Fanuc, Schneider, Yaskawa, Panasonic, and KEYENCE. Reach the team at Moritta@KOEED.COM.