Engineering Notes · PLC Architecture · 2026 Update
By KOEED Engineering Team · 2026-07-06 · 6 min read · Engineering Notes
> AI Summary
- Every general-purpose PLC is built from five blocks: CPU, memory, I/O, power supply, and communication interface.
- The 2026 update adds cybersecurity hardening, OPC UA over TSN, and energy-aware power supplies to the original five blocks.
- IEC 61131-3 still governs programming languages; PLCopen motion and OPC UA FX have become the default integration stack.
- Sourcing legacy 1747 / 6ES7 / FX3U parts remains a 24-hour BOM-quote operation at KOEED.
The five core PLC blocks are unchanged since 2024, but their internals (multi-core CPUs, OPC UA, secure boot) are not. This guide maps 2026 hardware and the parts KOEED keeps in stock.
Whether you are commissioning a ControlLogix line, replacing a 1990s SIMATIC S7-300 CPU, or quoting a BOM for a Mitsubishi retrofit, the hardware building blocks are the same five. This 2026 field guide refreshes the original 2024 article with current standards, refreshed model numbers, and what has changed in the last two years.
> TL;DR
- CPU, memory, I/O, power, and communication are still the five blocks.
- What is new in 2026: OPC UA over TSN, secure boot, multi-core ARM CPUs, energy-monitoring PSUs.
- IEC 61131-3 (3rd edition, 2013, reaffirmed) is still the language standard; PLCopen motion is now table stakes.
The five major components of a general-purpose PLC
A general-purpose PLC, whether it is a rack-mounted Allen-Bradley 1756-L73, a SIMATIC S7-1500 CPU 1515, or a Mitsubishi iQ-R, is always assembled from the same five hardware blocks. Understanding them is the fastest way to troubleshoot, quote, or migrate.
1. CPU (Central Processing Unit)
The CPU is the brain. It fetches inputs, runs the scan cycle, evaluates the logic, and writes outputs. In 2026, modern PLC CPUs are multi-core ARM Cortex-A class devices — for example the SIMATIC S7-1500 CPU 1518-4 PN/DP ships with a quad-core application processor, and the 1756-L8x ControlLogix family has moved to multi-core architectures for high-axis motion. The classic L73 remains widely deployed and in stock.
2. Memory
Memory is split into non-volatile program storage (flash / SD / MRAM) and volatile working memory (DDR RAM). IEC 61131-3 programs are loaded into program memory; tag data and process image live in work memory. A Mitsubishi FX5U uses 128 KB of program memory with a battery-less MRAM backup, while a 1756-L73 ships with 8 MB and supports an external 1784-SD2 card.
3. Input / Output (I/O) modules
Digital and analog I/O modules are the boundary between the controller and the field. Modern chassis such as SIMATIC ET 200MP and Allen-Bradley 1756 support channel-level diagnostics, per-channel over-temperature trip, and time-stamping to IEC 61131-3 function blocks. Specialty modules — high-speed counters, RTD / thermocouple, load-cell, and safety I/O — slot into the same backplane.
4. Power supply
The PSU converts mains 120/230 V AC or 24 V DC into the bus voltages the backplane needs. 2026 PSUs (for example 1756-PB75R and 6EP1334-3BA10) include peak-load reporting, brown-out ride-through, and efficiency above 92 %. For non-redundant sites, a 24 V DC UPS on the input rail is the cheapest way to ride through a 200 ms sag.
5. Communication interface
The fifth block connects the PLC to the rest of the plant. PROFINET, EtherNet/IP, and CC-Link IE TSN have displaced PROFIBUS and DeviceNet on greenfield lines, and OPC UA over TSN (IEC 62541) is now the recommended bridge to MES and cloud. KOEED keeps legacy adapters (1747-ASB, 6GK7 243-5DX30, QJ71C24N) in stock for brownfield retrofits.
! Warning
Hot-swapping a ControlLogix power supply without a UPS can corrupt program memory. Always export the project as .ACD and verify the SD card is seated before replacing a 1756-PBxx.
What changed since 2024
The five-block model has not changed — the silicon inside each block has. Three shifts matter for sourcing and migration in 2026.
| Block |
2024 baseline |
2026 update |
| CPU |
Single-core ARM Cortex-A8/A9 |
Multi-core A53/A55; hardware-accelerated OPC UA |
| Memory |
SD card + DDR3, battery-backed SRAM |
MRAM / pSLC NAND, battery-less, 10-year retention |
| I/O |
Channel diagnostics, time-stamping |
IO-Link master on every digital module; per-channel safety diagnostics |
| Power |
Fixed-output PSU |
Energy-reporting PSU, >92 % efficiency, brown-out ride-through |
| Communication |
PROFINET / EtherNet/IP, separate firewall |
OPC UA over TSN integrated; secure boot per IEC 62443-4-2 |
Translation for maintenance teams: the greenfield controller you spec in 2026 talks OPC UA and PROFINET TSN out of the box, but you can still run a 2008-era 6ES7 315-2EH14 alongside it on the same line — KOEED keeps the legacy CPU, the legacy I/O, and the modern TSN-capable 6ES7 516 in stock side by side.
Cross-Reference
SLC 500 chassis (1747-AIC, 1747-ASB) →
ControlLogix 1756-A4/A7 chassis with 1756-L73 CPU + 1756-EN2T EtherNet/IP bridge.
Migration difficulty: medium. Studio 5000 imports RSLogix 500 projects via the .RSS converter — budget two engineering days for I/O re-mapping.
Frequently asked questions
Are the five major PLC components still the same in 2026?
Yes. CPU, memory, I/O, power, and communication remain the canonical five blocks. What changed is the silicon: multi-core ARM CPUs, MRAM, IO-Link I/O, energy-reporting PSUs, OPC UA over TSN.
Which IEC standard governs PLC hardware today?
IEC 61131-3 (3rd edition, 2013, reaffirmed) governs programming languages. IEC 61131-2 covers electrical and mechanical requirements. IEC 62443-4-2 defines 2026 secure-boot rules.
Can I mix old and new PLCs on the same network in 2026?
Yes. A 1756-EN2T or 6GK7 243-5DX30 bridge can carry OPC UA tags from a 1756-L73 into a TSN backbone, and a QJ71C24N on a MELSEC-Q can be polled by a modern iQ-R via VLAN.
What is the most common failure point across all five blocks?
Power supply first, then communication modules and digital outputs. CPU and memory almost never fail in the first ten years. KOEED stocks 1756-PB75R, 6EP1334-3BA10, Q62P.
Working on a PLC retrofit or sourcing project?
Send your BOM to Moritta@KOEED.COM. Active stock, EOL stock, and cross-references — all in one quote within 24 hours.
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Related on KOEED Blog
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Engineering Notes archive — weekly field guides for PLC, HMI, and servo engineers.
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Migration Guides — SLC 500 to ControlLogix, S7-300 to S7-1500, MELSEC-Q to iQ-R.
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Allen-Bradley parts catalog — 1756, 1769, 1746, 1747 in active and EOL stock.
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Siemens SIMATIC catalog — S7-300, S7-1200, S7-1500 CPUs, I/O, and ET 200.
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PLC Error Code Database — search Siemens, Mitsubishi, and Omron fault codes.
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AI Diagnostic Tool — 24/7 cross-reference and obsolete-part lookup.
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Create a Quote — send a full BOM for a 24-hour quotation.
KOEED Engineering Team
Industrial automation editors at KOEED. Writes about PLC sourcing, cross-reference, and legacy system support. Reach the team at
Moritta@KOEED.COM.
