DIN Rail PC vs PLC: When to Use Each

Asking “PLC or PC?” used to mean choosing between a dedicated controller and an office-style computer on a shelf. Today both can clip onto the same DIN rail, take the same 24 V DC supply and run IEC 61131-3 control code, so the real question is which device should own which job.

A PLC runs one control program in a fixed scan cycle and does very little else. DIN rail PCs run Windows or Linux, so they can host an HMI, a historian, vision software and, with the right runtime, the control program as well. This page looks at that last option, the soft PLC, and at how to split work between the two.

A soft PLC is control software that turns a PC into a programmable controller. It provides the same scan cycle, task scheduling and IEC 61131-3 languages as a hardware PLC, but runs on an x86 or Arm processor under a general-purpose operating system.

Examples include CODESYS Control, which has been ported to many x86 and Arm platforms, open-source projects such as OpenPLC, and runtimes that PLC makers license for their own industrial PCs. Most are programmed from the same engineering tool as the vendor’s hardware controllers, so the logic, function blocks and diagnostics look familiar to automation engineers.

The runtime talks to field devices over the PC’s Ethernet ports. EtherCAT masters, for example, run on a standard network controller with no special card, and PROFINET, EtherNet/IP and Modbus TCP stacks are available for most runtimes. Local inputs and outputs come from the PC’s own isolated I/O, I/O modules or remote I/O near the machine.

A normal operating system can pause a program at any moment to deal with updates, drivers or other tasks. A soft PLC needs its cycle to start on time, every time, so PC-based control uses one or more of these methods:

A hardware PLC remains the safer default when any of these apply. Safety functions such as emergency stops, guard interlocks and light curtains belong on a safety PLC or safety relay certified to IEC 61508 or IEC 62061; some soft PLC runtimes offer certified safety options, but most projects keep safety on dedicated hardware. A PLC also suits machines that must run unchanged for ten years or more with no operating system patches, sites where maintenance staff are trained on one PLC family, and applications with simple logic and few signals, where a compact PLC is the smallest and simplest answer.

A DIN rail PC earns its place when control is only part of the job. Typical cases are a machine that needs an operator screen, recipe storage and production records beside its logic; vision inspection or AI models that run on the same camera feeds the control decision depends on; data collection from several PLCs, meters and drives for OPC UA, MQTT or a database; and multi-axis motion over EtherCAT, where a fast processor helps with complex kinematics.

Consolidation is the other driver. One DIN rail PC running a soft PLC, the HMI and the data software replaces a PLC, a panel computer and a gateway, which saves rail space, wiring and spare parts.

Most cabinets end up with both, split by job rather than by preference. Three patterns cover most projects:

In every pattern the two talk over the plant network, using Modbus TCP, PROFINET, EtherNet/IP or OPC UA. Our guide to how an HMI works with a PLC covers the tag exchange that sits underneath.

IEC 61131-3 defines the languages, Ladder Diagram, Function Block Diagram, Structured Text and Sequential Function Chart, with Instruction List deprecated in the 2013 edition. It does not make programs fully portable: each vendor adds its own libraries, I/O mapping and system functions. PLCopen XML can carry much of a project between tools, but plan to rework hardware-specific parts.

For systems spread across several controllers and PCs, IEC 61499 adds an event-driven function block model designed for distributed control, and is supported by open-source and commercial runtimes.

We supply our DIN rail PCs with the operating system, storage and wireless modules fitted and configured as standard, ready to install beside the PLC.