Synergy Logic SLM-RP4: A $249 Raspberry Pi PLC With OpenPLC

The Raspberry Pi has quietly become a serious tool on the factory floor, and the new Synergy Logic SLM-RP4 shows how far that has come. LinuxGizmos reported on October 21, 2026 that Synergy Logic has launched the SLM-RP4, a UL-listed industrial controller built around the Raspberry Pi 4 platform. It clips onto a DIN rail, ships with the open-source OpenPLC Runtime preinstalled and drives up to eight stackable I/O modules for a maximum of 128 I/O points.

• Price: $249 for the controller, with I/O modules sold separately; the earlier SL-RP4 is $149
• Compute: Broadcom BCM2711 with four 64-bit Arm Cortex-A72 cores, 2GB LPDDR4-3200 and a 16GB microSD card
• Expansion: up to 8 modules over an SPI backplane, chosen from 18 module types, for up to 128 I/O points
• Certification: UL 61010 and UL 61010-2-201 listed, CE-marked to EN 61131-2 and EN 61010, and approved under Powered by Raspberry Pi

What Is the Synergy Logic SLM-RP4?

A programmable logic controller, or PLC, is the rugged little computer that runs machines, pumps, conveyors and greenhouse irrigation. Traditional PLCs are reliable but often expensive and locked to proprietary software. The SLM-RP4 pairs the familiar Raspberry Pi 4 single board computer with an industrial enclosure and open-source software, giving makers and small integrators a far more approachable entry point.

According to LinuxGizmos, the controller runs from a 24V DC Class 2 supply inside a passively cooled aluminum case rated for 0 to 50°C. Connectivity covers Gigabit Ethernet, dual-band 802.11ac Wi-Fi, Bluetooth 5.0, two USB 3.0 and two USB 2.0 ports, dual micro-HDMI outputs and a 3.5mm audio jack. A front-panel toggle switch and RGB status LED can be used for PLC run control or custom applications.

How Do the Stackable I/O Modules Work?

The clever part is the backplane. Modules slide onto the 35mm DIN rail beside the controller and connect through built-in edge connectors, so there is no cabling between the Pi and its local I/O. LinuxGizmos lists the current module range:

• Digital: 16-channel inputs, 15-channel transistor outputs and mixed digital I/O
• Relays: 8- and 16-channel relay output modules
• Analog: eight channels of 0 to 10V or 0 to 20mA inputs, and eight channels of 0 to 10V or 4 to 20mA outputs, plus combo modules
• Temperature: the four-channel SLM-THM-4 thermocouple module with 16-bit voltage measurement

The backplane driver supports I/O scan intervals from 1 to 1,000ms, with 10ms as the default. As a safety measure, bus fault detection can halt the PLC program and set all outputs to zero if communication errors pass a configurable threshold.

Can You Program It With Ladder Logic or Python?

Yes, both. Synergy Logic's own documentation says the SLM-RP4 ships with OpenPLC Runtime plus its Python and C++ libraries preinstalled. Through OpenPLC Editor you can write programs in the IEC 61131-3 languages, including Ladder Diagram, Structured Text, Function Block Diagram and Instruction List, while OpenPLC handles backplane access. Alternatively, developers can skip PLC tooling entirely and write stand-alone Python or C++ programs that talk to the backplane directly, which suits gateways, data logging and custom IIoT edge apps. The docs include worked examples such as thermocouple logging and publishing to an HMI over Modbus.

Who Should Consider a Raspberry Pi PLC?

Small manufacturers, labs, schools teaching automation and home-shop tinkerers all stand to benefit. A UL listing matters when equipment has to pass inspection, and OpenPLC means skills learned on this box transfer to other IEC 61131-3 systems. It joins other DIN-rail options we have covered, such as the Luckfox Lyra PLC running Linux and the HomeMaster MiniPLC with ESPHome. Browse more boards in our mini computers section.

Sources: LinuxGizmos — October 21, 2026; Synergy Logic SLM-RP4 documentation — October 2026.