A Raspberry Pi in a DIN-rail case is nothing new, but this one comes with a full set of plug-in I/O modules. On 27 October 2026 CNX Software described the Synergy Logic SLM-RP4, an industrial controller (PLC) built around a Raspberry Pi 4 Model B. It costs $249, with I/O modules sold separately.

Hardware

  • Computer. A Raspberry Pi 4 with 2 GB of RAM and a 16 GB microSD card, Gigabit Ethernet, dual-band Wi-Fi 5, Bluetooth 5.0, two USB 3.0 and two USB 2.0 ports and two micro-HDMI outputs.
  • Power. 24 V DC through a terminal block. A USB-C port can power the controller alone for setup, but not the I/O backplane.
  • Case. Fanless aluminium for a 35 mm DIN rail, weighing 289 g, rated for 0 to 50°C. It is UL 61010 listed and CE marked to EN 61131-2.
  • Extras. A hardware watchdog, a programmable RGB LED and a toggle switch that can work as a run/stop switch.

Modules on an SPI backplane

The main difference from the earlier SL-RP4 ($149) is the backplane. Up to eight SLM modules snap onto the right side of the controller and connect through edge connectors, without extra cables. A module's position sets its slot address. There are 18 models, which gives up to 128 I/O points:

  • digital inputs for 12-24 V DC, 24 V AC/DC or mains voltage of 100-240 V AC,
  • sourcing or sinking DC outputs and 8- or 16-channel relay modules,
  • analog inputs and outputs for 0-10 V and current loops,
  • a four-channel thermocouple module and a board of switches for simulating inputs.

The modules are not hot-swappable, so power the unit down before changing them.

Software

The controller ships with Raspberry Pi OS, the OpenPLC Runtime and Synergy Logic's Python and C++ libraries. With OpenPLC Editor 4.3.1 or newer it can be programmed in the IEC 61131-3 languages: ladder diagram, structured text, function block diagram and instruction list. The backplane scans I/O every 10 ms by default, adjustable from 1 to 1000 ms. If communication errors exceed a set limit, it can stop the program and switch all outputs off. You can also drive the backplane directly from Python or C++, but not at the same time as OpenPLC.

The documentation also covers FUXA, an open web-based SCADA panel that talks to OpenPLC over Modbus TCP.

Why it matters for home automation

The SLM-RP4 is built for machines and factories, but Modbus TCP and Raspberry Pi OS make it interesting for a demanding home installation too: a boiler room, heating circuits, irrigation or a garage door, wired into the distribution board instead of running on Wi-Fi gadgets. Home Assistant has a Modbus integration, so it can read and set registers exposed by OpenPLC. The catch is the price, because modules come on top of the $249. CNX Software does not give module prices, and the controller is built on the older Raspberry Pi 4, not a Pi 5.