# Synergy Logic SLM-RP4 Raspberry Pi 4 PLC brings modular industrial I/O with open-source stack

> **Key takeaway:** The SLM-RP4 combines a Raspberry Pi 4 with an SPI-based modular backplane supporting up to 128 I/O points, pre-installed OpenPLC runtime, and open-source FUXA SCADA integration for under $250.

**Published:** 2026-10-27T09:00:19+00:00 
**Source:** CNX Software 
**Category:** open-hardware 
**Canonical URL:** https://fosswire.org/news/synergy-logic-slm-rp4-raspberry-pi-4-plc-brings-modular-indu.html 

## Summary
Synergy Logic has released the SLM-RP4, a DIN-rail mountable industrial programmable logic controller built around a Raspberry Pi 4 Model B with 2GB RAM. The fanless aluminum enclosure accepts 24V DC input and connects to up to eight stackable I/O modules via an SPI-based backplane, yielding up to 128 I/O points across 18 available module types including digital I/O, relays, analog voltage/current, and thermocouple inputs. The controller ships with Raspberry Pi OS preloaded alongside OpenPLC Runtime for IEC 61131-3 programming, plus Python and C++ libraries for direct backplane control.

The system targets industrial automation, machine control, data acquisition, and IIoT deployments. It includes Gigabit Ethernet, dual-band WiFi 5, Bluetooth 5.0, USB 3.0/2.0 ports, dual micro-HDMI outputs, and carries UL 61010 and CE certifications to EN 61131-2. The SPI backplane offers configurable scan intervals from 1ms to 1000ms, communication error detection with automatic PLC stop and output reset, and a hardware watchdog for failure recovery. Synergy Logic also provides integration documentation for FUXA, an open-source web-based HMI/SCADA platform using Modbus TCP.

## Hardware architecture and open-source software stack
- **Raspberry Pi 4 core:** Uses a Raspberry Pi 4 Model B with 2GB RAM, 16GB microSD storage, and standard Pi connectivity including Gigabit Ethernet, WiFi 5 (802.11ac), Bluetooth 5.0/BLE, two USB 3.0, two USB 2.0, and dual micro-HDMI.
- **SPI-based modular backplane:** Edge-connector backplane supports up to eight non-hot-swappable SLM modules with automatic slot addressing based on physical position, providing up to 128 total I/O points.
- **I/O module variety:** 18 module types span discrete 12-24V DC / 100-240V AC inputs, 15-channel DC sourcing/sinking outputs, 8/16-channel relay modules (SPST/SPDT), 13-bit analog inputs (0-10V/0-20mA), 12-bit analog outputs (0-10V/4-20mA), and four-channel thermocouple inputs (types J, E, K, R, S, T, B, N, C).
- **OpenPLC runtime and programming:** Pre-installed OpenPLC Runtime supports IEC 61131-3 languages (Ladder Diagram, Structured Text, Function Block Diagram, Instruction List) via OpenPLC Editor 4.3.1+, with a hardware support package for I/O module configuration and SPI backplane parameter adjustment.
- **Alternative control paths:** Python and C++ libraries allow direct backplane control without OpenPLC, though both methods are mutually exclusive; hardware watchdog and configurable error thresholds provide failure recovery mechanisms.

## Why it matters
The SLM-RP4 lowers the barrier to industrial automation by combining commodity Raspberry Pi hardware with an open-source PLC runtime and modular I/O at a $249 price point, competing with proprietary controllers that typically cost significantly more. Its support for OpenPLC and FUXA SCADA integration aligns with growing demand for open, auditable industrial control systems, while the Python/C++ libraries appeal to developers bridging traditional automation with modern software practices. For Linux and open hardware advocates, the platform offers a certified, mainline-friendly foundation that avoids vendor-locked programming environments, though the lack of mainline real-time Linux or explicit RTOS support may limit deterministic applications without additional kernel configuration.