Peripherals

Peripherals#

Front panel#

SLM-RP4 front panel with the USB-C, micro-HDMI and audio ports, the PWR, BASE and LED indicators, and the T/F toggle switch

Element

Function

PWR LED

Lit when 24 VDC is present at the power terminal.

BASE LED

Lit when the base controller is signed on and the backplane is being scanned. When off, the backplane is inactive and the base cannot communicate with the modules.

LED (RGB)

Status indicator. Its behavior depends on the programming path; see below.

T/F toggle switch

Two-position input. Its behavior depends on the programming path; see below.

USB-C

5 VDC power input for the controller only (not the backplane). USB device (OTG) port.

micro-HDMI 0

Primary display output.

micro-HDMI 1

Secondary display output.

Audio

3.5 mm analog audio output. Unavailable while the RGB LED driver is running.

RGB LED and toggle switch#

The toggle switch is a general-purpose input rather than a dedicated run/stop control: T reads as true and F as false. What each position means is decided by the software that owns the switch. OpenPLC Runtime uses it as a run/stop switch; a Python or C++ program can use it for anything.

Path

T/F toggle switch

RGB LED

OpenPLC Runtime

Reserved by the runtime. T runs the PLC program; F stops it. Not readable from the PLC program.

Reserved by the runtime. Green: program running. Red: fault. Off: runtime stopped or no program loaded.

Python / C++

User-controllable. Read through the slm_rp4_helpers package (Python) or GPIO 13 (C++).

User-controllable through the Adafruit NeoPixel library (Python) or rpi_ws281x (C++). Requires root.

To use the switch or the LED from Python or C++, stop the PLC or stop the OpenPLC Runtime. See Sharing the Hardware and Onboard Peripherals.

Connectors#

SLM-RP4 end view with two USB 2.0 ports, two USB 3.0 ports and the Ethernet port

Connector

Function

24 VDC terminal block

Primary power input for the controller and the backplane.

Ethernet

10/100/1000 Mbit interface used for programming, fieldbus traffic, and general network access.

USB 2.0 (×2)

USB-A host ports.

USB 3.0 (×2)

USB-A host ports.

Backplane edge

Right-hand connector mating with slot 1 of the module stack.

SD card slot

Holds the SD card with the operating system and OpenPLC Runtime. The card ships inserted and must stay in place for the device to boot.

Raspberry Pi status LEDs#

Two LEDs next to the SD card slot belong to the stock Raspberry Pi 4 board inside the SLM-RP4. They report the state of the Raspberry Pi itself, not of the PLC program.

The SD card slot and the ACT and PWR status LEDs on the SLM-RP4, labeled

LED

Meaning

PWR (red)

Lit steadily when the Raspberry Pi is receiving adequate power. Off or flashing indicates a supply undervoltage.

ACT (green)

Flashes during SD card activity. Dark once the system has fully shut down; wait for this before removing power.