# Peripherals

## Front panel

```{image} ../../images/hardware/slm-rp4-front.jpg
:alt: 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
:width: 380px
:align: center
```

| 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](../advanced/shared-hardware-rules.md) and [Onboard Peripherals](../advanced/onboard-peripherals.md).

## Connectors

```{image} ../../images/hardware/slm-rp4-ports.jpg
:alt: SLM-RP4 end view with two USB 2.0 ports, two USB 3.0 ports and the Ethernet port
:width: 350px
:align: center
```

| 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)=
### 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.

```{image} ../../images/hardware/slm-rp4-sd-and-leds.png
:alt: The SD card slot and the ACT and PWR status LEDs on the SLM-RP4, labeled
:width: 380px
:align: center
```

| 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. |