Raspberry Pi / Device Setup

The SLM-RP4 runs Raspberry Pi OS on an SD card. The sections below cover the operational practices required to keep the device reliable in the field.

Clean shutdown

Removing power while the filesystem is being written can corrupt the SD card. The journaled filesystem recovers from most interruptions, but a clean shutdown is preferred.

The preferred method, when SSH or local shell access is available, is:

sudo shutdown -h now

Wait for the green activity LED next to the SD card slot to go dark before removing power. See Raspberry Pi status LEDs.

If no shell access is available, stop the PLC application before cutting 24 VDC, either with the OpenPLC Editor’s Stop button or by moving the toggle switch to F. Removing power with the OpenPLC Runtime in stop mode is normally safe.

Network configuration

The device requests an IP address over DHCP on first boot. The current address is reported by:

hostname -I

To assign a static address, use nmtui or edit the NetworkManager connection directly. See the Raspberry Pi networking documentation for the full procedure.

The default hostname is slm-rp4. Networks that resolve .local hostnames reach the device at slm-rp4.local.

SSH access

The device accepts SSH connections on port 22.

Username	slm-rp4
Password	slm-rp4
Hostname	slm-rp4

ssh slm-rp4@<device-ip>

Change the password on every unit before deployment:

passwd

For long-term use, configure key-based authentication and disable password authentication in /etc/ssh/sshd_config. See Accessing the Device for the full procedure.

SPI interface

The backplane uses /dev/spidev6.0, enabled by an overlay in /boot/firmware/config.txt:

dtoverlay=spi6-2cs,cs0_pin=9,cs1_pin=10

Verify SPI after system updates

A Raspberry Pi OS update may reinstate the default SPI configuration and disable the backplane. After apt upgrade or any change to /boot/firmware/config.txt, verify that /dev/spidev6.0 is present:

ls -l /dev/spidev6.0

If the device is missing, confirm the overlay line above is still in config.txt and reboot.

Do not enable Interface Options → SPI in raspi-config. That toggle sets dtparam=spi=on, which enables SPI0 on GPIO 7–11 (CE1, CE0, MISO, MOSI, SCLK). GPIO 9 and 10 are the chip selects of the backplane’s SPI6 overlay (cs0_pin=9,cs1_pin=10), so enabling SPI0 disables backplane communication.

Reserved GPIO pins

The backplane uses the following GPIO pins. Driving any of them from user code breaks backplane communication for both OpenPLC Runtime and the direct libraries.

Pin	Function
GPIO 19	SPI6 MISO
GPIO 20	SPI6 MOSI
GPIO 21	SPI6 SCLK
GPIO 9, 10	SPI6 chip selects assigned by the overlay (cs0_pin=9,cs1_pin=10)
GPIO 27	Base controller chip select
GPIO 25	Base controller acknowledge
GPIO 17	Base controller enable

The front-panel controls also use fixed pins:

Pin	Function
GPIO 13	T/F toggle switch
GPIO 18	RGB LED

Check pin assignments before claiming additional GPIO:

pinctrl

Timekeeping

The SLM-RP4 has no battery-backed real-time clock. The system clock is set over the network through NTP and is accurate whenever the device has network access.

A device that boots without network access starts with an approximate time until synchronisation completes. Record an elapsed counter alongside timestamps if timestamp accuracy at boot matters.

timedatectl