PiPLC Mainboard

PiPLC

This is a Raspberry Pi breakout board + DIN rail case to use Pis in PLC-like industrial situations.

The design relies on a Raspberry Pi 4 for all functions to work. Pi 5 does also work but won't fit the 3D printed case, while Pi 3 and below don't allow for all functions to be used.

The PiPLC is intended to be used with Home Assistant, as well as OpenPLC. Obviously, since it's just a breakout board for any Pi, you can run whatever software you want, but Home Assistant and OpenPLC are the ones inspiring the design of this project.

For an example use with your own software, check out Grass!

IO featured on board the PLC are:

• 8 x 24 V tolerant optocoupled inputs
• 8 x 16A Relays (250 V AC / 24 V DC)
• KNX TP
• Modbus RTU / RS-485
• I²C
• 2 x analog 3.3 V PWM outputs

All the parts are or will be stocked at Tindie!

If you like my work please consider supporting me!

Table of contents

• GPIO mapping
• Pinout
• I/O
  • J1 | Power Input
  • J2 | Modbus
  • J3 - J6 | Digital Outputs (Q1-8)
  • J7 | PWM
  • J8 | I²C
  • J9 | 1-Wire
  • J10 - J12 | Digital Inputs (I1-8)
  • Wago header | KNX / NCN5121
• On board devices
  • PCF8523 RTC
• Example Hardware
• Software
  • OpenPLC
    • Installation
    • Example
  • Home Assistant
    • Installation
    • Example
  • Codesys
    • Installation
    • Example
• Raspberry pi configuration
  • /boot/config.txt
  • /boot/cmdline.txt
• Making your own
  • Boards
    • Mainboard
    • HMI
    • Frontpanel
    • Pi Riser
    • HMI Riser
  • 3D printing
  • Impressions
  • Testing
• Tools used
• Sources
• Donations
• License

GPIO mapping

Pins marked unusable with OpenPLC are either not broken out, or differ too much from OpenPLC's mapping. As OpenPLC's mapping is immutable, these pins might not be used if PiPLC is used with the OpenPLC runtime and Pi default hardware layer.

Pinout

I/O

J1 | Power Input

The PiPLC is to be supplied with 24 V and requires at least a 1 A power supply under full load.

While it will theoretically work at 12 V, the relays used for Q1-8 require 24V coil voltage to trigger. If you are not placing the relays, feel free to supply your PLC with 12 V though.

Internally, 5V is generated by a TPS54331D based DC-DC converter. At J10 12/24V is passed through unprotected from the power input. At J2 and J8 5 V from the DC-DC can be used to draw up to 500mA (protected through a polyfuse). At J3, 3V3 from the RPI can be used to draw up to 500mA (protected through a polyfuse)

J2 | Modbus

Modbus is an industrial communication protocol often used with PLCs communicating with I/O extensions or other PLCs. The wire based one used here is called Modbus RTU and is based on half-duplex RS-485 so the header can also be used for that if you dislike Modbus.

OpenPLC natively supports Modbus to talk to I/O, while Home Assistant has a Modbus integration.

[!NOTE]
To enable Modbus on your Pi, make sure to enable UART3 at GPIO5/06. This UART is not enabled by default and is only present on Raspberry Pi models 4 and up!

We do NOT enable the ctsrts option as we don't want to run with hardware handshakes. We do however need to set/unset the DE/NRE pins of the RS485 transceiver IC. How to do this differs based on the chose OS.

In /boot/config.txt add:

dtoverlay=uart3

and reboot afterwards

After rebooting you should see the new serial port /dev/ttyAMA3 when executing ls /dev/tty*

For OpenPLC, if you followed the installation guide you should have installed my fork that includes DE/NRE control via GPIO_6 so there should be nothing else for you to do

J3 - J6 | Digital Outputs (Q1-8)

J3

J4

J5

J6

8 digital outputs are found at the pottom of the device. These outputs are driven by G5RL-1A-E-TV8 relays theoretically capable of 16 A at 250 V AC / 16 A at 24 V DC.

As with the digital inputs, each output features a status LED found on the HMI subboard.

[!WARNING] As this the PiPLC is an open source passion project and not externally rated by TÜV or a similar agency, usage of these relays in mains powered systems is at your own risk.

All components chosen are rated for 16 A at 250 V AC and should be capable of driving everything in a home environment.

Only certified electricians should ever be performing mains work, and home-built devices should never be connected to mains power unless you know what you are doing.

J7 | PWM

The PWM outputs are intentionally bare, unprotected GPIO connections of GPIO_18 for PWM 0 and GPIO_19 for PWM 1 so take care not to damage the Pi here. If choosing to use the GPIO as inputs, bear in mind that the Pi's GPIO are only 3.3 V tolerant.

Choosing to actually use the PWM outputs, you have both hardware PWM's available to you here.

[!WARNING] Unprotected access to GPIO_18 and GPIO_19. Take care not to damage your Pi!

J8 | I²C

Here you have a protected, somewhat isolated 5 V tolerant I²C header with integrated pullups. This was at first intended for connecting to extensions, but the Modbus header takes care of that now. You do have the option however, to attach whatever I²C device you want, provided the runtime you are using supports I²C communication.

[!WARNING] The I²C contacts are already pulled up to 5 V so do not connect any 3.3 V only I²C devices directly without isolation!

[!NOTE] To enable I²C, go into /boot/config.txt and uncomment dtparam=i2c_arm=on, or alternatively enable I²C through sudo raspi-config

You might need to install i2c-tools with sudo apt install i2c-tools to test your I²C connections.

i2cdetect -y 1 should show the adresses of all found I²C devices afterwards

J9 | 1-Wire

This header contains GPIO_04 for native 1-Wire capability.

1-Wire is usually used for temperature sensors, like DS18B20, or battery voltage sensing, but is sometimes also used for access control with iButtons.

[!WARNING] Unprotected access to GPIO_04. Take care not to damage your Pi!

[!NOTE] To enable 1-Wire, add the following to /boot/config.txt:

dtoverlay=w1-gpio,gpiopin=23

reboot

You should be able to find any connected 1-Wire devices automatically with ls /sys/bus/w1/devices/ afterwards

J10 - J12 | Digital Inputs (I1-8)

J10

J11

J12

8 digital inputs are found at the top of the device. These inputs are 5-24 V tolerant current sinking inputs through EL817C optocouplers. J10 features 24 V outputs to easily supply switches or other sensors feeding back into J11-J12 as inputs.

All inputs feature a status//Order is intentional to match the GPIO mapping table of PiPLC at https://github.com/chrismettal/piplc#gpio-mapping //while having all relays mapped "in order" from left to right LED driven by the optocoupler output, thus not loading the input.

On the Raspberry side, pullups need to be enabled on the GPIOs to enable reading from these input stages.

The inputs require 5mA of input current to reliably trigger at 24 V.

Wago header | KNX / NCN5121

Utilizing knxd to run a NCN5121 KNX transceiver chip the PiPLC can talk to KNX networks natively, without going through an actual KNX-IP gateway first. (Internally, knxd looks like a KNX-IP gateway, but it won't route through your actual LAN)

For Home Assistant, there is an addon for knxd, so Home Assistant sees the internal NCN5121 as a KNX-IP gateway @ localhost.

See the knxd documentation for specifics on how to use the driver if you are not using Home Assistant.

In this board, the NC5121 is NOT supplied through the KNX supply, but through the Pi's 3v3 rail. This means that it doesn't need to be taken into account when calculating the KNX power supply needs.

[!NOTE]
To enable KNX, make sure to disable bluetooth and the serial console on your Pi This is required to enable full UART0 at GPIO14/15

In /boot/config.txt add:

dtoverlay=pi3-disable-bt

In /boot/cmdline.txt remove:

console=serial0,115200 or console=ttyAMA0,115200 if found

reboot

In your OS, /dev/ttyAMA0 can now be used as your KNX interface

On board devices

PCF8523 RTC

A PCF8523 based realtime clock is present on the board so the PiPLC can keep track of time even when its 24 V input drops.

[!NOTE]

To enable the real time clock on your Pi, add the following to /boot/config.txt:

dtoverlay=i2c-rtc,pcf8523

And reboot afterwards.

Enabling RTC in the Pi and setting the time correctly is explained well in Adafruit's RTC guide.

Example Hardware

This is the testing "schematic" used for all software examples. It demonstrates all interfaces present on-board and all testing of PiPLC was done in this configuration.

There is an example chinesium Modbus I/O module on the testing board with the following information:

Software

OpenPLC

OpenPLC is a Fully IEC 61131-3 compliant, open source, multi-hardware programmable logic controller suite.

Consisting of a separate runtime and editor, it can run on regular PCs as a soft PLC, as well as a multitude of embedded systems. (Think Beckhoff XAR / XAE)

I am currently creating a hardware layer for PiPLC at https://github.com/Chrismettal/OpenPLC_v3. This should allow easy adressing of all IO while enabling native use of Modbus and other interfaces. The regular "Rpi" layer should also be compatible, but I²C, Modbus and KNX might not work, and I1-8 / Q1-8 adressing might be a bit unintuitive (See GPIO mapping table)

Installation

Example

Check out /src/OpenPLC!

Home Assistant

Home Assistant is an open source smart-home automation software, that provides logic, visualization and automation in a single place. It allows you to connect practically everything to your home under one interface, awakening your smart home.

Home Assistant seems to move away from local hardware IO a bit, instead focussing on devices in your LAN / WiFi. That doesn't mean though, that it isn't fully capable of running on an actual PLC, giving you control of your devices without a middle man or WiFi shenanigans. This is where the NC5121 KNX interface comes into play, which enables you to talk to a vast variety of off-the-shelf decentralised ACTUAL smart home components (No supplier apps, no accounts, no telemetry) without needing to go through an KNX-IP interface first.

Installation

mkdir ~/TempMount

sudo mount -t vfat /dev/sdx1 ~/TempMount

Example

TODO Upload example configuration.yaml

Codesys

Codesys, like OpenPLC is a IEC 61131-3 compliant PLC runtime, compatible with PiPLC. It is not open source and the license for the RPI runtime costs about 55€. PiPLC was designed for OpenPLC but Codesys (as of 2024-01) is a much more robust runtime with more features.

As Codesys is proprietary, I won't be focussing much on it, but I want to make sure that most functions work well with PiPLC.

Installation

Example

Check out /src/CODESYS!

Raspberry pi configuration

/boot/config.txt

# PiPLC
dtoverlay=uart3                     # Enable UART3 for Modbus
dtoverlay=pi3-disable-bt            # Disable Bluetooth so that UART0 is free for KNX
dtoverlay=w1-gpio,gpiopin=23        # Enable the 1-Wire pus on pin 23
dtoverlay=i2c-rtc,pcf8523           # Enable the I²C RTC
gpio=24,25,8,7,12,16,20,21=op,dl    # Set all relay pins to outputs and turn them off
gpio=17,27,22,10,9,11,13,26=ip,pu   # Set all digital input pins as pulled-up inputs

remove any mention of I²S or SPI if found, otherwise relay GPIOs might be driven HIGH on boot. Sometimes dtparam=spi=on is set which will ruin your day so comment that one out if found.

/boot/cmdline.txt

remove console=serial0,115200 and/or console=ttyAMA0,115200 if found.

Making your own

Boards

A full PiPLC consists of several subboards that need to be manufactured individually. Theoretically only the mainboard is technically required, as you could use wires to manually connect the Pi to the PLC as well as just not using the HMI as it is only a status display at the moment, but the existing 3D printable case assumes all boards to be used.

Only the main board has SMD parts that need to be assembled, so only one board will need to go through PCBA.

All boards are designed to be manufactured by JLCPCB, with BOMs using LCSC partnumbers attached. ALso all boards are to be manufactured with regular 1.6mm thickness, lead free HASL and 1oz copper weight.

Mainboard

Interactive Bom / Online view

This board houses all logic and is the only one that benefits from a PCBA service. All other boards plug into here, including the Raspberry Pi.

HMI

Interactive Bom

While not strictly required, this board houses frontpanel LEDs for status view of digital inputs and outputs, as well as 5V and 24V power status LEDs.

Additionally, the Wago header to plug into a KNX network is housed on this board.

Frontpanel

Interactive Bom / Online view

Purely for decoration / labelling. No electrical connections here. Can theoretically be replaced by a sticker.

Pi Riser

Interactive Bom / Online view

Riser board to give the Pi enough clearance to connect face-down to the mainboard. Can theoretically be replaced by a ribbon cable.

HMI Riser

Interactive Bom / Online view

Riser board to connect the HMI board to the mainboard. Can theoretically be replaced by a ribbon cable

3D printing

A 3D printable case is provided in /3d_printing.

The case just snapfits together, after all boards were installed using M3x5mm screws.

Currently it will only fit a Raspberry Pi 4 and will not work with a Pi 5! You could modify the case to make a Pi 5 fit but I haven't done that yet.

The DIN-Rail mount on the back is not optimal yet so the compliant snap-action might now work, but since all boards I have manufactured are now deployed, I haven't yet gone around to updating the case.

The case is intended to be printed in a isolating and/or flame retardant filament like PC or PETG VO.

Impressions

Testing

The following commands can be used to test your hardware configuration without installing PLC code or a full Home Assistant install. These are to be executed on a regular Pi OS install with raspi-gpio installed.

• Enable pull ups on all input pins

raspi-gpio set 17,27,22,10,9,11,13,26 pu

• Set all output relay pins to be outputs

raspi-gpio set 24,25,8,7,12,16,20,21 op

• Read state of all input pins

raspi-gpio get 17,27,22,10,9,11,13,26

• Set individual relays HIGH/LOW (See GPIO mapping)

raspi-gpio set 24 dh / raspi-gpio set 24 dl

• Detect connected I²C devices (Should show found addresses in the grid if properly connected)

i2cdetect -y 1

Tools used

• FreeCAD 0.21.2
  • KicadStepUp Workbench 10.20.7
• KiCAD 7.0.9

Sources

• Wago 243-211 KNX Connector 3D file - https://grabcad.com/library/knx-connector-1
• Raspberry Pi 3D file - https://grabcad.com/library/raspberry-pi-4-model-b-1
• PCB Standoffs 3D files - https://grabcad.com/library/ettinger-standoff-nickel-plated-brass-m-f-m2-m8-1/details?folder_id=12725833

Donations

If you like my work please consider supporting me!

License

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.