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    How to build a rPLC Coupler

    Step by step process of building an open source and open hardware Coupler running OPC-UA
    • Last Update:2024-05-22
    • Version:004
    • Language:en


    • Purchase required hardware
    • Print with 3D printer needed cases
    • Assemble coupler
    • Assemble MOD-IO
    • Connect the coupler and MOD-IO
    • Power up devices and connect peripherals


    • A real physical Coupler device and one MOD-IO device
    • a 3D printer
    • USB keyboard and mouse
    • basic assembly skills


    This how to explains how to build yourself an OPC UA coupler from scratch. At end of this how to you will have a real physical Coupler device to use in automation projects as seen below.

    Figure 1: a rPLC Coupler (with attached MOD-IO to the right.) running the "Hello World!" PLC program here.

    Above is the Coupler device which you will build (cover plates are removed for visibility).

    Purchase required hardware

    A coupler consists of at two cases:

    • The Coupler itself (in left part in the image) which consists of a SOC board (STMP157-OlinuXinu-LIME2-EXT) and a shield on top (STMP15X-SHIELD) where the shield that provides UEXT connectivity to MOD-IO devices
    • one of many (up to 127) MOD-IO boxes which are connected to coupler over UEXT cable (the right part in the image).

    Purchase list

    1. STMP157-OlinuXinu-LIME2-EXT (the coupler's main board)+ power supply
    2. STMP15X-SHIELD (an expansion top on SOC board)
    3. MOD-IO + power supply for it
    4. UEXT cable
    5. Micro SD card with recommended  capacity of minimum 16Gb
    6. LAN cable
    7. (optional) external battery
    8. Screws (Phillips type): 4 pieces 4 x 40 mm, 4 pieces 4 x 30 mm and 8 pieces 4 x 10 mm

    Print with 3D printer needed cases

    Two cases need to be printed each with a top and bottom part. This makes overall of 4 different parts.

    Below you may find links to .stl files which can be used with proper transformation with any 3D printer.

    1. STMP157-OlinuXinu-LIME2-EXT top
    2. STMP157-OlinuXinu-LIME2-EXT bottom
    3. MOD-IO top
    4. MOD-IO bottom
    5. (optional, needed only if you mount on DIN rail) DIN rail clips from https://remoteqth.com/3d-din-rail-mount-clip.php

    Please note that these casings are not yet DIN mountable. To make them DIN mountable you need to attach a DIN rail clips.

    Assemble coupler

    Step1: Install the micro SD card first into the coupler

    The first thing we need to do is to insert the micro SD card into the designated slot as show in the picture below.

    Step 2: Connect coupler to its bottom case

    The second step is to connect the coupler bottom case with board from step 1. For this we use 4 screws (4 x 10mm) as shown in picture below.

    Step 3: Connect the shield

    We need to connect the shield board by placing it on top of the case and board from step 2. Mind to properly match all the pins as this potentially can lead to an electrical damage.

    Step 4: Connect the UEXT cable

    As later one we will close the case we need to install the connecting cable (UEXT) to MOD-IO now. Please connect to UEXT1 slot.

    Step 5: Put top cover plate

    The last step is to close the case with its top cover plate by 4 screws (4 x 30mm).

    The coupler is ready.

    Assemble MOD-IO

    Step 1: Attach to MOD-IO's bottom case the MOD-IO board using 4 screws (4 x 10mm) in the designated montage holes as shown below.

    Step 2: Place top MOD-IO plate using designated screws (4 x 40mm)

    The MOD-IO is ready.

    Connect the coupler and MOD-IO

    Step 1: Connect the boxes using the UEXT cable

    1. Stick the two boxes next to each other (to the most left side the coupler and next to it MOD-IO)
    2. Connect using UEXT cable the boxes (XXX: define name opf the ports on the boards)
    3. Place the top cover of the coupler and screw the four connecting screws.
    4. Place the top cover of the MOD-IO and screw the four connecting screws.

    Power up devices and connect peripherals

    Please follow the following procedure:

    1. Connect a 5V  power supply to the coupler.
    2. (optionally) if you have a battery connect it as well to the designated port.
    3. Connect a 12V power supply to MOD-IO
    4. Connect an USB keyboard and a mouse
    5. Connect the coupler to the local LAN

    At the end it should be same as picture below.


    Now you have all needed hardware up and ready. Next step is install OS on the coupler.