Balanced Trio

The PiXtend board [1] opens numerous possibilities for the Raspberry Pi and provides additional flexibility, not only by the hardware, which meets industry standards, but also because of its support for professional software. Recently, the new PiXtend version 2.0 [2] was released (Figure 1).

Figure 1: A PiXtend V2 with an assembled Raspberry Pi 3.

Alternative

Manufacturer Qube Solutions provides the new board in three forms: as a plain vanilla extension board and as the ePLC Basic and the ePLC Pro. The last two alternatives include a Raspberry Pi 3 (RPi3) and a pre-installed operating system tailored to the respective application.

The ePLC Basic (~EUR240) is intended for easy installation in devices and for developing programs. The Pro version has everything you need for deployment as a professional-grade industrial programmable logic controller (PLC; a digitally programmable device for controlling or regulating a machine or system): a metal case suitable for mounting on a top-hat rail, which explains why the Pro version costs around EUR290. If you are unsure, start with the Basic version and retrofit the housing later. The PiXtend V2 S extension board is aimed at users who already own a Raspberry Pi: Neither the RPi3 nor an operating system are included, but it only costs around EUR165.

The PiXtend V2 is not intended to replace the previous version 1.3 board. Instead, it offers users who want to use the board in control systems a more cost-effective and space-saving alternative. A glance at the technical data of the PiXtend V2 will reveal whether it is suitable for the desired purpose.

Technology

The PiXtend V2 hardware is designed for practical use; the board is well thought out, like its predecessor, and components are almost exclusively surface mount, so that it is a good third smaller than its predecessor and accordingly costs a little less than version 1.3. Table 1 provides an overview of the various inputs and outputs.

The digital outputs deserve special attention: Version 2 works in PNP mode (version 1.3 uses NPN mode). PNP mode is used in industrial controls (PLC standard), and the operating voltage is at the output on switching, whereas in NPN mode, it switches to ground. The advantage of PNP mode is that the systems react in a more fault-tolerant way, because no permanent operating voltage is present at the consumer ends, in contrast to NPN mode.

Another novelty is persistent memory, which can store status information in the controller permanently, even if you switch it off or the power fails (i.e., an unforeseen power failure means that there is not enough time to write the data to an SD card). If the operating voltage drops below 19V, the PiXtend V2 microcontroller saves the residual data in flash memory in just 5msec. This technology can otherwise only be found in very expensive industrial controls.

Supported Software

Codesys. As with the predecessor, version 2 of the PiXtend board has extensive software support: Codesys [3] is a hardware-independent programming system supported by many manufacturers in the field of professional industrial controls.

Codesys implements the IEC 61131-3 [4] standard and supports the creation of web visualizations that deliver data as HTML from a web server for display on all common browsers. The open Codesys system allows for an easy exchange of programs developed on different hardware.

Linux Tools. The functions of PiXtend V2 can be tested with Linux tools, and the board can be programed in C. SD card images for the PiXtend come with these tools pre-installed, which use Gordon Henderson's WiringPi library, also integrated into the images.

The pxauto2s tool provides a graphical interface for the Linux console, where you can read and set all values and settings of the board. It exchanges data with the hardware 100 times per second, which makes it very easy to test the basic functions of the board.

With the pixtendtool2s tool, you can change and read out the settings directly in the shell, allowing you to develop small scripts quickly that perform simple control tasks, for example, with FHEM [5], a home automation server that allows easy builds of smart home solutions. FHEM is open source software that controls components from almost all professional manufacturers. If required, you can install a large number of modules for various items of hardware on the server and connect them to each other in a central interface.

Python. The popular Python scripting language is often used for development in the IoT area. It has evolved over the past decades, with libraries for almost every scenario imaginable to make your work easier.

OpenPLC. This completely free software serves as the basis for PLCs. Because it is very easy to install and operate, it is often used in training and studies. The OpenPLC project supports programming language standards according to IEC 61131-3:

• IL (instruction list): text-based, comparable to Assembler.
• LD (ladder diagram): graphical, like an electric circuit diagram.
• FBD (function block diagram): graphical, similar to a logic circuit diagram.
• SFC (sequential function chart): graphical, a kind of state diagram.
• ST (structured text): text-based, high-level language based on Pascal.

FourZero. The FourZero [6] platform supports the development of IoT and automation applications. It abstracts dependencies on the deployed hardware, enables system-level tests, and prevents redundant developments. All these features significantly accelerate the development process and reduce project costs. FourZero uses a decentralized approach.

STEP. The Standard for the Exchange of Product (STEP) Model Data [7] – ISO 10303, the computer-aided design (CAD), manufacturing (CAM), and engineering (CAE) standard for describing product data and partly adopted in German DIN standards – is a text-based CAD format that virtually any design software supports. With the STEP model, it is possible to design the PiXtend board for a control cabinet or a system, making it easier for mechanical engineering companies to integrate the hardware into their systems. The STEP model also makes it very easy for 3D printer owners to create precisely tailored housings for the board.