I'm building a GUI for the electronic load we made here on element14.

The development will be in a few incremental steps.

  • communication with the instrument
  • basic GUI with a SCPI command line and results

In the next iterations:

  • add error logging
  • make the LCD display emulation work
  • a few dedicated controls to set current and enable / disable input


Functionality in the 1st Iteration


The application has its basic layout and one function implemented.

I want to start with a layout because it's easier to dry-run that and see if it would be up to the job when there isn't any application logic yet.

In this case, I used the NetBeans IDE's (see next section for more info) graphic designer to lay out the application.

Like most Java dialog designers, it lets you place elements on the window frame and choose what objects stay fixed, who moves and who resizes if the window size changes.

You can see below how I want the application to behave upon resize:


Here you can see the first iteration in action. The video is edited for brevity, but all actions are real.




The intended functionality:

  • the top frame emulates the LCD screen on the eLoad. It should always display the same info as the physical screen on the instrument. No user interaction.
  • then four controls that work together: the SCPI command line interface.
    • a command entry box with send button. When you click the button, the command in the entry box is sent to the instrument
    • then the instrument reply is read and the result shows in the text display button
    • the clear button empties the reply display.
  • at the bottom, the error log. Here, SCPI engine errors will be displayed. At this moment, I plan to only call this when you submit a command.
    The clear button next to it clears the log.
  • GUI and instrument functions separated. GUI should only ask and retrieve.


Functionality in this blog: the layout done and SCPI command + response working. Separate object to handle instrument logic.


Development  Environment: Java and NetBeans IDE


I've chosen Java as language. Because I know it and I find it the easiest solution to have cross-platform GUI with little dependencies.

The graphic framework is Swing.

I'm using two additional libraries: JSerialComm for USB access and commons-cli for command line parsing.

The editor is NetBeans. I created a project with the utility Maven as its build engine and dependency manager.

The NetBeans + Maven combination allows you to select add the two additional libraries as dependencies. The toolset then arranges download and version management of those.

The Jave JDK I use is openjdk version "1.8.0_212".

I'm developing on Windows 10. I expect that the program will work without adaptions on most Linux flavours (including on a Pi if you didn't install the headless Raspbian OS).


Instrument Object


The important methods are connect(), disconnect(), write() and read():


package net.cumps.eloadgui;

import com.fazecast.jSerialComm.SerialPort;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.Reader;
import java.util.logging.Level;
import java.util.logging.Logger;

 * @author jancu
public class Instrument {

    private static final Logger log = Logger.getLogger(Instrument.class.getName());
    private SerialPort comPort = null;
    private final char[] buffer;

    public Instrument() {
        buffer = new char[100];

    public void init() {

    public boolean connect(USBParams usbParams) {
        comPort = SerialPort.getCommPort(usbParams.getConnectionName());
//        comPort.setComPortTimeouts(SerialPort.TIMEOUT_READ_SEMI_BLOCKING, 1000, 0);
        comPort.setComPortTimeouts(SerialPort.TIMEOUT_READ_BLOCKING, 100, 0);
        boolean bResult = false;
        if (comPort.openPort()) {
            bResult = true;
            log.log(Level.INFO, "Open port succeeeded");
        } else {
            comPort = null;
            log.log(Level.SEVERE, "Open port failed");
        return bResult;

    public void disconnect() {
        if (comPort != null) {
            if (comPort.closePort()) {
                log.log(Level.INFO, "Closing port succeeeded");
            } else {
                log.log(Level.INFO, "Closing port failed");
            comPort = null;

    public String read() {
        Reader in = new InputStreamReader(comPort.getInputStream());
        String s = null;
        try {
            int len = in.read(buffer, 0, buffer.length);
            s = new String(buffer, 0, len);
        } catch (IOException e) {
            s = null;
//            e.printStackTrace();
        return s;

    public int write(String s) {
        OutputStream out = comPort.getOutputStream();
        int retval = -1;
        try {
            retval = 0;
        } catch (Exception e) {
        return retval;



I have taken a shortcut here to deal with SCPI commands that don't give a reply.

Instead of checking the command to see if there's a question mark; or checking if the serial port has data available, I let the reply reader block with timeout.

If the timeout happens, I consider that as a "no reply". Lazy.


Application and Main Window Object



The layout is done with the graphic editor.

I then added initialisation code.


In the main method, before the visual objects are set up, the startup command line is checked for COM port name and baud rate.

Then the Instrument object is primed.


     * @param args the command line arguments
    public static void main(String args[]) {

        USBParams usbParams = new USBParams();
        new Cli(args).parse(usbParams);

        if (instrument.connect(usbParams)) {

        /* Set the Nimbus look and feel */
// generated GUI code here ...


Then I create event handlers for the Send and Clear button.


    private void jButtonSCPISendActionPerformed(java.awt.event.ActionEvent evt) {                                                
        instrument.write(jTextFieldSCPICmd.getText() + '\n');
        String s = instrument.read();

    private void jButtonSCPIResultClearActionPerformed(java.awt.event.ActionEvent evt) {                                                       


I've attached the zipped NetBeans project - source, project settings and compiled JAR. The code is also on GitHub: https://github.com/jancumps/eLoadGui.

To test it, you can import it into NB and run from there.


To execute it without the editor on Linux or Windows, use these commands:


If you run it from Linux , you can use a command prompt similar to this (assuming you copied all the jars in the current working directory):

java -cp ./eloadGui-1.0-SNAPSHOT.jar:./commons-cli-1.4.jar:jSerialComm-2.5.0.jar net.cumps.eloadgui.MainFrame -usb "/dev/ttyACM0" -baud 9600  


If you run on WIndows (here I assumed the dependency jars are in a maven managed location):

D:\users\jancu\Documents\NetBeansProjects\eLoad\eloadGui\target>java -cp .\eloadGui-1.0-SNAPSHOT.jar;D:\NetBeans\maven_libs\commons-cli\commons-cli\1.4\commons-cli-1.4.jar;D:\NetBeans\maven_libs\com\fazecast\jSerialComm\2.5.0\jSerialComm-2.5.0.jar net.cumps.eloadgui.MainFrame -usb "COM9" -baud 9600




Related Blog
Programmable Electronic Load
Programmable Electronic Load - Java GUI Part 1: Basic Functionality
Programmable Electronic Load - Java GUI Part 2: Support for Current Mode, Input On/Off, Error Log