Language - Python SBC Linux: Difference between revisions

Revision as of 19:22, 13 December 2018

Python Development Environments
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Windows
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Phidget SBC Linux

Language - Python

SBC Linux with Python

Welcome to using Phidgets with Python! By using Python, you will have access to the complete Phidget22 API, including events.

Something about python on the SBC here

Getting Started With the Phidget SBC

Welcome to using the Phidget SBC. If you haven't already, check out the user guide in order to set up the following:

Networking
Administrator password

If you are ready to go, the first step will be deciding how you will use the SBC:

Use a more powerful external computer to develop your code, and then simply copy the files to the SBC.
Use the SBC like any other Linux computer, simply connect a monitor and a keyboard and begin your development.

This guide will cover development using an external machine. For development using the SBC itself, check the Terminal (Linux) page.

To begin, this video will help you get started:

Developing with an External Computer

There are two main ways in which you can access your SBC from an external computer:

SBC Web Interface
Secure Shell (SSH)

The user guide covers the SBC Web Interface in detail, so we will review SSH below.

SSH

If you are unfamiliar with SSH, it is a simple yet powerful tool that allows you to log into a remote machine in order to execute commands. You can also transfer files using the associated SCP tool.

In order to use SSH, you need to know the following things about the SBC:

IP address (e.g. 192.168.3.195) or the link local address (e.g. phidgetsbc.local)
The administrator password

You must also enable SSH on the SBC. You can do this via the SBC Web Interface which is shown in the image below:

SSH on Windows

To use SSH on Windows, we recommend PuTTY. Use the images below as a guide for configuring PuTTY (use the IP address or the link local address interchangeably):

After clicking open, simply login as root and provide the administrator password:

To transfer files between your SBC and Windows machine, we recommend either of these programs:

You will follow a similar process to access the SBC as described for SSH.

SSH on Linux and macOS

SSH is available on Linux and macOS by default. To run SSH, simply open the terminal and type the following:

ssh root@phidgetsbc.local

Or, something like this (you will need to know the IP address of your SBC):

ssh root@192.168.3.195

You will then be prompted for the password in order to gain access to the SBC:

To copy a file from the SBC to your development machine using SCP, simply open the terminal and type the following:

scp root@phidgetsbc.local:/path/to/source /path/to/destination

You can reverse this if you want to transfer a file from your development machine to your SBC:

scp  /path/to/source root@phidgetsbc.local:/path/to/destination

Installing Packages for Development

Installing support for Python has three steps:

Ensure Include full Debian Package Repository is checked on the SBC Web Interface (System->Packages)
Install Python
Install Phidget Python module

You will need to run commands on the SBC to install support for Python. You can either use SSH to issue the commands, or you can connect directly to the SBC via a monitor and keyboard.

Basic Python

The base Python functionality can be downloaded and installed in one step:

apt-get install python

Next, install the Phidget Python module.

Method 1: Use the Internet

First, install wget and unzip:

apt-get install wget
apt-get install unzip

Next, copy the web link address for the Python Libraries and use it in the following command (right click to copy into a terminal):

wget http://copied_link

The Phidget Python libraries should now be downloaded in the folder you ran the previous command in. The next step is to unzip the file:

unzip filename

Finally, change directories to the unzipped folder:

cd /path/to/unzipped/folder

and install the Phidget Python libraries:

python setup.py install

You're now ready to begin programming! Continue through this guide for code examples and directions on where to go next.

Method 2: Use a USB Key

Copy the Python Libraries onto a USB key. Unpack the zip file into a folder on the USB key. Insert the key into the SBC.

You will have to figure out where the USB key (and the Phidget Python library folder) is now located. We describe how in the general Using USB Data Keys section. Next, run the following commands (be sure to modify the usb directory number if necessary):

cd /media/usb0/
python setup.py install

You're now ready to begin programming! Continue through this guide for code examples and directions on where to go next.

Use Our Examples

For simplicity, if you have not used Phidgets before, we recommend trying them out directly on an external development machine like a desktop computer. For Python development, check the Command Line (Windows), Terminal (MacOS), and Terminal (Linux) pages.

One of the best ways to start programming with Phidgets is to use our example code as a guide.

Select an example that will work with your Phidget:

Python Examples

When developing on an external computer, you will write, and test your programs on that machine. When you are ready, you will then upload your programs to the SBC to run them.

Follow these steps to get our examples running on an SBC:

1. Download the example files onto the development machine.

2. Using the SBC Web Interface, create a new project:

3. Transfer all the example files from the development machine to the SBC, either using the SBC Web Interface or a tool like WinSCP.

4. Use SSH to access the SBC terminal

5. Navigate to the project folder using the command:

cd /usr/userapps/ProjectName

6. You can now run the program with the command:

python ExampleName.py

Success! The example is running on your SBC. If you aren't interested in developing directly on the Phidget SBC, jump ahead to running a program automatically.

You should now have the example up and running for your device. Your next step is to look at the Editing the Examples section below for information about the example and important concepts for programming Phidgets. This would be a good time to play around with the device and experiment with some of its functionality.

Editing the Examples

To get our example code to run in a custom application, simply remove the calls to AskForDeviceParameters and PrintEventDescriptions, and hard-code the addressing parameters for your application.

If you are unsure what values to use for the addressing parameters, check the Finding The Addressing Information page.

For instance:

channelInfo = AskForDeviceParameters(ch)
        
ch.setDeviceSerialNumber(channelInfo.deviceSerialNumber)
ch.setHubPort(channelInfo.hubPort)
ch.setIsHubPortDevice(channelInfo.isHubPortDevice)
ch.setChannel(channelInfo.channel)   
        
if(channelInfo.netInfo.isRemote):
    ch.setIsRemote(channelInfo.netInfo.isRemote)
    if(channelInfo.netInfo.serverDiscovery):
        Net.enableServerDiscovery(PhidgetServerType.PHIDGETSERVER_DEVICEREMOTE)
    else:
        Net.addServer("Server", channelInfo.netInfo.hostname,
            channelInfo.netInfo.port, channelInfo.netInfo.password, 0)

Might become:

ch.setDeviceSerialNumber(370114)
ch.setHubPort(2)
ch.setIsHubPortDevice(1)

Notice that you can leave out any parameter not relevant to your application for simplicity.

You can then manipulate the rest of the code as your application requires. A more in-depth description of programming with Phidgets can be found in our guide on Phidget Programming Basics.

Write Code

By following the instructions for your operating system and compiler above, you now have working examples and a project that is configured. This teaching section will help you understand how the examples were written so you can start writing your own code.

Remember: your main reference for writing Python code will be the Phidget22 API Manual and the example code.

Step One: Create and Address

You will need to declare your Phidget object in your code. For example, we can declare a digital input object like this:

ch = DigitalInput()

Next, we can address which Phidget we want to connect to by setting parameters such as DeviceSerialNumber.

ch.setDeviceSerialNumber(496911)

Although we are not including it on this page, you should handle exceptions of all Phidget functions. Here is an example of the previous code with error handling:

try:
  ch = DigitalInput()	
except RuntimeError as e:
  print("Runtime Error: %s" % e.message) 
try:
  ch.setDeviceSerialNumber(496911)
except PhidgetException as e:
  print (“Phidget Exception %i: %s” % (e.code, e.details))

Step Two: Open and Wait for Attachment

After we have specified which Phidget to connect to, we can open the Phidget object like this:

ch.openWaitForAttachment(5000)

To use a Phidget, it must be plugged in (attached). We can handle this by calling openWaitForAttachment, which will block indefinitely until a connection is made, or until the timeout value is exceeded. Simply calling open does not guarantee you can use the Phidget immediately.

Alternately, you could verify the device is attached by using event driven programming and tracking the attach events.

To use events to handle attachments, we have to modify our code slightly:

ch = DigitalInput()	
ch.setOnAttachHandler(onAttachHandler)
ch.openWaitForAttachment(5000)

Next, we have to declare the function that will be called when an attach event is fired - in this case the function onAttachHandler will be called:

def onAttachHandler(e):
    print("Phidget attached!")

We recommend using this attach handler to set any initialization parameters for the channel such as DataInterval and ChangeTrigger from within the AttachHandler, so the parameters are set as soon as the device becomes available.

Step Three: Do Things with the Phidget

We recommend the use of event driven programming when working with Phidgets. In a similar way to handling an attach event as described above, we can also add an event handler for a state change event:

ch = DigitalInput()	
ch.setOnAttachHandler(onAttachHandler)
ch.setOnStateChangeHandler(onStateChangeHandler)
ch.openWaitForAttachment(5000)

This code will connect a function to an event. In this case, the onStateChangeHandler function will be called when there has been a change to the channel's input. Next, we need to create the onStateChangeHandler function:

def onStateChangeHandler(self, state):
    print("State %f" % state)
    return 0

If you are using multiple Phidgets in your program, check out our page on Using Multiple Phidgets for information on how to properly address them and use them in events.

If events do not suit your needs, you can also poll the device directly for data using code like this:

state = ch.getState()
print("State %f" % state)

Important Note: There will be a period of time between the attachment of a Phidget sensor and the availability of the first data from the device. Any attempts to get this data before it is ready will result in an exception. See more information on this on our page for Unknown Values.

Enumerations

Some Phidget devices have functions that deal with specific predefined values called enumerations. Enumerations commonly provide readable names to a set of numbered options.

Enumerations with Phidgets in Python will take the form of EnumerationType.ENUMERATION_NAME.

For example, specifying a SensorType to use the 1142 for a voltage input would look like:

VoltageSensorType.SENSOR_TYPE_1142

and specifying a K-Type thermocouple for a temperature sensor would be:

ThermocoupleType.THERMOCOUPLE_TYPE_K

The Phidget error code for timing out could be specified as:

ErrorCode.EPHIDGET_TIMEOUT

You can find the Enumeration Type under the Enumerations section of the Phidget22 API for your device, and the Enumeration Name in the drop-down list within.

Step Four: Close and Delete

At the end of your program, be sure to close and delete your device:

ch.close()

Setting up a New Project

When you are building a project from scratch, or adding Phidget functionality to an existing project, you'll need to configure your development environment to properly link the Phidget C library.

When developing on an external computer, you will write, compile, and test your programs on that machine. When you are ready, you will then upload your programs to the SBC to compile and run them.

To include the Phidget Python library, add the following imports to your code:

from Phidget22.PhidgetException import *
from Phidget22.Phidget import *

Then, you will also have to import the class for your particular Phidget. For example, you would include the following line for a DigitalInput:

from Phidget22.Devices.DigitalInput import *

Once your code is written, follow these steps to get your program running on the SBC:

1. Using the SBC Web Interface, create a new project:

2. Transfer all the example files from the development machine to the SBC, either using the SBC Web Interface or a tool like WinSCP. The project directory will be /usr/userapps/ProjectName

3. Use SSH to access the SBC terminal

4. Navigate to the project folder using the command:

cd /usr/userapps/ProjectName

5. You can now run the program with the command:

python ExampleName.py

Success! The program is running on your SBC. Next, you may want to learn about running a program automatically.

Running a Program Automatically

After testing your program, you will likely want it to run on boot, or on a schedule, without your input.

To run a Python script as a standalone application, you will need to add a line called a "shebang" to the top of the script, with the path to your Python executable. If you have followed the steps in this guide, the line will be:

#!/usr/bin/python

Running a Program from the SBC Web Interface

To quickly test whether a program can be run automatically, you can try starting it from the SBC Web Interface.

1. To start the program, navigate to Projects->ProjectName->Startup Settings in the SBC Web Interface.

2. Select your program in the drop-down menu labeled Executable/Class Name.

3. Click the Start button on the SBC web interface.

4. You'll note that as it runs, there are two links below the Stop button which can be used to view the program output:

stdout: view the program output like you would in a terminal or command prompt
stderr: view the program error output

Run on Boot

Running on boot ensures that your program will never miss an event. As long as the SBC is running, your code will be running. This section assumes you have written and compiled your program on an external computer, and have uploaded it to the SBC Web Interface.

To have your program run on boot, navigate to Projects->ProjectName->Startup Settings in the SBC Web Interface. After selecting your project, copy the settings from the image below:

We will review some of the options that are shown in the image above:

Startup Order: lower numbers boot first. Booting later means more programs are available for use, booting earlier means other programs can use your program.
Run as a daemon: starts the program as a daemon. Unless you have explicitly written your program as a daemon, leave this checked, or else your SBC may hang on boot.
Executable/Class name: your main Java class or C file.
Arguments: any command line arguments the program needs.

After saving your changes, your program will run automatically whenever your SBC boots.

Run on a Schedule

Running your program on a schedule allows you to perform your task once a week, or once a minute without worrying about memory management issues or instability problems that may arise. It executes, and then gets cleaned up. To run your program on a schedule, we recommend using Cron. Cron can automatically schedule programs (known as jobs, or cron jobs). Cron simply reads a crontab file and runs whatever programs are listed, with whatever timing they are listed with. Cron runs continuously in the background, but the cron jobs only run as long as they naturally would, and then they exit.

Let's set up your first cron job. We will use nano to edit the crontab file, but feel free to use whatever editor you prefer.

First, set your editor to nano:

export EDITOR=nano

Next, edit your crontab file:

crontab -e

Finally, schedule your cron job:

#cron job that will run at 5AM every week:
0 5 * * 1 /root/code/myprogram argument1

After entering your task, simply save and exit the file.

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