Selecting I/O Units
In this article, I introduce the I/O unit, the hardware that allows Linux to interact with the physical world. I/O units have different characteristics that make them appropriate for virtually any monitoring or automation task, and initially, it may seem difficult to choose the right one. I introduce terms for those just starting out in this field and describe features that will help you select the right device for the application. Finally, I outline steps to follow for implementing the I/O system, whether small or large.
I/O is a commonly used term referring to data input and output devices. Disk drives, video displays, keyboards and mice are typical computer devices associated with I/O. In the automation industry, the term I/O refers to a device that permits a computer to monitor and control physical elements in the physical world. To allow Linux-based or other systems to reach into the physical world, I/O units are coupled with sensors and actuators. Sensors are electronic devices attached to inputs; they convert quantities in the physical space into quantities computers understand. Actuators, attached to outputs, are devices that translate computer commands into responses in the physical world.
As with any technical field, there is a vast ocean of terminology used with I/O units. I present some of that terminology now and refer back to it when I later discuss the design features that drive selection decisions. Look out! Here comes the tide.
When I mention the word signals, I'm referring to an I/O unit's capability to receive and send analog and digital electrical signals, not the signal programming sort of concept. Understanding the nature of signals and the differences in signal types permits proper selection and design for I/O units. These signals fall into two primary categories, analog and digital, with a smaller category for a rapidly developing group of ``smart sensors'' that report encoded digital data.
Digital or discrete signals are used by a two-state device, which is a device that has only two possible states: on or off. For example, switch sensors may provide a signal that tells us a tank level is full or the power is on. Another example of digital sensors are the so-called idiot lights on the dashboard of an automobile.
These signals are driven by an excitation source, either a power supply or a battery. Discrete loop is another name for the discrete circuit. While discrete loops are typically powered with 12 to 48 volts direct current (VDC), many applications use a convenient excitation source. In the United States, this may be 120 volts alternating current (VAC) or another voltage comparable to the regional supply ratings. In other words, don't grab any wiring until the power to the control box is turned off!
Other sensors and effectors are analog or proportional in nature, meaning that they have not just two states but a range of possible values. Examples of analog values are temperature gauges, analog clocks, speed, sound volume and pressure. These proportional quantities are converted to analog signals. The value of the signal, whether current or voltage, reflects the reading at the sensor. While the signal may be proportional to a physical value, it may not be linear--that is, if the signal doubled in value, the physical value would not necessarily double as well. There are many types of nonlinear analog sensors; thermocouples, devices that measure temperature via a voltage produced by dissimilar metals, are a popular example.
Other sensor technologies require substantial processing and generally have their own computers. While these units may send data in a proportional analog format, they also have the ability to send the sensor reading as digital data, using a digital interface in which the data is encoded and transmitted. RS-232, RS-422 and RS-485 are common interfaces used. Examples of digital data sensors include intelligent accelerometer monitors, laser extensometers, flow meters and stepper or synchronous motor controllers. With USB and ``biscuit'' Ethernet devices, these sensors can be directly connected to Ethernet networks and other high-bandwidth communication media.
|Dynamic DNS—an Object Lesson in Problem Solving||May 21, 2013|
|Using Salt Stack and Vagrant for Drupal Development||May 20, 2013|
|Making Linux and Android Get Along (It's Not as Hard as It Sounds)||May 16, 2013|
|Drupal Is a Framework: Why Everyone Needs to Understand This||May 15, 2013|
|Home, My Backup Data Center||May 13, 2013|
|Non-Linux FOSS: Seashore||May 10, 2013|
- Dynamic DNS—an Object Lesson in Problem Solving
- Making Linux and Android Get Along (It's Not as Hard as It Sounds)
- Using Salt Stack and Vagrant for Drupal Development
- New Products
- A Topic for Discussion - Open Source Feature-Richness?
- Drupal Is a Framework: Why Everyone Needs to Understand This
- Validate an E-Mail Address with PHP, the Right Way
- RSS Feeds
- Readers' Choice Awards
- Tech Tip: Really Simple HTTP Server with Python
- BASH script to log IPs on public web server
39 min 25 sec ago
4 hours 15 min ago
- Reply to comment | Linux Journal
4 hours 47 min ago
- All the articles you talked
7 hours 11 min ago
- All the articles you talked
7 hours 14 min ago
- All the articles you talked
7 hours 15 min ago
11 hours 40 min ago
- Keeping track of IP address
13 hours 31 min ago
- Roll your own dynamic dns
18 hours 44 min ago
- Please correct the URL for Salt Stack's web site
21 hours 56 min ago
Enter to Win an Adafruit Pi Cobbler Breakout Kit for Raspberry Pi
It's Raspberry Pi month at Linux Journal. Each week in May, Adafruit will be giving away a Pi-related prize to a lucky, randomly drawn LJ reader. Winners will be announced weekly.
Fill out the fields below to enter to win this week's prize-- a Pi Cobbler Breakout Kit for Raspberry Pi.
Congratulations to our winners so far:
- 5-8-13, Pi Starter Pack: Jack Davis
- 5-15-13, Pi Model B 512MB RAM: Patrick Dunn
- 5-21-13, Prototyping Pi Plate Kit: Philip Kirby
- Next winner announced on 5-27-13!
Free Webinar: Hadoop
How to Build an Optimal Hadoop Cluster to Store and Maintain Unlimited Amounts of Data Using Microservers
Realizing the promise of Apache® Hadoop® requires the effective deployment of compute, memory, storage and networking to achieve optimal results. With its flexibility and multitude of options, it is easy to over or under provision the server infrastructure, resulting in poor performance and high TCO. Join us for an in depth, technical discussion with industry experts from leading Hadoop and server companies who will provide insights into the key considerations for designing and deploying an optimal Hadoop cluster.
Some of key questions to be discussed are:
- What is the “typical” Hadoop cluster and what should be installed on the different machine types?
- Why should you consider the typical workload patterns when making your hardware decisions?
- Are all microservers created equal for Hadoop deployments?
- How do I plan for expansion if I require more compute, memory, storage or networking?