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.

Figure 1. Automation System Portrait

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.

Figure 2. Linear and Nonlinear Graph

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.