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All electronics, including PLCs, rely on signals to collect, store, and transmit information. Electronic designs are based on the types of signals — analog, digital (discrete), or both — that each device will use for their inputs and outputs (I/O). These signals can transmit all kinds of messages, including audio, video, or encoded text.
Derived from the Greek word analogos, meaning proportional, analog devices rely on a continuously variable signal. As the information in a message varies, the signal varies proportionally with the message. Analog signals oscillate between two levels, with smooth transitions between each level. These oscillations are typically described by their amplitude and frequency.
Typically, the natural world is entirely analog. For example, sound waves are inherently analog; beautiful harmonies in music are due to the combinations of analog audio signals that our ears are optimized to capture.
Signals with sharp, sudden transitions between discrete levels are called digital signals. This name stems from the Latin root digitus, or finger, as discrete quantities can be counted on one’s fingers. Most digital signals can take on exactly two values, making digital systems a natural choice for boolean logic. The boolean system of true/false values to encode information leads to a binary system of zeroes and ones — forming the computing power of nearly all digital devices.
Converting Between Analog and Digital Signals
Often, the same information can be captured in either analog or digital form. Audio is a classic example: consider music, where technology has moved from mixtapes on analog cassettes to digital streaming playlists. The same songs and artists can be captured in either medium.
Specialized devices can convert analog signals to digital and vice versa. Given that most of the natural world exists in analog, analog-to-digital converters (ADCs) are much more popular, but digital-to-analog converters (DACs) are also on the market. As shown in the image below, ADCs typically discretize analog signals into “ones” at the maximum amplitude of the signal, and “zeros” at the minimum amplitude of the signal. This allows analog signals to be utilized with traditional binary computing processes.
Comparing Analog and Digital PLCs
While analog may seem to align with the natural order of physics, analog circuits are much more complicated to design in practice. However, analog components are usually cheaper than their digital counterparts.
Since analog signals can take on an infinite range of values, analog devices are also much more susceptible to extraneous noise that clutters the signal. Digital signals can effectively round values up to one or down to zero, greatly reducing signal interference issues.
From a more technical standpoint, digital signals require less power and more bandwidth than analog signals. Frequency filters are also much easier to configure on analog circuits.
As a more concrete example, consider a PLC that reads an input voltage. A digital PLC will have cutoffs for “on” and “off” values, and all other values will be rounded to either the on or off conditions. The operator is shown a simple on or off reading. The wide boundary between these two conditions helps reduce errors; this improved accuracy is a tradeoff for less precision. An analog PLC will record the input value as any possible integer from 0 to 32,767 (i.e. the largest number that a 16-bit processor can store) to maximize precision. The PLC will then convert this intermediate number into a scaled value — in this example, representing volts — to a specific number that the operator can read. Since this is more error prone, additional processing, such as filtering, is necessary to ensure accuracy.
Although there are many tradeoffs, the primary benefit of digital devices is their interoperability with advanced computing — the easy connection from discrete digital signals to binary logic opens endless opportunities for additional processing and analysis. Analog systems excel at capturing real-world audio and video data; thus, they’re a popular design choice for many sensors. The selection of analog and/or digital I/O is heavily application-dependent, and PLC configuration can be customized to fit specific needs.
This entry was posted on October 23rd, 2020 and is filed under General. Both comments and pings are currently closed.
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