I recently started an internship with a company that heavily utilizes PLCs, but I am completely new to this technology. As I delve into learning how PLCs have replaced relays, I came across a section in a website's "book" discussing creating programs with relay and replacing relay. While I grasp the concept of how a relay works internally, I am struggling to understand how it is translated into ladder logic within the PLC. Why are the inputs represented by a symbol of two bars resembling a contact in a relay? Shouldn't the input be the switch? The terminology and symbols used in PLC programming are leaving me confused. I thought the relay receives voltage at the input, creates a magnetic field to activate the contact, and the contact touching is the output. However, in PLC logic, it seems different. Can anyone provide insights into the history of PLC symbols and terminology? I wonder if it's just a matter of semantics. Your assistance would be greatly appreciated.
For valuable insights into XIO and XIC programming concepts, check out the series of informative videos by Ron Beaufort on YouTube. The videos cover the derivation and application of XIO and XIC, including variations. The explanation is detailed and comprehensive, but it also offers a convenient shortcut for those short on time. Visit www.plctalk.net for more information and discussions on this topic.
The symbols —| |— and —|/|— serve as visual representations of conditions, particularly in the context of relay contacts in a PLC system. These symbols can signify a normally open or normally closed contact, which the PLC system interprets accordingly. These conditions can be related to inputs, outputs, memory bits, or comparisons between values. The use of these symbols varies depending on how one wishes to interpret the conditions at hand. In practice, these symbols are employed in diverse ways to represent a wide range of physical and memory-related concepts.
The symbols —| |— and —|/|— are commonly associated with specific conditions, serving as a representation rather than being something more complex. It is crucial to avoid analogies like "passing voltage" or "logic flow" when explaining PLC ladder logic, as these may lead to confusion down the line. User programs run during the execution of a program rely on the values of bits in memory, with I/Os facilitating the conversion between these bit values and the corresponding physical input and output signals. Even immediate I/O instructions that access physical I/O directly only update the I/O image, as the user program continues to work with bits in memory. For a comprehensive understanding of how PLCs function, consider watching the video series by @Ron Beaufort, which is less than two hours long. Repeated viewing of this series will make PLC ladder logic much easier to grasp and comprehend.
The reason for the use of "TWO" bars in contact symbols may have historical significance, stemming from various styles of drawings in old relay systems. For example, in PLCs, the two dots and a bar can represent a switch, with the bar connected at an angle indicating a normally open state, or connected horizontally indicating a normally closed state. However, these symbols can be confusing when monitoring a program. Many systems now use colored bars to indicate the true or false state of a contact, with some, like Siemens, even animating the connections to show the flow of current. This approach can lead to clarity issues, especially when representing normally closed contacts that are true when energized. Different programming languages, such as FBD and ST, offer alternative ways of depicting logic symbols like AND, OR, and EXOR. The choice between American and European styles of digital symbols can also impact the design of the diagrams.
Absolutely, I can understand how the transition from physical components like relays to their representation in PLC programming can be a bit jarring. The key is to think about how relays function as a whole and how they've been abstracted in ladder logic. In essence, when you see that two-bar symbol in PLC programs, it's effectively representing an entire relay circuit - essentially a shorthand. When the input conditions of that circuit are met (the relay receives voltage), the "contacts" close (the two bars connect), simulating the same behavior of a physical relay. It's definitely odd at first if you're used to thinking of these in terms of physical components, but it does get easier with time. I hope this clarifies things a bit! It's all a part of the PLC's history of trying to streamline and update the cumbersome relay logic systems of the past.
Absolutely, it can be confusing since terminologies slightly shift in context. Essentially when you move to PLC, you migrate from physical to logical operations. The PLC input you're referring to mimics the action of a relay contact. In traditional relay, when 'voltage applied' or 'switch activated', the contact moves. This same brief narrative is translated in ladder logic as 'when there's an input (whether it's a switch, sensor or other), do something.' The two bars symbolizing a contact in a relay represent how we might visualize a switch process in a conventional wiring diagram. So, don't think of 'an input' as the switch itself but rather the 'state' of the switch. Yes, in essence, it's a matter of semantics. The terminologies and symbols have been adapted in a way that practitioners who worked with relays could easily understand and use. Once you adapt those concepts to a PLC context, it should be more straightforward.
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Answer: Answer: In PLC programming, the two bars resembling a contact in a relay symbolize the concept of a logical input being energized or active, similar to how a physical relay contact would operate in a traditional relay system. This representation is a common convention in ladder logic to indicate the status of inputs.
Answer: Answer: Relay conversion refers to the process of converting traditional relay-based control systems into PLC-based systems. This transition involves replicating the logic and functionality of relays using ladder logic programming in PLCs. Understanding this conversion is essential for effectively working with PLCs and developing control programs.
Answer: Answer: The symbols and terminology used in PLC programming have evolved to represent the functionality and logic of control systems in a more standardized and systematic manner. While they may differ from traditional relay terminology, they are designed to streamline programming and make it more efficient and intuitive for users.
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