In the world of PLC programming, Set and Reset instructions are often regarded by novice engineers as versatile tools. When first stepping into this field, many are captivated by their simplicity, as if they were a "Swiss Army Knife" that could solve all problems.
However, as your programming experience grows, you may find that the reality is far from simple. Those seemingly flawless Set and Reset instructions gradually reveal their shortcomings over time. So, why do senior PLC engineers increasingly discourage the use of these instructions? Let's dissect this from several perspectives.
Execution Efficiency Issues
Consider the classic example of motor start-stop control programs. On the surface, Set and Reset instructions can easily complete start and stop operations, but delving into the underlying mechanism reveals problems. In a typical scanning cycle, the output Q0.0 might be written to twice—once to set it to 1, and once to reset it to 0. This redundant operation not only slows down program execution but can also lead to unstable output states, akin to a tug-of-war over operational rights.
Programming "Pitfalls"
Set and Reset instructions typically need to appear in pairs during programming. However, no one is infallible, and as programs grow, oversights can occur. If you miss one of them, trouble arises. Forgetting a Set instruction could keep the output perpetually at 0, while omitting a Reset instruction might keep it perpetually at 1. More alarmingly, debugging such programs often requires sifting through code repeatedly to find this "hidden bomb."
Scan Cycle "Black Hole"
Devices may not fully "see" the instructions. Set and Reset instructions are based on conditional judgments, but within a scanning cycle, if neither the start nor stop button is pressed, Q0.0 might not be written to at all. This seemingly harmless skip can trigger unpredictable situations under certain conditions. This "black hole" of skipping makes it impossible to determine the device's state, leading to potentially flawed logic.
Logical Consistency Challenges
PLC control systems often need to process a combination of multiple input signals, especially in complex industrial environments. Using Set and Reset instructions can easily lead to inconsistent device execution logic due to minor timing differences between signals. For example, in a very short time, some input signals might miss the logic judgment window, leading to incorrect device operations.
A Better Choice: Self-Latching Logic for More Reliable Control
Since Set and Reset instructions are fraught with issues, how should we tackle these tricky scenarios? The answer is simple: use self-latching logic. Compared to traditional Set and Reset instructions, self-latching logic ensures that the device maintains the correct state after each operation, without the uncertainty caused by multiple writes or omitted operations.
Set and Reset instructions may seem convenient, but they hide problems such as low execution efficiency, the likelihood of missing instructions, and flawed logic, often costing more in debugging. For senior PLC engineers who pursue efficient and reliable control, self-latching logic is undoubtedly a better choice. After all, in the complex environment of industrial automation, what we need is control programs that can run stably and reliably, not commands that seem simple but are fraught with risks.
The next time you write a PLC program, try self-latching logic; it might save you a lot of headaches in debugging.
