Hello everyone! Today, let’s talk about standard function blocks in PLCs. To be honest, many engineers have used PLCs for years but are still a bit confused about using standard function blocks. What a pity! These little blocks are the secret weapon to improve programming efficiency—it’s incredible!
Just last week, I encountered a situation where a production line suddenly stopped because an engineer misused a timer! Can you believe it? The entire factory was down for two hours… the losses were significant. Upon careful analysis, it was entirely due to a misunderstanding of the differences between TON and TOF, which led to chaotic logic in the conveyor belt start-stop sequence. Isn’t this a typical case of a weak foundation in function blocks?
Honestly, many problems are not complicated, but if you don’t truly understand the characteristics and applicable scenarios of each function block, it’s easy to fall into various inexplicable pitfalls. Alright, let’s get to the good stuff today!
Sequential Control: A Deep Dive into Timer Function Blocks
TON, TOF, and TP are the three most commonly used timers. What’s the difference? TON is on-delay, TOF is off-delay, and TP is pulse timing. But many people confuse them—it’s too easy!
TON On-Delay is the Most Practical. Its logic is: when the input signal becomes TRUE, the timer starts counting, and after reaching the preset time PT, the output Q becomes TRUE. It is particularly suitable for scenarios like device start delays and alarm delay triggers.
A simple example:
TON_1(IN:=Start_Button, PT:=T#5S);
Motor_Start := TON_1.Q;
But on second thought, TOF can also be incredibly powerful when used well! It starts counting when the input signal becomes FALSE, keeping the output TRUE during the counting period, and after the counting ends, the output becomes FALSE. This characteristic makes it an excellent choice for delayed stopping of conveyor belts and delayed turning off of lights—surprisingly practical!
Counting and Pulse Processing: CTU and CTD Function Blocks
Counting function blocks… Honestly, if used well, they can solve many production statistics problems! CTU counts up, CTD counts down, and CTUD combines both.
The CTUD function block is particularly common in the packaging industry. It can handle the counting logic for products coming in and out simultaneously, solving both forward and reverse counting with one block—what a time saver!
Code example:
CTUD_1(CU:=Product_In, CD:=Product_Out, PV:=100);
Full_Alarm := CTUD_1.QU; // Alarm when upper limit is reached
The problem is that many beginners do not understand the difference between the R and LD terminals. R resets the count value to 0, while LD loads the preset value PV into the current value of the counter. This detail—it’s really easy to overlook!
Data Comparison Function Blocks: Simplifying Logical Judgments
Comparing sizes. Is this simple? Wrong! I often wonder why so many engineers write complex logic instead of using the ready-made comparison function blocks?
GT (greater than), LT (less than), GE (greater than or equal to), LE (less than or equal to), EQ (equal to), NE (not equal to)—these comparison function blocks are truly code simplifiers. One line of code does the trick, no need to complicate things!
Temperature control example:
GT_1(IN1:=Current_Temp, IN2:=Max_Temp);
Cooling_Fan := GT_1.Q; // Start fan when temperature is too high
Data Selector SEL is Extremely Useful. Imagine: selecting different parameters based on different conditions, writing it with IF-THEN would be long and cumbersome, but SEL solves it in one line! Input G determines whether the output is IN0 or IN1, it’s that simple… astonishingly concise!
Edge Detection Function Blocks: Capturing the Moment
R_TRIG and F_TRIG. They detect rising and falling edges, respectively—fundamental function blocks that must be mastered! But strangely, many automation problems actually stem from here.
Want to detect the moment a button is pressed? Use R_TRIG. Want to execute an operation at the moment a signal disappears? F_TRIG is your go-to.
They only output TRUE during the scanning cycle when the signal changes, automatically resetting in the next cycle—this feature is crucial! Avoiding repeated triggers in the program relies on them. Without this, manually clearing the signal? How much would the code bloat…
Practical Application: Function Block Combinations
Theory is understood by everyone. The key is to apply it flexibly! For example—counting control in a filling machine:
1. Use CTU to count the number of products
2. Use R_TRIG to capture the rising edge of the sensor signal
3. Use TON to set the filling delay
4. Use SEL to switch filling parameters between different batches
Looking at it from another angle, these function blocks combined can solve complex control requirements, and the code is concise and easy to read—why not?
If we delve deeper, many automation problems are essentially about “time” and “sequence.” Mastering these standard function blocks equates to mastering the ability to handle 80% of industrial automation issues!
The True Advantage of Standard Function Blocks lies in their reusability, high readability, improved programming efficiency, and simple maintenance. Compared to writing logic from scratch, using existing function blocks can reduce at least 60% of the code volume and 80% of debugging time—doesn’t that sound appealing?
Alright, everyone, that’s it for today’s valuable content. Do you also have any unique tips for using function blocks? Hurry up and share them in the comments, let’s improve together! Give a thumbs up and follow for more PLC practical experiences coming your way…
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