What is a Control or Function Block?

Have you ever heard of a control block? How about a function block? Different programmers call them by different names, and that can be confusing, but I hope to clear things up for you in this article.

So, what exactly is a control or function block? They give us extra functionality in our PLC system. You have probably seen or used a basic timer or counter while looking through the program of a PLC.

When looking at the ladder logic, it literally looks like a block in the middle of a rung. These are examples of the control or function blocks that we are talking about.

The great thing about using one of these PLC control blocks is the fact that it can replace a complete physical component.

PLC Timers as Control or Function Blocks

Let’s discuss some of the most basic and commonly used control or function blocks. Probably the most common is a “timer”. More specifically the two basic types;

– Timer “ON delay”.

– Timer “OFF delay”.

A timer “ON delay” will wait to turn an output on after the set period of time.

A timer “OFF delay” will turn the output off after the set period of time.

Both timers must be triggered by an input. Once the input is energized, the timer will start.

OFF-Delay Timers and Car Wash

Anyone that has ever washed their vehicle at a car wash has experienced a timer off delay.

You put your money into the machine to turn the car wash equipment on. Adding the money into the machine acts as your input.

A timer immediately starts counting down as the pumps and car wash equipment turn on. Once the timer ends, the equipment will turn off. This is all possible with a simple control block known as the timer off delay.

ON-Delay Timers and Locking your Vehicle

An “ON delay” timer works similarly to an off delay timer except the timer will delay turning on the output instead of turning it off. You may have experienced the effects of this by simply locking your vehicle.

Many vehicles have a feature that will delay the locking of the vehicle by 3 to 5 seconds after pushing the lock button. In this scenario, the lock button acts as an input that triggers the timer. After the set amount of time, the output is energized and the vehicle then locks.

As you can imagine, timers such as these, are extremely useful in an industrial setting. Being able to program the timer in the PLC instead of using a hardwired mechanical timer saves time, money, and is far more reliable in the long run.

ON-Delay Timers and Box Filling Station

“ON delay” timing is used frequently in automated equipment. Let’s look at a box filling station. A box has to move from one set of rollers to a weight scale to be filled with a product. Once the box is filled, it triggers an “ON delay” timer for about one second.

This then triggers a pneumatic cylinder programmed to the timer’s output. This cylinder pushes the box to the next set of rollers. The timer allows that extra second to make sure nothing else is going to fall into the box and make sure the coast is clear for the box to move.

OFF-Delay Timers and Vacuum Systems

Vacuum pumps are commonly programmed with “OFF delay” timing control blocks. One example that I am familiar with is in the manufacturing of adhesives. Once the adhesive is mixed with all of its ingredients, the manufacturer must remove all of the extra air bubbles from the adhesive.

This is made possible with a vacuum pump. The operator will start the vacuum pump while the mixer is turning and let it run for the programmed period of time, usually half of an hour or longer.

Once the timing period ends, it will turn off the vacuum pump.

Let’s take a look at what we have learned until now.

Control blocks and function blocks are the same things called by different names. They perform a function in our programming of PLCs.

Some of the simple functions are Timers and Counters. More specifically are “ON delay” timing and “OFF delay” timing. These are examples that we talked about in more detail.

“ON delay” timers will turn on an output after the set period of time.

“OFF delay” timers will turn the output off after the set period of time.

These are extremely helpful and useful in our PLC programming. They can also replace hardwired physical components to give us more reliability and flexibility.

I truly hope this article has helped you in some way. There is a lot to learn when it comes to PLCs and programming. Getting a grasp of the simple control blocks here can ease your learning.

Thanks again for reading. Leave your questions and comments and we’ll chat with you soon!

Happy learning,

The RealPars Team

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