TOF Timer Off Delay Symbol Definition. Counts time base intervals when the instruction is false. The Timer Off Delay instruction begins to count time base intervals when the rung makes a true to false transition. As long as rung conditions remain false, the timer increments its accumulated value (ACC each scans until it reaches the preset value (PRE).
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Industrial automation technologies have made manufacturing processes increasingly more efficient. Programmable logic controllers (PLCs) are commonly used in industrial automation, and many of these industrial computer control systems use ladder logic as their main programming language. In this course, engineer and educator Zahraa Khalil covers the basics of PLC ladder logic programming. Explore fundamental concepts such as rung operation, timer and counter instruction, and the main types of input and output instructions used in ladder logic programming. Along the way, Zahraa shares case studies that can help you grasp the real-world application of these concepts.Related courses
Course Transcript
- The main two types of timers are, Timer ON Delay and Timer OFF Delay. And it's usually written on the timer instruction, what type of timer that is. Timer On Delay instruction, usually use to turn on an output, on or off after the timer has been on for a preset value. So let's take an example of a Timer On Delay that is set for 20 second. And, we use an input like a switch. I'm going to use a timer diagram, in order to explain the concept and to show the difference between the two timer types. This is the time diagram for the Rung. It goes from False to True. And if we have an input in the Rung, like in our case we have a switch, then when the input is True the Rung is True. This is showing how long the Rung is being True, or how long we have kept this switch on. Let's say that the Rung stays on for 40 seconds. Whenever the Rung is True, the Enable bit becomes True as long as the Rung is on. The timer is set to time for 20 seconds, so the timer is timing for 20 seconds only. This…Download courses and learn on the go
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Let's say I have a Boolean input which can switch its state automatically. Swichting from 0 to 1 and from 1 to 0 sets a cycle. To determine if that cycle is valid, the cycle time must be at least 10 seconds.
I can determine if one semicicle is invalid, but I can't figure out how could to be able to determine at the end of the cycle if it has been invalid.
This is what I have:
Aldridge1991Aldridge1991
1 Answer
You seem to be saying if the status of the input signal is not stable for 10 seconds, you have an invalid cycle and you want to check for that.
To do that, you obviously need a timer triggered by each transition. (I'm not specifically familiar with your PLC but they're all pretty similar so I'll assume that's what you have, and you appropriately have two, triggered on different transitions.
The invalid transition condition for positive-transition is:
That is, the timer must be running and the input must go to the wrong state.
Now, the input might spike low then go high again immediately; you probably don't want lose the fact that you had an invalid transition. So what you really want to do is latch the detection of invalid transition.
I'm not good at pretty ladder logic graphics so I'll write in old style:
A corresponding ladder rung is needed for negative transition.
Presumably, some other bit of code will notice the InvalidPositive, respond appropriately, and then unlatch the condition.
Ira BaxterIra Baxter