Momentary Push button

plcnovel

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Join Date
Feb 2006
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Hi All
how does a momentary push button in Panel View 1000 work? If its hold time is set to 500 ms, does it mean if some one pushes it on the screen, it will be set (1) for 500 ms and writes to the tag that value? Once the 500 ms elapses, it will reset (0) and writes to the tag '0' associated to it?

Also , how does a TOF instruction in SLC Processor works? I will appreciate your time if you answer to my queries. Thanks in advance
 
It means that when the button is pressed, the input is true

When the button is let off, the input stays on for 500 ms then off.

Because of delays in communications, this is sometimes necessary so the PLC will see that the operator has pressed the button. When you have a button used to jog something, the hold time can be a real pain in the rear
 
plcnovel said:
Hi All
Also , how does a TOF instruction in SLC Processor works?

The TOF (Timer Off Delay) is just as it says it is. When the rung containing the TOF is true, the timer is reset and not timing. When the rung goes false, the timer begins timing and will time to it conclusion unless the rung goes true again, at which time it resets and stops timing. Try one in a program and do not control anything with it, just watch it run. You will get the gist of it pretty quick.
 
Sorry, missed the TOF question. Had a flash back to the hold time on one of my jog buttons crashing a machine. They have situations that they are good and situations that are bad
 
TOF's are as Randy described, but you need to do as he advised... See how one operates in your PLC.

In some PLC's, they start timing based on a TRUE to FALSE transition of the rung logic, in others, they operate only on the state of the rung logic. That can cause some seriously wierd behavior when cycling from STOP to RUN mode.

Just something to be aware of.
 
This is just to reiterate what others have already said here, but to give a little more info.

When using a TON timer, when the rung is true the EN bit goes true and stays true as long as the rung is true. After the rung is true the TT will become true as long as the timer is timing. If the rung goes false so does the TT bit. The DN bit goes true at the conclusion of the preset time and will remain true until the rung goes false.

Now with the TOF timer, when the rung is true the EN bit is true (just like the TON) and will stay true as long as the rung is true. Once the rung goes false the TT bit will be true as long as the timer is timing, while the EN will be false. Now the DN bit will go true when the rung is true and will remain true after the rung goes false until the timer times out.

I hope that explains things well enough. Again, try it without controlling anything first, it can be a bit tricky until you get to know the instruction. This is a very useful instruction if used correctly.

Be careful, too, when controlling an output if say an EStop is pressed. The output will turn off, but if the EStop is released before the TOF has timed out, the output will go true again. (I learned this the hard way .... and got a nickname because of it!! LOL) Something that you can do, if this were the case, is to monitor the EStop input and if the EStop has been pressed, then MOV the PREset into the ACCumulator, this will stop the TOF from timing.
 
To clarify the hold time on the PV.....

The hold time will guarantee the input stays in the "pressed" state for a minimum amount of time.

So, if you momentarily press and release the button the hold time should ensure the PLC sees the pressed state.

HOWEVER......

If you hold the button down longer than the hold time then as soon as you release the button it will immediately release.

The hold time only guarantees a minimum, it does not ensure an off delay.

OG
 
The Screens will set/reset bits assuming ladder logic leaves the bits alone. For example:

If a momentary PB is set for .5 second hold on, the panel will set the bit, then reset the bit .5 sec later. It writes the bit twice and twice only. It will not hold the bit on. If the ladder resets the bit .1 sec after the screen sets it, the screen will not set it again.

If the PB is maintained, the screen sets it to one and leaves it alone. If the ladder resets it, it's off and the screen will not set it again unless the PB is pressed again.

I try to always use maintained PB's and reset the bits in my ladder when I'm done with them. Then all my PB's work the same way throughout a project. I know what to expect from the MMI.
 
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