What is a one shot ?

Mickey,
Siemens don't. They just go to 999. You can obviously cascade them to make a higher count but it's just easier (for me) to use an 'add'.

;-)

Cheers

John

Nice to know. Most of this thread was using AB terms. But we really don't
know what brand the OP is using.
 
Very good point.

I tend to use an 'ADD' for a lot of my counts though as I find the 999 maximum counter value a limitation for a lot of what I'm doing.

;-)

In siemens s7 this is naturally applicable only to s5 type counters, IEC counters can count to 32767.

I like just plain add too.
 
One Shot

Dear Folks,

Beside all what have been said,an important usage of the one shot , is: it's used as a shift pulse in the Shift Register instruction

Cheers to all,

Hagos
 
We(my employer) uses one shots on our reset buttons so that bubba the operator cannot bypass alarms by taping reset buttons in the pressed position.....
 
Controllogic counters go to 4294967295 because it is based on a DINT
on 32 bits.

Incorrect !

The DINT is a 32-bit data-type, but bit 31 is a Sign bit.

Therefore the maximum positive count it can hold is 2,147,483,647, which is 2#0111_1111_1111_1111_1111_1111_1111_1111

Because the counter uses pure binary addition (CTU), the next false-to-true rung transition adds 1, to give 2#1000_0000_0000_0000_0000_0000_0000_0000, which is the signed binary for -2,147,483,648.

This causes the Overflow (OV) flag to be set, but the counter will continue to count in the postive direction, through zero, all the way up to +2,147,483,647 a further 4,294,967,296 counts.

By taking the overflow bit into account, the counter can actually handle a count of 2,147,483,647 + 4,294,967,296 = 6,442,450,943 before we lose track of the magnitude, since the next count will not be able to flag itself, as the overflow bit is already set.
 
We(my employer) uses one shots on our reset buttons so that bubba the operator cannot bypass alarms by taping reset buttons in the pressed position.....

Or the machine cabinet wears a broom handle - looks funny.
sorry - no photos - cameras not aloud
 
Or the machine cabinet wears a broom handle - looks funny.
sorry - no photos - cameras not aloud

I guess that's so Bubba doesn't have to reach out too far.....

Reminds me of a premature failure of a PanelView membrane keypad - completely wrecked in 3 months !

I discovered (because I saw it happen), the operator was leaning over and using a 4inch RJT spanner to hit start and stop buttons o_O šŸ™ƒ
 
yep - the only reason we are leaving it that way is until we resolve some other issues. soon though - just need a shutdown window.
 
I have been around industrial automation for 24 years and I cannot believe that I still don't fully understand a "One Shot". Could someone explain that to me , and also an example of a "Real World" situation .

Thanks , in advance. Have a great Thanksgiving.



I find it hard to believe that after programming PLC's for 24 years, you've never understood the value or importance of a One-Shot. It's not an esoteric high-level concept like Modbus or communication protocols.

Let me guess, after 24 years, you've also never managed to understand the function of SET or RESET coils either? I only presume because one-shots are essential in the proper functionality of SET and RESETs. Perhaps you meant you have been programming for 2.4 years? Still considerably longer than I've been a controls engineer, and I'm not still fumbling with the basics. For someone who takes their skills or profession seriously, it's an embarrassment of an admission to not know this essential functionality, and you probably should have tried Google before throwing up your hands in defeat on this forum and wasting the time of others.

Regardless, although you receive no compassion from me for your laziness, I will still provide an answer to your post as their may be noobie amateurs who will stumble across the same question, and THEY deserve an answer (inexperience is an excuse I can accept):

One-shots are essential any time you have logic where you want to output a single action based on conditions which need to be met, but when you don't necessarily want to HOLD that output true the entire time the conditions solve:

"Debounce" for holding a pushbutton down is a good example (as another commentor mentioned, you could have a pushbutton held down for 100 PLC scans, and you wouldn't want to register 100 presses for each time that rung evaluates).

A practical application I implemented just yesterday involved putting a valve state in either AUTO or MANUAL. The operator can switch between AUTO and MANUAL at his command, but they also wanted the valve to automatically switch to AUTO based on certain conditions. If I did not use a one-shot for this logic, the rung would HOLD the valve in AUTO the entire time the conditions were true, and therefore the operator would be unable to put the valve back into manual. Therefore, a one-shot was appropriate because you want to initiate the change in state, but not maintain that state forever.


If it still doesn't make sense to you, there are some great jobs in the Amazon delivery field of work these days! 24 years is far too long to struggle with a simple concept. If nobody around you is telling you to change your profession, then I am your only true friend.
 
I find it hard to believe that after programming PLC's for 24 years, you've never understood the value or importance of a One-Shot. It's not an esoteric high-level concept like Modbus or communication protocols.

Let me guess, after 24 years, you've also never managed to understand the function of SET or RESET coils either? I only presume because one-shots are essential in the proper functionality of SET and RESETs. Perhaps you meant you have been programming for 2.4 years? Still considerably longer than I've been a controls engineer, and I'm not still fumbling with the basics. For someone who takes their skills or profession seriously, it's an embarrassment of an admission to not know this essential functionality, and you probably should have tried Google before throwing up your hands in defeat on this forum and wasting the time of others.

Regardless, although you receive no compassion from me for your laziness, I will still provide an answer to your post as their may be noobie amateurs who will stumble across the same question, and THEY deserve an answer (inexperience is an excuse I can accept):

One-shots are essential any time you have logic where you want to output a single action based on conditions which need to be met, but when you don't necessarily want to HOLD that output true the entire time the conditions solve:

"Debounce" for holding a pushbutton down is a good example (as another commentor mentioned, you could have a pushbutton held down for 100 PLC scans, and you wouldn't want to register 100 presses for each time that rung evaluates).

A practical application I implemented just yesterday involved putting a valve state in either AUTO or MANUAL. The operator can switch between AUTO and MANUAL at his command, but they also wanted the valve to automatically switch to AUTO based on certain conditions. If I did not use a one-shot for this logic, the rung would HOLD the valve in AUTO the entire time the conditions were true, and therefore the operator would be unable to put the valve back into manual. Therefore, a one-shot was appropriate because you want to initiate the change in state, but not maintain that state forever.


If it still doesn't make sense to you, there are some great jobs in the Amazon delivery field of work these days! 24 years is far too long to struggle with a simple concept. If nobody around you is telling you to change your profession, then I am your only true friend.

Calm down there, necroposter.
 

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