different types of one shot

plc noob

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i am trying to learn plcs ( going to school at night ) and working for an automation contractor by day doing small pieces of large projects . i have not needed to do a one shot instruction as of yet but i want to be prepared when i do to use one . my question is what is a one shot ? how does it work ? and what is the difference in the AB brand one shots . i see they have ONS,OSF,OSR for the different processors . which is the best to use when i have more than one option ?
 
Ab one shots

ONS
The ONS instruction is an input instruction that makes teh rung true for one program scan upon a false-to-true transition of the conditions preceding the ONS instrution on the rung. Use the ONS instruction to start events that are triggered by a pushbutton, such as pulling values from thumbwheel switches or freezing rapidly displayed LED values.

This instruction is used in Micrologix 1200 and 1500

OSF
The OSF is conditional output instruction that triggers an event to occur one time. Use the OSF whenever an event must start based on the change of state from true-to-false,not on the resulting rung status. You must enter a bit address for the OSF storage bit. Use either a binary file or integer file address. This is the bit address that remembers the rung state from the previous scan. This address you use must be unique. Do not use it elsewhere in the program.

This instruction is used in Micrologix 1200 and 1500

OSR
The OSR is conditional input instruction that triggers an event to occur one time. Use the OSR when and event must start based on the change of state of the rung from false-to-true, as triggered by a pushbutton. An example would be to use the OSR to freeze rapidly display LED values.
Place the OSR on the rung immediately before the output instruction. Then reference the ouput as the one-shot in your program. The address that you give to the OSR input instruction is where the previous state of the rung is retained. You must enter a bit address for the OSR. Use either binary file or integer file address. The bit address you use must be unique. Do not use it elsewhere in the program.
The Address assigned to the OSR instruction is not the one-shot address referenced by your program, nor does it indicate the state of the OSR instruction. This address allows the OSR instruction to remember its previous rung state.

This instruction is used in all SLC and Micrologix 1000
 
It depends a great deal on which AB PLC you are using

In the SLC-500 series PLCs and the MicroLogix 1000 the only one shot instruction is the OSR. It is an input instruction. You need to be aware that on SLC5/01 and SLC5/02 PLCs this instruction has some restrictions on how it can be used, particularly in rungs with branches. The restrictions are covered in detail in the SLC 500 instruction set reference manual available at AB's online documentation library.

In the PLC/5, the MicroLogix 1100, 1200, 1500 there are three one shots. The ONS instruction is an input instruction and OSR and OSF are output instructions. The output bits for OSR and OSF instructions program a little differently in the PLC/5 than in the Micrlogix PLCs but otherwise function basically the same way.

In the control logix PLC there are five one shot instructions: ONS, OSR, OSF, OSRI, and OSFI. ONS, OSR, and OSF are similar in operation to the ones on the Micrologix PLCs. OSRI and OSFI are one shot function blocks.





When the input conditions preceeding a one shot rising instruction are true then the instruction operates like this:

If (Storage_bit) then
Output_bit:= false
else {
Storage_bit := true
Output_bit:= true
}




If the rung consition preceeding the one shot rising instruction are false, then the instruction does this:

Storage_bit := false
Output_bit := false


So, when the rung becomes true after it was false, the storage_bit is not set, so the storage bit is turned on and the output bit is turned on (Or true is passed in the case of the inline ONS instruction). On the next scan, if the rung is still true, the instruction finds that the storage bit is already set so it sets the output false - thus the output bit is on for only one scan.


A one shot falling bit works in a similar fashion, except it triggers the output bit for one scan when the rung changes from true to false.

Since you have asked this question for learning purposes, here is a little homework for you: Construct a short program that creates a one shot rising bit without using the ONS, OSR, OSF instructions, using only instructions from the set XIC, XIO, OTE, OTL, OTU.

Feel free to search the site, because constructing generic one shot logic has been discussed quit a bit.
 

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