PLC one-shot low level (OSL) ladder diagram instruction

A one-shot low level (OSL) instruction is a normally output true instruction. Whenever its input is false then it output false for a PLC scan cycle.

The one shot low level instruction symbol

For more details let see its timing diagram.

The OSL timing diagram

This instruction is useful in the case that the user needs to trigger the action on the falling to low level of the input signal.

In the following example, we create a sample monitoring of this instruction. Whenever the user switch the input contact to the ground, the contact makes a input false signal to the OSL. This OSL again trigger a low level pulse of one PLC scan cycle of 10 mS.

The OSL ladder diagram example

The output coil Y0 is normally output true in the overall PLC program. The output coil Y1 outputs true when there's nothing happen.

Circuit Diagram

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PLC one shot rising ladder diagram instruction

In the PLC ladder diagram a one shot instruction creates a one shot signal output. For the one shot rising OSF instruction, it output a high pulse for one PLC cycle time - for example 10 mS, whenever its input true. This output pulse only create one's until the system is reset.

The one shot rising instruction symbol

This instruction is an auxiliary to other instruction in the same rung. It's place in front of other instructions to make them run only once when the program started.

Let take a look at a simple ladder diagram example. 

OSF ladder diagram example program

An OSF activate the turn off delay T0F that create a high delay time for 10 second before it's fall. So the output coil Y0 turned on for only 10 second in the PLC program.

The ladder diagram programming example of the OSF

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PLC turn-off delay Tdoff ladder diagram instruction

Referring to the timer delay related instructions in the PLC ladder diagram instructions, we are now discussing about another timer delay called turn-off delay (Tdoff). 

Turn-off delay instruction symbol

When ever the input to this instruction changes from false to true, it will output true for a specific delay time. The input high time must greater or equal to the PLC cycle time.

Timing diagram for Turn-off delay instruction

Let see a very simple example. I use an input contact to turn on an output coil for 10 seconds.

Turn-off delay ladder diagram instruction example

We configure the Tdoff instruction as below.

The delay time is set to 10000 mS or 10 second.

It's convenience to add a one-shot rising to the rung to prevent the rung become true while the input contact X0 is in a long press duration.

The remanding open-circuit rungs are just an example of cutting the rungs. 

For a serious PLC programmer they always need to test the program in both hardware and software. However, we already know that the overall program is completedly work. So It's not necessary to test this program in another software simulation and in the hardware.

Schematic diagram

Click here to download the zip file of this ladder diagram example.