How to Make a One-Shot in the PLC

A one-shot in the PLC will turn an output on for one scan. This is used to trigger events that should only happen once. An example of this would be to increment a value in memory. If a one-shot is not used, then every scan of the PLC will increment the value.


What is a one-shot in the PLC?

One-shots are known by several other names. Differential Up (DIFU), Differential Down (DIFD), One Shot Relay (OSR), Powerflow Modifier, Leading edge contact, Trailing edge contact, etc. This all relates to the programmable controller that you are programming.

PLC Examples of One-Shots

Let’s take a look at programming a one-shot using bit logic only. We will program both a leading edge one shot and a trailing edge one-shot bit. This program will work in all PLCs.
Note: The white background in the increment (INC) instruction just indicates the reset for the animation.

How to Make a One-Shot in the PLC

Leading Edge One-Shot Bit Example

Leading-edge one-shot bit: This will turn on a bit for one scan when the input condition makes a transition from 0 to 1. (Off to on)
When input X0 turns on C0 is turned on for one scan. This is because it is in series with the C1 lead work bit. The next rung will latch this on and not unlatch it until the input condition X0 turns off. C0 will only be on for one scan when X0 turns on.

How to Make a One-Shot in the PLC

Trailing-Edge One-Shot Bit Example

Trailing edge one-shot bit: This will turn on a bit for one scan when the input condition makes a transition from 1 to 0. (On to off)
When input X0 turns off C2 is turned on for one scan. This is because it is in series with the C3 trail work bit. The next rung will latch this on and not unlatch it until the input condition X0 turns off.

The Do-more PLC has several different ways to do the leading and trailing edge one-shots. Here is a couple:

How to Make a One-Shot in the PLC

The leading or trailing edge contact instruction will allow logic flow for one scan from a transition. (On to off / Off to on)

How to Make a One-Shot in the PLC

The leading and trailing edge Powerflow Modifier is placed before the output. It will turn multiple input signals into a one-shot for the output.

Watch on YouTube: How to Make a One-Shot in the PLC
If you have any questions or need further information please contact me.
Thank you,
Garry



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