Tag Archives: ladder logic programming

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.


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.

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.

One Shot Bit Leading EdgeLeading 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.

One Shot Bit Trailing EdgeTrailing 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 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 are a couple:

Leading and Trailing Edge Contact

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

Leading and Trailing Edge Powerflow Modifier

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



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

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Creating a Flip Flop Circuit in the PLC

A flip flop circuit in a PLC usually has one input and two outputs. When the input is activated, the two outputs latch on/off opposite to each other alternately.  Basically, it is used to toggle (latch) an output on and off with just one input. In the PLC it is a single input that will toggle an output on and off each time the input signal is activated.


Here is an example of a hard-wired flip flop circuit using relays.
C9C_pulse_toggle_relays

The PLC program will be a little different than the relays because of the way in which the PLC scans. Scanning takes place from left to right, top to bottom. The output conditions from the logic are available to the next rung as the logic is solved. Outputs and inputs are read usually only once at the end of the scan. Remember to think of the outputs in the PLC as make before break. This is the opposite of the relay logic presented above which is break before make.

Let’s look at the logic. This is programmed using the Do-More Programming Software which comes with a simulator. This full programming package is free of charge and can be downloaded here.

Flip Flop PLC Circuit 1

The input is on leading-edge instruction. (One Scan) If output 2 is on then it will set output 1. If output 2 is not on then it will reset output 1. The third line of code will determine the state of output 2 based upon output 1.

You may be asking yourself why do we not just use the conditions from output 1 to control output 1.  This is because if we substituted output 1 for the conditions on the input then the output 1 would never turn on/off. The output conditions are available for the next line of PLC code. This would allow the output to be set and reset within the scan without being updated. Using output 2 is the only way in which this logic would work.

Here is an automated picture to show the input toggling on / off and the outputs flip-flopping.

Flip Flop PLC Circuit S

Note: An emergency condition can be added to the set or reset rungs to automatically control the output either way.

Watch on YouTube: Creating a Flip Flop Circuit in the PLC
If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.