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PLC Programming Fundamentals – OSF | One Shot Falling | Instruction

Vladimir Romanov
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The OSF, also known as One Shot Falling, instruction allows a programmer to create a scenario where an output is energized for a single scan when a transition from a logic HIGH to a logic LOW is detected on the input side of the instruction. The instruction allows an experienced programmer to capture key components based on such a transition and allow a one-time execution of particular bits of logic. This instruction can be found on the right side of a ladder logic rung and has two bits tied into it. One of these bits, labeled as “Output Bit”, can be used elsewhere in the code to energize other instructions when the OSF is enabled.

Example & Usage of OSF

Here’s a real-world scenario of an OSF instruction:

  1. A Micrologix 1100 Allen Bradley PLC is used to control a process.
  2. A “Heating Process Start” status is tied to I:0/0.
  3. The F8:0 register is used to read the temperature of the process.
  4. The F8:3 register is used to store the final temperature of the process.
  5. An OSR Instruction is tied to the output of the“Heating Process Start” XIC instruction.
  6. The “Output Bit” of the OSF instruction is used on an XIC instruction in rung “0003”
  7. Once the I:0/0 is pulled LOW, the OSR is executed once.
  8. Once the OSF executes, the “Output Bit” is set to HIGH for one scan of the PLC.
  9. The MOV instruction moves the contents of the F8:0 register to the F8:3 register.

Programming example in RSLogix 500:

The OSF instruction is energized by the “Heating Process Start” condition transitioning from TRUE to FALSE. As soon as this condition is met, the “Output Bit” of the OSF is set to HIGH for a single scan of the PLC. During this scan, the bit allows the MOV instruction to energize through an XIC instruction. The MOV instruction transfers the contents of a register into another at the end of the process.

Data Types Allowed for OSF

The OSF instruction will work with the following data types within the RSLogix 500 environment:

  • Boolean – The OSF requires two bits to operate. One is called the “Storage Bit” and the other the “Output Bit”. The “Output Bit” should be the only one used outside of the instruction.

Important Notes

  • Note 1 – The OSF instruction requires two non-input boolean bit to function. The “Storage Bit” should not be used anywhere else in the program. The “Output Bit” should be used as a condition to execute other instructions.
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