Introduction to Basic Ladder Logic Instructions

Introduction

In this section we will cover some of the basic instructions used in PLC programming. These instructions are some of the most used instructions in PLC programming. Once we explain in detail the instructions then we will have a look at how we can implement and used those instructions in real life application.

Definition

The following is a list of the basic instructions in SLC 500:

• XIC - Examine if Closed
• XIO - Examine if Open
• OTE - Output Energize
• OTL - Output Latch
• OTU- Output Unlatch
• OSR - One-Shot Rising

XIC Examine if Closed

Symbol

Definition

• Examines a bit for an On condition
• Use the XIC instruction in your ladder logic to determine if a bit is ON.
• 0 = False
• 1 = True

Devices

• Start/Stop push buttons
• Selectors
• Limit switch
• Proximity switch
• Light
• Internal bit

XIO Examine if Open

Symbol

Definition

• Examines a bit for an off condition.
• Use an XIO instruction in your ladder logic to determine if a bit if off.
• 1 = True
• 0 = False

Devices

• Start/Stop push buttons
• Selectors
• Limit switch
• Proximity switch
• Light
• Internal bit

OTE Output Energize

Symbol

Definition

• Turns a bit on or off
• Use OTE instruction in your ladder logic to turn on a bit when rung condition is evaluated as true.

Devices

• Light
• Motor run signal
• Internal bits

OTL Output Latch

Usually we latch a signal with a condition and then unlatch it when a different condition becomes true. Most of time the Latch / Unlatch go together.

Symbol

Definition

• Turns a bit on when the rung is executed, and this bit retains its state when the rung is not executed or a power cycle power occurs.
• OTL is a retentive output instruction. OTL can only turn on a bit. This instruction is usually used with OTU with both OTL and OTU addressing the same bit.
• Ladder logic can examine a bit controlled by OTL as often as necessary.
• When you assign an address to the OTL instruction that corresponds to the address of a physical output, the output device wired to the screw terminal is energized when the bit is set. When rung conditions become false, the bit remains set and the corresponding output device remains energized.
• Actuating the latch input turns the function on or causes it to change state. The function then stays on even if the latch input is turned off. To turn the function off, another input must unlatch which turns the function off.

OTU Output Unlatch

Symbol

Definition

• Turns a bit off when the rung is executed, and this bit retains its state when the rung is not executed or when power cycle occurs.
• OTU is a retentive output instruction. OTU can only turn off a bit. This instruction is usually used with OTL with both OTL and OTU addressing the same bit.
• Ladder logic can examine a bit controlled by OTU as often as necessary.
• When you assign an address to the OTU instruction that corresponds to the address of a physical output, the output device wired to the screw terminal is de-energized when the bit is cleared.
• The unlatch instruction tells the controller to turn off the addressed bit. Thereafter, the bit remains off, regardless of the rung condition, until it is turned on.

OSR One-Shot Rising

Symbol

Definition

• Triggers a one-time event.
• The OSR instruction is a retentive input instruction that triggers an event to occur only one time. Use the OSR instruction when an event must start based on change of state of the rung from false to true.
• When the input instruction goes from false to true, the OSR instruction conditions the rung so that the output goes true for one scan. The output goes false and remains false for successive scans until the input makes another false to true transition.