Ladder logic is a graphical programming language commonly used in programmable logic controllers (PLCs) for industrial automation. It uses symbols to represent logic functions, input/output devices, and control elements in a ladder diagram. Here are some of the most common ladder logic symbols:

1. Contacts (Normally Open and Normally Closed):

   • Normally Open Contact (X): Represents a condition that must be true (input energized) for the associated logic to be executed.
   • Normally Closed Contact (X): Represents a condition that must be false (input de-energized) for the associated logic to be executed.

2. Coils:

   • Output Coil (Y): Represents an output device or action that is controlled by the logic in the ladder diagram. When the associated logic is true, the coil energizes, causing the output device to activate.

3. Seal-In (Latch):

   • Seal-In Contact (Y): Represents a self-holding or latching circuit. Once the associated coil is energized, the seal-in contact maintains continuity even if the original input condition changes.

4. Timers and Counters:

   • Timer (T): Represents a timer function that delays the execution of logic based on a preset time interval.
   • Counter (C): Represents a counter function that increments or decrements a count value based on input conditions.

5. Special Function Blocks:

   • Rising Edge Trigger (↑): Represents a condition that is true only when the input signal transitions from false to true (rising edge).
   • Falling Edge Trigger (↓): Represents a condition that is true only when the input signal transitions from true to false (falling edge).
   • Set/Reset (S/R): Represents a set/reset function that toggles the state of an output coil based on input conditions.

6. Mathematical Operations:

   • Math Operations (+, -, *, /): Represents basic mathematical operations (addition, subtraction, multiplication, division) used to manipulate data values in ladder logic.

7. Comparison Operations:

   • Greater Than (>): Represents a comparison operation that checks if one value is greater than another.
   • Less Than (<): Represents a comparison operation that checks if one value is less than another.
   • Equal To (=): Represents a comparison operation that checks if two values are equal.

8. Function Blocks:

   • Function Block (FB): Represents a reusable block of logic or a subroutine that performs specific control functions.

These symbols are arranged in a ladder diagram format, with vertical rails representing power rails (usually labeled L and N for line and neutral) and horizontal rungs representing logical sequences of operations. The symbols are interconnected by vertical lines representing electrical continuity.