Ladder logic diagram
A ladder logic diagram is so-called because the relay logic runs in parallel lines between two power lines and the whole diagram resembles a ladder. This diagram consists of one vertical line running down the left side, with the horizontal lines branching off to the right.

The line on the left is called the bus bar, while horizontal lines are instruction lines or rungs. Along the instruction lines conditions are placed, that lead to other instructions on the right side. Power flow is always from left to right.

Therefore, the logical combination of these conditions from left to right side determines when and how the instructions at the right side are executed.

In a ladder logic diagram, instruction lines can have multiple branches. The vertical pairs of lines are called conditions. Conditions without diagonal lines through them are called NO conditions that correspond to AND, LOAD, or OR instruction.

The conditions with diagonal lines through them are called NC conditions that correspond to AND NOT, LOAD NOT, or OR NOT instruction. Each condition has a number above/below each condition that indicates the operand bit for the instruction. Operand bit (Input/ Temporary bit) is associated with that

The status of the bit determines the execution condition for the following instructions.

Basic terms used in ladder logic
Normally open and normally closed conditions
Each condition in a ladder logic diagram is either ‘ON’ or ‘OFF’ depending on the status of the operand bit that has been assigned to it. A NO condition is ‘ON’ if the operand bit is ‘ON’ and it is ‘OFF’ when the operand bit is ‘OFF’. On the other hand, a NC condition is ‘ON’ if the operand bit is ‘OFF’ and it is ‘OFF’ when the operand bit is ‘ON’.

In short, an NO condition simply follows the bit status (on => on and off => off) and an NC condition follows inverted bit status (on => off and off => on).

Execution conditions
In a ladder logic program, the logical combination of ‘ON’ and ‘OFF’ conditions before an instruction determines the conditions under which the instruction is executed. This condition is called the execution condition for the instruction. Except for the ‘LOAD’ instruction, all other instructions have execution conditions.

The operands designated for any of the ladder logic instructions can be I/O bits, flags, work bits or flags, timers, or counters, etc. In a ladder logic diagram, these conditions can be determined using these operands.

Logic blocks
The manner in which the conditions correspond to instructions is determined by the relationship between the conditions, within the instruction lines that connect them. Any group of conditions that go together to create a logic result is called a logic block.

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