Free Form Ladder Diagram (FFLD)

A Ladder Diagram is a list of rungs.

Each rung represents a Boolean data flow from a power rail on the left. The power rail represents the TRUE state. The data flow must be understood from the left to the right. Each symbol connected to the rung either changes the rung state or performs an operation.
These are possible graphic items to be entered in FFLD diagrams:
- Power Rails
- Contacts and Coils
- Operations, Functions and Function A function calculates a result according to the current value of its inputs. A function has no internal data and is not linked to declared instances. blocks, represented by rectangular blocks.
  - See Call a Function or Call a Function Block.
- LABELS and Jumps JMP JMPC JMPNC JMPCN
- Use of ST Expressions in Graphic Language

Use of EN Input and ENO Output for Blocks

The rung state in a FFLD diagram is always Boolean.

Blocks are connected to the rung with their first input and output.
- This implies that special EN and ENO input and output are added to the block if its first input or output is not Boolean.
The EN input is a condition.
- It means that the operation represented by the block is not performed if the rung state (EN) is FALSE.
The ENO output always represents the sane status as the EN input.
- The rung state is not modified by a block having an ENO output.

This is an example of XOR / XORN with Boolean inputs and outputs and requiring no EN or ENO pin.
- First input is the rung.
- The rung is the output.

This is an example of the GT > (greater than) with non-Boolean inputs and a Boolean output.
- This block has an EN input in FFLD language.
- The comparison is executed only if EN is TRUE.

This is an example of the SEL with a first Boolean input but an integer output.
- This block has an ENO output in FFLD language.
- The input rung is the selector.
- ENO has the same value as SELECT.

This is an example of Addition + having only numerical arguments.
- This block has both EN and ENO pins in FFLD language.
- The addition is executed only if EN is TRUE.
- ENO has the same value as EN.