Introduction

The structure of an LD program corresponds to a rung for relay switching.

The left power rail is located on the left side of the LD editor. This left power rail corresponds to the phase (L conductor) of a rung. With LD programming, in the same way as in a rung, only the LD objects which are linked to a power supply, that is to say connected with the left power rail, are "processed". The right power rail corresponds to the neutral conductor.

All contacts and FFB inputs must be directly or indirectly connected with the left power rail and all coils and FFB outputs must be directly or indirectly connected with the right power rail to allow current flow.

A group of connected objects that are not connected to other objects (except for the power rails), is called a network or rung.

Properties of an LD program

Properties of an LD program:

• An LD section contains 11-63 columns and 17-3998 lines.

  The default number of columns for a new section is set via the Tools > Project Settings dialog box, in the Editors tab in the Number of columns text box.

  The number of columns can be increased or reduced during the creation of section contents.

  The number of lines can be increased or reduced during the creation of section contents.

• The width of the cells can be managed from one to three standard cell widths.

• LD programs are cell oriented, i.e. only one object can be placed in each cell.

• The processing sequence of the individual objects in an LD program is determined by the data flow within the section. Networks connected to the left power rail are processed from top to bottom (connection to the left power rail). Separate networks within the section are processed in the order in which they are placed (top to bottom). Refer also to section Execution order and signal flow).

• A syntax and semantics check is performed immediately after the statement is entered. The result of this check is displayed in colored text and objects.

• Syntactically or semantically incorrect sections can be saved.

LD program objects

The objects of the LD programming language help to divide a section into a number of:

• Contacts,

• Coils,

• EFs and EFBs (Elementary Functions and Elementary Function Blocks),

• DFBs (derived function blocks)

• Procedures

• operate blocks,

• compare blocks,

• jumps within the section and

• subroutine calls

These objects can be linked with each other through:

• Links or

• actual parameters (only FFBs).

Comments for the logic of the program can be added using text objects.

Edit and view functions

Edit and view functions of the LD editor:

• Selecting objects

• Deleting objects

• Cutting, copying and pasting objects

• Moving objects (also between different LD/FBD sections)

• Replacing objects

• Undo and Redo

• Using bookmarks

• Searching and replacing variables and function blocks and DFB instances

Input assistants

Input assistants for the LD editor:

• Mnemonics for contacts, coils, blocks, links and jumps

• Function input assistants for functions, function blocks and procedures

Online functions

Online functions of the LD editor:

• Display the current value.

• Setting breakpoints

• Setting watchpoints

• Step by Step