Basic Ladder Diagram Symbols

Power Rails & Links

A ladder diagram contains rungs with functions and objects. Each rung must be complete for proper operation (should connect to both power rails with continuity using horizontal and vertical links between objects.

Ladder Diagrams are limited on the left and right side by vertical lines called power rails (the left power rail and the right power rail). The figure below shows a typica ladder diagram utilizing power rails and links.

Creating links will be discussed later.

Connection Types

There are multiple types of connections (diagram circuits) that can be created. These are created by combining vertical and horizontal links. Objects can be combined into series or parallel circuits and combinations of serial and parallel circuits. Below are figures that represent series and parallel ladder diagram circuits.

Series Ladder Diagram Circuit

Parallel Ladder Diagram Circuit

Contacts & Coils

A ladder diagram can use many objects and functions blocks. The basic symbols for ladder diagrams are contacts and coils. Contacts and coils represent the real world inputs and outputs as well as any internal I/O (control relays).

The basic contacts and coils are :

	DIRECT CONTACT The direct contact may represent real world inputs or internal relay contacts (CRs). A TRUE condition on the real world input (or CR) will cause the contact to be TRUE. A FALSE condition on the real world input (or CR) will cause the contact to be FALSE.
	INVERTED CONTACT The inverted contact operates the same as the direct contact, with the exception that the actual logic is opposite of the direct contact. When the real world input (or CR) is FALSE, the contact is TRUE and when the real world input (or CR) is TRUE, then the contact is FALSE.
	DIRECT COIL The direct coil may represent real world outputs or internal relay coils (CRs). A TRUE condition on the direct coil will cause a TRUE condition on the real world output (or CR), while a FALSE condition on the direct coil will cause a FALSE condition on the real world outputs (or CR).
	INVERTED COIL The inverted coil operates the same as a direct coil with the exception the logic is the opposite of the direct coil. A FALSE condition on the inverted coil will cause a TRUE condition on the real world output (or CR), while a TRUE condition on the inverted coil will cause a FALSE condition on the real world outputs (or CR).