Ladder logic has contacts that make or break circuits to control coils. Each coil or contact corresponds to the status of a single bit in the programmable controller’s memory. Unlike electromechanical relays, a ladder program can refer any number of times to the status of a single bit, equivalent to a relay with an indefinitely large number of contacts.
So-called “contacts” may refer to physical (“hard”) inputs to the programmable controller from physical devices such as pushbuttons and limit switches via an integrated or external input module, or may represent the status of internal storage bits which may be generated elsewhere in the program.
Each rung of ladder language typically has one coil at the far right. Some manufacturers may allow more than one output coil on a rung.
- Rung input : checkers (contacts)
—[ ]—Normally open contact, closed whenever its corresponding coil or an input which controls it is energized. (Open contact at rest)
—[\]—Normally closed (“not”) contact, closed whenever its corresponding coil or an input which controls it is not energized. (Closed contact at rest)
- Rung output: actuators (coils)
—( )—Normally inactive coil, energized whenever its rung is closed. (Inactive at rest)
—(\)—Normally active (“not”) coil, energized whenever its rung is open. (Active at rest)
The “coil” (output of a rung) may represent a physical output which operates some device connected to the programmable controller, or may represent an internal storage bit for use elsewhere in the program.
A way to recall these is to imagine the checkers (contacts) as a push button input, and the actuators (coils) as a light bulb output. The presence of a slash within the checkers or actuators would indicate the default state of the device at rest.