PLC ladder diagrams
PLC are programmable logic controllers, mainly used in industrial automation. A PLC is cheaper
than a relay logic circuit and reduces the chance of failure.
Besides the working of a machine can be modified without rewiring the electric circuit.
The most used PLC programming language is ladder logic.
A ladder diagram is similar to a relay logic
circuit schematic: its main elements act like electromechanical components, though they are just logic instructions.
So we can talk about virtual relays, coils and contacts.
A PLC takes information by input terminals, on which we can connect (for example) button switches, dump switches and
By output terminals the PLC drives contactors, pneumatic or hydraulic valves, lamps etc.
Ladder logic contacts can be driven by input terminals or by coils.
Basically there are two kinds of ladder logic coils: outputs and merkers.
Output coils are so called because they drive PLC output terminals.
Merkers are internal relays, that drive each other and output coils.
To fully understand basic ladder logic have a look at the animated image below, that represents a
basic start-stop configuration.
Two buttons are connected to the input terminals I1 and I2 of the PLC, while a contactor coil is
connected to the output terminal O1.
The ladder diagram consists of the output relay O1 and the input logic contacts
(dashed line = contact closed).
When the start button is pressed the input terminal I1 activates, so the logic contact I1 closes;
the coil O1 turns on and keeps in that condition when the start button is released.
By pressing the stop button the input terminal I2 activates, so the logic contact I2 (which is NC) opens
and turns off the relay O1.