PLCs or Programmable Logic Controllers are utilized in complex automation systems to
carry out a series of process tasks.
Ladder Logic is used to program these PLCs in order for them to communicate a specific
sequence to each part of the system.
Ladder logic was originally a written method to document the design and construction of
relay racks.
Each device in the rack would be represented by a symbol on the ladder diagram along with
other, external items in the system like pumps and chillers.
These diagrams were used so relay rack setups could be replicated at a later date or in
another location.
As technology advanced, traditional relay racks gave way to the more efficient and expandable
PLC and old ladder diagrams evolved into the programming language that we know today as
ladder logic.
The motivation behind utilizing the ladder diagram as the base for the new programming
language was pragmatic.
Since it was using something that technicians were already familiar with, it did not require
intensive training in complex computer programming languages like FORTRAN.
Ladder Logic notation is best suited for systems where binary values are required.
For example, a contact is either open or closed.
Here is a sample of a very simple Ladder Logic Program:
Here, we see a ladder logic program with two regular contacts illustrated by open and closed
brackets, that represent key switch 1 and key switch 2, and one regular coil represented
here by open and closed parentheses, that represents a door motor.
A program of this nature could be utilized for physical security within a system.
Since Key Switch 1 and Key Switch 2 are aligned in sequence, both of these conditions must
be true in order to engage the door motor.
So, both switches must make contact in order for the door to open in this scenario.
Ladder Logic has an interesting past and clearly illustrates how necessity is the mother of
invention.
Ladder logic programs in PLCs can be very power and have the ability to control complex
industrial processes.