Ladder logic diagrams provide a standard representation of circuits that are convenient
for design, analysis, and troubleshooting. They can be thought of as a means to describe
outputs based on inputs. The vertical rails (the leftmost and rightmost vertical lines)
indicate the power source, while the horizontal and vertical lines (the symbols placed
on the rungs of the diagram) indicate the possible current flow. Buttons, contacts, limit
switches, and coils each have a representative symbol that can be placed on the rungs of
the ladder. Upon activation of appropriate contacts, the coil is energized and the
appropriate contacts are in either a opened or closed state.
A simple implementation of ladder logic is illustrated in the two images below. Below and
to the left is an electrical diagram of a switch and a motor, then below and to the right
is the equivalent ladder logic diagram.
SYMBOLS & NOTATION:
Relay contacts or switches that are normally opened are depicted by two parallel vertical
lines. When activated the switch or contact closes and power flows. Relay contacts or
switches that are normally closed are depicted by two parallel vertical lines with a
diagonal line running accross them. When activated the switch or contact opens allowing
power to flow. The figure to the right illustrates the appropriate symbols.
Outputs allow switches to close; thus, supplying or cutting off power to control devices.
Ladder logic indicates what to with an output, regardless of what is attached. Outputs
can go to electrical outputs, or to memory.
Below are some output symbols and their respective meanings.
Considering the leftmost symbol above, when it is energized, the output turns on. In
contrast, considering the middle symbol above, when it is energized, the output is turned
off. The symbol above and to the right represents a one-shot relay. This is where an
input will cause the output to go ON for one scan.
When L is energized, X (below, left) will be toggled ON and will stay on until U (below,
middle) is energized. This is similar to a flip-flop and will stay set even when the PLC is
turned off. Some PLCs allow immediate outputs and do not wait for a program scan to complete
before setting an output. The rightmost symbol below represents this characteristic.
The notational identifiers for ladder logic diagrams are X, Y, and C, which is representative
of inputs, outputs, and control relays, respectively.
Switches and relay contacts can be wired into the various logic functions, such as OR, AND,
and combinations of the two.
An example of the OR arrangement is illustrated in the figure to the left. In this scenario,
each switch is located on seperate rungs, each having a power path to the output. Therefore,
if either switch, X1 or X2, is turned ON, then the appropriate contact closes and the output
Y1 is energized.
The figure to the right illustrates the AND condition. In this case, the switches are lined
up on the same rung; thus, there is only a single power path to the output. In order for the
output Y1 to become energized, both the switches, X1 and X2, must be turned on.
As the diagram indicates, the switches are processed sequentially. For instance, if switch X1
is turned ON and switch X2 is turn OFF, power terminates at switch X2. Likewise, if switch X1
is turned OFF and switch X2 is turn ON, power terminates at switch X1 and switch X2 does not
The figure on the left illustrates a slightly more complicated combination of the logical
operations and switches. This scenario combines AND and OR operations, as well as switches
that are normally opened and normally closed.
To explain, when switch X4 is OFF, the motor is energized through a set of normally closed
contacts (X4) on the switch. In the case that X4 is turned ON, its normally closed contacts
open and the motor can only be energized by turning ON X1 or X3, which in turn closes the
appropriate contact, X1 or X3.
If both switches X2 and X4 are turned ON, then their normally closed contacts will be open.
In this state, only when switch X3 is turned ON and its contacts are closed will the motor
In addition, timers can be introduced into a control system. The figure to the right
illustrates how a timer would be included in a ladder logic diagram. Since the discussion
of ladder logic is relative to PLCs, a brief description of PLC timers is appropriate.
Programmable controllers can effectively duplicate electromechanical sequencing drum timers,
time-out timers, and counters. They are built into the controller and only need be programmed
to be utilized. No additional hardware is required; thus, the electronic equivalent of the
aforementioned hardware can often simplify and improve the performance of a control system.
This can be accomplished without increasing cost.
CONTROL RELAY SCHEME:
Control relays not only provide decision-making through logic circuit connections, they also
allow for the control of power at remote locations and/or allow for the control of a high-voltage
or a high-current device with a low current switch.
A schematic representation of a control relay is in the figure to the left. As the figure shows,
X1 is open, C1 is de-energized, and Y1 is connected to the 120V source through the normally
closed contacts of C1.
In contrast, when X1 is closed, the coil C1 is energized and the movable contact moves to close
the normally open contacts. This has the effect of removing the power from Y1 and applying power
to Y2. The ladder logic equivalent is illustrated in the figure above and to the right. Note
that all parts relative to the same relay are designated by the same number.