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CHAPTER - INTRODUCTION TO PLCS
Advantages of PLCs
Cost effective for controlling complex systems.
Flexible and can be reapplied to control other
systems quickly and easily.
Computational abilities allow more sophisticated
Trouble shooting aids make programming easier and
Reliable components make these likely to operate for
years before failure.
- Developed to replace relays in the late 1960s
- Costs dropped and became popular by 1980s
- Now used in many industrial designs
In a PLC
Reading Ladder Logic
INP UT S
OUT P UT S
Note: Power needs to flow through some combination of the inputs
(A,B,C,D,E,F,G,H) to turn on outputs (X,Y).
A Ladder Logic Example
Note: When A is pushed, the output B will turn on, and
the input B will also turn on and keep B on perma
nently - until power is removed.
Note: T he line on the right is being leftf intentionally
and is implied in these diagrams.
the mnemonic code is equivalent to
the ladder logic below
Note: The notation shown above is
not standard Allen-Bradley
notation. The program to the
right would be the A-B equiva
Execut ion follows
i := 0;
i := i + 1;
UNTIL i >= 10
PLC in a Control Loop
The Four Stages of a PLC Scan
P LC program changes outputs
by examining inputs
Set new outputs
P ower turned on
Process changes and P LC pauses
while it checks its own operation
Ladder Logic Inputs
Normally open, an active input x will close the contact
and allow power to flow
Normally closed, power flows when the input x is not open.
immediate inputs will take current values, not those from
the previous input scan. (Note: this instruction is actually
an output that will update the input table with the current
input values. Ot her input contacts can now be used to
examine the new values.)
Ladder Logic Outputs
When power is applied (on) the output x is activated
for the left output, but turned
off for the output on the right.
An input transition on will cause the output x to go on for one scan
(this is also known as a one shot
When the L coil is energized, x will be toggled on, it will stay on until
the U coil
is energized. T his is like a flip-flop and stays set even when the P LC is turned
Some P LCs will allow immediate outputs that do not wait for the program scan to
end before setting an output. (Note: T his instruction will only update the outputs using
the output table, other instruction must change the individual outputs.)
Note: Outputs are also commonly shown using parentheses ’-( )-’ instead
of t he circle. T his is because many of the programming systems are text
based and circles cannot be drawn.
Try to develop (without looking at the solution) a relay
based controller that will allow three switches in a room
to control a single light.