PLC Logic Samples

I/O Devices

PLC Ladder Elements Sample

PLC Motor Flow Sample

Circuit Diagram Sample

PLC Motor Control Sample

HardWired Motor Control Sample

What is PLC scan time?

PLC Scan Time:

The PLC program is executed as part of a repetitive process
referred to as a scan. A PLC scan starts with the CPU
reading the status of inputs. Next, the application program is
executed. Then, the CPU performs internal diagnostics and
communication tasks. Finally, the CPU updates the status of
outputs. This process repeats as long as the CPU in the run
mode. The time required to complete a scan depends on the
size of the program, the number of I/Os, and the amount of
communication required.

Programming:

Programming Language:

Ladder logic (LAD) is one programming language used with
PLCs. Ladder logic incorporates programming functions that are
graphically displayed to resemble symbols used in hard-wired
control diagrams.
While ladder logic programs are still the most common, there are many
other ways to program PLCs. Two other common examples are
statement list and function block diagrams.

Statement list (STL) instructions include an operation and an
operand. The operation to be performed is shown on the left.
The operand, the item to be operated on, is shown on the right.

Function block diagrams (FBD) include rectangular functions
with inputs shown on the left side of the rectangle and outputs
shown on the right side.

I/O Types

Types of I/O:

  Discrete inputs and outputs, also referred to as digital
inputs and outputs, are either on or off. Pushbuttons, toggle
switches, limit switches, proximity switches, and relay contacts
are examples of devices often connected to PLC discrete
inputs. Solenoids, relay and contactor coils, and indicator lamps
are examples of devices often connected to PLC discrete
outputs.
In the on condition, a discrete input or output is represented
internal to the PLC as a logic 1. In the off condition, a discrete
input or output is represented as a logic 0.

 Analog inputs and outputs are continuous, variable signals.
Typical analog signals vary from 0 to 20 milliamps, 4 to
20 milliamps, or 0 to 10 volts.

PLC Facts:

What is a PLC?

A programmable logic controller (PLC), also referred to as
a programmable controller, is the name given to a type of
computer commonly used in commercial and industrial control
applications. PLCs differ from office computers in the types of
tasks that they perform and the hardware and software they
require to perform these tasks. While the specific applications
vary widely, all PLCs monitor inputs and other variable values,
make decisions based on a stored program, and control
outputs to automate a process or machine.

Prior to PLCs, many control tasks were performed by
contactors, control relays, and other electromechanical devices.
This is often referred to as hard-wired control. Circuit
diagrams had to be designed, electrical components specified
and installed, and wiring lists created. Electricians would then
wire the components necessary to perform a specific task. If
an error was made, the wires had to be reconnected correctly.
A change in function or system expansion required extensive
component changes and rewiring

Some of the more popular PLC's!

Allen Bradley   (The best in my opinion)
Modicon
Siemens
DirectLogic
Bechoff
Pilz