6.3.2 Explain
the PLC languages
a.
Mnemonic Boolean
•
mnemonic instructions.
•
These instructions can be derived directly from
the ladder logic diagrams and entered into the PLC through a simple programming
terminal.
•
An example of mnemonics is shown in See An
Example of a Mnemonic Program and Equivalent Ladder Logic.
b.
Ladder Diagram
•
To interpret this diagram imagine that the power
is on the vertical line on the left hand side, we call this the hot rail. On
the right hand side is the neutral rail.
•
In the figure there are two rungs, and on each
rung there are combinations of inputs (two vertical lines) and outputs
(circles).
•
If the inputs are opened or closed in the right
combination the power can flow from the hot rail, through the inputs, to power
the outputs, and finally to the neutral rail.
•
An input can come from a sensor, switch, or any
other type of sensor.
•
An output will be some device outside the PLC
that is switched on or off, such as lights or motors.
•
In the top rung the contacts are normally open
and normally closed. Which means if input A is on and input B is off, then
power will flow through the output and activate it.
•
Any other combination of input values will
result in the output X being off.
•
The second rung of See A Simple Ladder Logic
Diagram is more complex, there are actually multiple combinations of inputs
that will result in the output Y turning on.
•
On the left most part of the rung, power could
flow through the top if C is off and D is on. Power could also (and
simultaneously) flow through the bottom if both E and F are true. This would
get power half way across the rung, and then if G or H is true the power will
be delivered to output Y.
c.
Flow chart
•
A flowchart is ideal for a process that has
sequential process steps.
•
The steps will be executed in a simple order
that may change as the result of some simple decisions.
•
The symbols used for flowcharts.
•
These
blocks are connected using arrows to indicate the sequence of the steps.
•
The different blocks imply different types of
program actions.
•
Programs always need a start block, but PLC
programs rarely stop so the stop block is rarely used.
•
Other important blocks include operations and
decisions.
•
The other functions may be used but are not
necessary for most PLC applications.
•A flowchart is shown
is for a control system for a large water tank.
•When a start button
is pushed the tank will start to fill, and the flow out will be stopped.
•When full, or the
stop button is pushed the outlet will open up, and the flow in will be stopped.
•In the flowchart the
general flow of execution starts at the top.
•The first operation
is to open the outlet valve and close the inlet valve.
•Next, a single
decision block is used to wait for a button to be
pushed.
•when the button is
pushed the yes branch is followed and the
inlet valve is opened, and the outlet
valve is closed.
•Then the flow chart
goes into a loop that uses two decision
blocks to wait until the tank is full,
or the stop button is pushed.
• If either case
occurs the inlet valve is closed and the outlet
valve is opened.
•The system then goes
back to wait for the start button to be
pushed again.
•When the controller
is on the program should always be
running, so only a start block is needed.
•Many beginners will
neglect to put in checks for stop buttons.
d.
Logic Diagram
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