Flowcharts are helpful in understanding a complicated process. This is especially true if you have to make decisions and do different steps depending on those decisions. By looking at a flowchart you can visually follow different paths through the chart. For each step on a flowchart you can ask yourself "Is this step necessary? Can it be improved?" . Flowcharts are very essential for you to complete your ladder diagram. Without a flowchart you will face difficulties in order to complete your ladder diagram.
You can make your flowcharts easier to understand and less subject to errors by using only a fixed set of structures. These structures include:
- Sequence
- Decision
- Loop
- Case
Sequence
The reader would start at the Start shape and follow the arrows from one rectangle to the other, finishing at the End shape. A sequence is the simplest flowcharting construction. You do each step in order.
If your charts are all sequences, then you probably don't need to draw a flowchart. You can type a simple list using your word processor. The power of a flowchart becomes evident when you include decisions and loops.
Decision
Another structure is called a decision, "If Then.. Else" or a conditional. A question is asked in the decision shape. Depending on the answer the control follows either of two paths. Example, if the temperature is going to be less than freezing (32 degrees Fahrenheit) the tomatoes should be covered.
Loop
This structure allows you to repeat a task over and over. It is not important that you remember whether the loop is a "Do While" or "Repeat Until" loop, only that you can check the condition at the start of the loop or at the end. You can also have the conditions reversed and your loop is still a structured design loop.
Ladder Diagram
Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.
Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting).
A ladder diagram consists of two basic parts : left section also called conditional and a right section which has instructions. When a condition is fulfilled, instruction is executed :
-> Normally Open (NO) and Normally Close (NC)
-> NO switch won't conduct electricity until it is pressed down
-> NC switch will conduct electricity until it is pressed.
-> Examples - doorbell and house alarm.
Anatomy of a ladder diagram
* Typically
flows from left to right.
* Can
be divided into sections called rungs, each rung typically consists of a combination
of input instructions.
* Each
input or output instruction is assigned an address indicating the location in
the PLC memory where the state of that instruction is stored.
* The
numerical format of the address depends on the scheme used by the particular
manufacturer
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