Ladder diagrams are specialized schematics
commonly used to document industrial control logic systems. They are called
"ladder" diagrams because they resemble a ladder, with two vertical
rails (supply power) and as many "rungs" (horizontal lines) as there are
control circuits to represent. If we wanted to draw a simple ladder diagram
showing a lamp that is controlled by a hand switch, it would look like this:
Figure 2.1 Simple ladder diagram
The "L1" and "L2" designations refer to the two poles of a 120 VAC supply, unless otherwise noted. L1 is the "hot" conductor, and L2 is the grounded ("neutral") conductor. These designations have nothing to do with inductors, just to make things confusing. The actual transformer or generator supplying power to this circuit is omitted for simplicity. In reality, the circuit looks something like this:
Figure 2.2 Actual ladder diagram circuit
The language itself can be seen as a set
of connections between logical checkers (contacts) and actuators (coils). If a
path can be traced between the left side of the rung and the output, through
asserted (true or "closed") contacts, the rung is true and the output
coil storage bit is asserted (1) or true. If no path can be traced, then the
output is false (0) and the "coil" by analogy to
electromechanical relays is considered
"de-energized"..
Ladder logic has contacts that make or
break circuits to control coils. Each coil or contact corresponds to the status
of a single bit in the programmable controller's memory. Unlike
electromechanical relays, a ladder program can refer any number of times to the
status of a single bit, equivalent to a relay with an indefinitely large number
of contacts.
So-called "contacts" may refer
to physical ("hard") inputs to the programmable controller from
physical devices such as pushbuttons and limit switches via an integrated or external input module, or may represent the
status of internal storage bits which may be generated elsewhere in the
program.
Each rung of ladder language typically
has one coil at the far right. Some manufacturers may allow more than one
output coil on a rung.
·
—( )— A
regular coil, energized whenever its rung is closed.
·
—(\)— A
"not" coil, energized whenever its rung is open.
· —[ ]— A
regular contact, closed whenever its corresponding coil or an input which
controls it is energized.
· —[\]— A
"not" contact, closed whenever its corresponding coil or an input
which controls it is not energized.
Sources :
Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.1-2
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