Ladder Diagram（Ld - SMB MAT-LC-C Series User Manual

6.2.6 Ladder Diagram（LD）
Ladder diagram is also a graphics-based programming language, which is close to the structure
of electronic circuits. On the one hand, ladder diagram is very suitable for building logic switches.
On the other hand, it can also create network diagrams like FBD. Useful when controlling calls to
other POUs.
The ladder diagram contains a series of networks. There is a vertical current line on the left and
right sides. The network diagram is only limited to the range between the left and right bus bars.
In the middle is a circuit diagram composed of coil contacts and connecting lines.
Each net contains a series of contacts on the left that communicate the on and off states from left
to right according to the TRUE and FALSE values of Boolean variables. Each contact is a
Boolean variable. If the variable value is TRUE, the circuit is connected from left to right through
the connecting line. Otherwise the right side receives the value "off".
Contact
There are contacts on the left side of each network diagram in the ladder diagram (contacts are
represented by two parallel lines | |), which are used to indicate the "on" and "off" states of the
circuit.
These states correspond to Boolean variables TRUE and FALSE. Boolean variables belong to
each contact. If the value of the variable is TRUE, then the state can be passed from left to right
by connecting lines. Otherwise, "disconnect" is received on the right.
Contacts can be used in parallel, and one of the parallel branches must pass the "on" state
before the parallel branch can pass "on". Or the contacts are connected in series. At this time,
when the contacts must transmit the "on" state, the last contact transmits "on", which is
consistent with the circuit connected in series and parallel.
Coil
On the right side of the ladder network diagram there are some so-called coils, which are
represented by ( ) and can only be connected by horizontal lines. The coil transfers the
connection state from left to right, and copies the state into a boolean variable, which can
describe the state of the entry wire as "on" (corresponding to TRUE for a boolean variable) or
"off" state (corresponding to FALSE for a boolean variable).
Contacts and coils can also be negated (contact SWITCH1 and coil %QX3.0 are negated in the
example above). If the contact is negated (represented by "/" in the contact symbol), then it
copies the negated value to the corresponding boolean variable. If a contact is negated, the
circuit can be connected only when the corresponding Boolean variable is set to FALSE.
Function Blocks in Ladder Diagram
Function blocks and programs can be added to the network diagram, but they must have
Boolean-valued inputs and outputs and can be used on the left side of the ladder diagram like
contacts.
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