PLC Programming

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Contact
Each network in LD consists on the left side of a network of contacts (contacts are represented by two
parallel lines: | |) which from left to right show the condition "On" or "Off".
These conditions correspond to the Boolean values TRUE and FALSE. A Boolean variable belongs to
each contact. If this variable is TRUE, then the condition is passed on by the connecting line from left
to right, otherwise the right connection receives the value "Out".
Contacts can be connected in parallel, then one of the parallel branches must transmit the value "On"
so that the parallel branch transmits the value "On"; or the contacts are connected in series, then
contacts must transmit the condition "On" so that the last contact transmits the "On" condition. This
therefore corresponds to an electric parallel or series circuit.
A contact can also be negated, recognizable by the slash in the contact symbol: |/|. Then the value of
the line is transmitted if the variable is FALSE.
Coil
On the right side of a network in LD there can be any number of so-called coils which are represented
by parentheses:( ). They can only be in parallel. A coil transmits the value of the connections from left
to right and copies it in an appropriate Boolean variable. At the entry line the value ON (corresponds
to the Boolean variable TRUE) or the value OFF (corresponding to FALSE) can be present.
Contacts and coils can also be negated (in the example the contact SWITCH1 and the coil %QX3.0 is
negated). If a coil is negated (recognizable by the slash in the coil symbol: (/)), then it copies the
negated value in the appropriate Boolean variable. If a contact is negated, then it connects through
only if the appropriate Boolean value is FALSE.
Function blocks in the Ladder Diagram
Set/Reset coils
LD as FBD
Along with contacts and coils you can also enter function blocks and programs In the network they
must have an input and an output with Boolean values and can be used at the same places as
contacts, that is on the left side of the LD network
Coils can also be defined as set or reset coils. One can recognize a set coil by the "S" in the coil
symbol: (S)) It never writes over the value TRUE in the appropriate Boolean variable. That is, if the
variable was once set at TRUE, then it remains so.
One can recognize a reset coil by the "R" in the coil symbol: (R)) It never writes over the value FALSE
in the appropriate Boolean variable: If the variable has been once set on FALSE, then it remains so.
When working with LD it is very possible that you will want to use the result of the contact switch for
controlling other POUs. On the one hand you can use the coils to put the result in a global variable
which can then be used in another place. You can, however, also insert the possible call directly into
your LD network. For this you introduce a POU with EN input.
Such POUs are completely normal operands, functions, programs, or function blocks which have an
additional input which is labeled with EN. The EN input is always of the BOOL type and its meaning is:
The POU with EN input is evaluated when EN has the value TRUE.
An EN POU is wired parallel to the coils, whereby the EN input is connected to the connecting line
between the contacts and the coils. If the ON information is transmitted through this line, this POU will
be evaluated completely normally.
Starting from such an EN POU, you can create networks similar to FBD.
2-22 CoDeSys V2.3