Satec Expertmeter PM180 Reference Manual page 11

(AND logic). Alternative paths can be present with other contacts in
them (in parallel), which can be used to build OR logic.
An example of ladder diagram is shown in the figure below.
Basic concepts of LD implemented in PLC Configurator are the following:
 There are two rails: left rail (so called "+" rail) and right rail (so
called "-" rail).
 Between rails can be contacts and one coil. They are situated along
horizontal so called "rung".
 Contacts represent BOOL input variables, whereas coils represent
BOOL output variables.
 There are two types of contacts and coils: normally closed and
normally opened.
 Serial connection of contacts is used to create AND logic.
 Parallel connection of contacts is used to create OR logic.
 If all the contacts on the way from the left rail to a coil are closed,
the coil is energized and its state is TRUE.
 Coils can have their own contacts like shown in the figure below.
Figure 12 - LD
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