Programming instructions

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For example, a light switch is usually in the off position (non-actuated), no current
is flowing (the switch is open) until someone turns it on (actuates it and energizes
it). At this point, electricity starts to flow (the switch is conducting) and the lights
turn on. A light switch is a normally open contact.
The normally closed contact is the opposite in every respect. Current flows until
the switch is actuated. A common example of this is an E-stop. It allows current
to flow until an operator hits the switch in an emergency. The switch is actuated
and the current flow stops.
Example: Normally Open & Normally Closed Contacts
10.2 Ladder Basics
The alignment of contacts and coils determine how a rung of ladder logic is
processed.
If two or more devices are positioned in series (one after another), they are
operated as an AND instruction. In order for power to flow through the rung, all
the conditions must be made true.
If the devices are in parallel, they are operated as an OR condition. OR
conditions provide multiple paths for the power to flow.