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How to program normally closed inputs in programmable logic controllers (plcs) to behave as normally closed devices. It discusses the difference between examine-on and examine-off instructions and their impact on the input behavior. The document also provides examples and illustrations to clarify the concepts.
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will^ be^ logic^0 (power
not flowing to the module). The processor’s examination for an OFF condition atreference^10 will^ then^
be^ TRUE;^ therefore,^ the instruction^ will^ provide^ continuity
to^ the^ rung^ and turn output 100 and PL1 ON.
closed^ input^ condition^ has^
been programmed as an examine-ON instruction. During operation figure (a), if PB1 is not pushed, the inputmodule 10 will read an ON status. When the processor evaluates the ladder rung, its examinationfor an ON condition at reference 10 will be TRUE. Therefore, contact 10 will close to provide power to the rung,turning output 100 and PL1 ON.
input^ device^ to^ behave^ as
a normally closed device when connected, it must be programmedas an examine-ON, or normally open, contact instruction. Discrete inputs to a PLC can be made to act as normally open ornormally^ closed^ contacts,^
regardless^ of^ their^ original configuration. This ability to examine a single device for either an open orclosed state is the key to the flexibility of PLCs—no matter howa^ device^ is^ wired^ (normally^
open^ or^ normally^ closed),^ the controller can be programmed to perform the desired actionwithout changing the wiring. NOTE: The programming state of an input depends not only onhow it is wired, but also on the desired control action.
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closed^ device^ and^ vice versa. However, for fail-safe reasons, normally closed inputdevices^ should^ be^ wired
to^ the^ input^ module^ as normally^ closed^ devices^
and^ then^ programmed^ as examine-ON^ instructions,
so^ that^ they^ behave^ like normally closed devices. A^ wired^ normally^ open
device^ must^ not^ be programmed^ to^ act^ as^
a^ normally^ closed^ device, especially if it is being used to interrupt continuitywhen a device is pushed or closed.
Program/Flow Control Instructions^ ^ Direct the flow of operations, as well as the executionof instructions, within a ladder program.^ ^ They perform these functions using branching andreturn instructions, which are executed when certainalready programmed control logic conditions occur.^ ^ Program/flow^ control^
instructions^ form^ a^ “fence”within a program. ^ This^ fence^ contains
groups^ of^ other^ ladderinstructions that are used to implement the desiredfunction.
Figure Illustrating A Fence Created UsingProgram/Flow Control Instructions
Table Showing Some Of The MostCommonly Used Program/Flow ControlInstructions.
is^ TRUE,^ it^ activates^ the referenced output, allowing all rung outputs within the zone tobe controlled by their respective rung input conditions. When the MCR output is turned OFF, it de-energizes all nonretentive (non latched) outputs within the zone.
or^ zone^ control^ last^ stateinstruction. ^ This^ instruction^ is^
usually^ unconditional^
(i.e., programmed without any conditions to energize). An end instruction reference address may or may notreference a MCR or ZCL. If a reference is included, the END instruction willend that particular MCR or ZCL. If^ the^ instruction^ does
not^ include^ a^ reference address,^ it^ will^ terminate
the^ latest^ MCR^ or^ ZCL instruction.
