Diferent FC & RLO

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What is the difference between a signal edge evaluation and an RLO edge evaluation?

Entry ID:7228342 Date:2011-06-14

STEP 7 -- Creating S7 programs -- Configuring monitoring

What is the difference between a signal edge evaluation and an RLO edge evaluation?

Description A signal edge is present when the signal status concerned changes. An RLO edge occurs if the resultof the logic operation of the operand to be evaluated has changed compared with the result of thelogic operation before. The table below explains the difference between the two edge evaluations.

No. Difference between signal and RLO edge evaluation

1 Signal edge evaluation In the function block diagram (FBD) and in the ladder diagram (LAD) the edge evaluation isrealized by the functions "POS" (for rising edges) and "NEG" (for falling edges). The signalstatus of the first operand (for example, input bit E1.1) is compared with the signal status (for

example, E1.1) of the query before. If the actual signal status = 1 and the signal status of thequery before = 0, then a rising edge is recognized and the output of the function "POS" is set to"1". In order for "POS" to be able to compare the actual signal status with the previous status ofE1.1, for each query the actual signal status is buffered for the next query in a marker bit.

Example of a signal edge evaluation of the positive edge and of the negative edge in thefunction block diagram.

Fig. 01

Example of a signal diagram with signal edge evaluation with rising and falling edges. Thesignal "E1.2" works as a release signal.

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Fig. 02

2 RLO edge evaluation In FBD and in LAD this edge evaluation is realized by the functions "P" (rising edge) and"N" (falling edge). If the result of logic operation changes, for example from 0 to 1, then the

query of "P" delivers the signal status "1" (see signal diagram) for the duration of the cycle. Forthe function to recognize the change of edge, the RLO must be stored in an edge marker (forexample, M1.1).

Example of an RLO edge evaluation of the positive edge and of the negative edge as functionblock diagram.

Fig. 03

Example of a signal diagram with RLO edge evaluation for rising and falling edges.


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Keywords Switch pulse, Signal pulse

Fig. 04

Entry ID:7228342 Date:2011-06-14

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Diferent FC & RLO