PLC program low-level mistake often made summary -

In the PLC controller program, whether novice or veteran, will make such a low-level mistakes. Because this kind of mistake is grammatical, so use also can't check out the error of the programming software. Once this error occurred, sometimes is hard to find, debugging and running, until computer control equipment can not run or stop running to a certain location, to realize there is a problem, again to the PLC program to every search a detailed analysis, or to take the program passes sentence by sentence, time consuming work.

so what problem is easy that we make such a low-level mistakes? Relay electrical control of the inherent thinking in the program, one or more input point using the normally closed contact of physics ( Such as stop switch, limit switch) , in the program, is still the continuation of relay electrical control mode, which still use the normally closed contacts used as conducting conditions.

below with a simple start-stop and self-locking circuit examples to illustrate.

according to the above, I can not run the PLC program is as follows:

after the PLC to electricity, X000, X002 normally closed point will disconnect. The logical value of '0'

Y0 = ( Y0 + X001) * X002 X000

digital logic expressions from above knowable, after press the start button SB1 X001 logic value is '1', and Y0 logic value will never change, always be '0'. Reason is related to the PLC internal input circuit, the following is a PLC internal input equivalent circuit:

correct PLC program is as follows:

PLC after power on, X000, X002 normally open points will be closed. The logical value of '1'

Y0 = ( Y0 + X001) * X002 X000

as long as you press the start button SB1, X001 logic value is '1', Y0 logic value for the '1'. Loosen the start button SB1, X001 logic value of '0' but Y0 logic value is '1', Y0 and X001 is or relationship, to ensure the Y0 logic value is always '1', namely the self-locking. Until you press the stop button or overload ( FR0 action) Y0 logic value to '0'.

through the simple example above, the beginners may not make sense of the external for the normally closed type, after the PLC internal input circuit logic value has changed 'non'. Electrical control and relay inherent thinking influence, veteran was due to negligence. Although this is a low-level mistakes, also easily occurs in the process of programming.

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