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I've seen contacts assigned with a % symbol on ladder diagrams and plc structured text. For example a normally open start button would be %I1.02 and a coil would be %Q5.3 I understand the I and Q is input and output, respectively... But what is the % symbol for?

Any information on this will be appreciated.

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    \$\begingroup\$ Do you have any documentation for your PLC device? \$\endgroup\$
    – user103380
    Commented Sep 4, 2019 at 4:36
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    \$\begingroup\$ While the use of the % symbol in PLC addresses is part of the IEC 61131 standard, that isn't a law. The only way to know for sure, is by reading the documentation for your particular PLC. Over the past 51 years, many manufacturers have been making their own languages, rules, and syntax. \$\endgroup\$
    – R Drast
    Commented Sep 4, 2019 at 9:39

1 Answer 1

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In IEC 61131 the percent symbol means it refers to an a fixed hardware address. Eg: input/output or some other feature of the PLC hardware.

These addresses may never change, regardless of program size. In contrast to other variables that the compiler may put anywhere within the specified retention area.

I suggest you get yourself a copy of the IEC 61131 standard.

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