Electronic – What does the % symbol mean on plc addresses


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.

Best Answer

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.