Understanding the IEC 1131-3 / CEI 61131-3 addressing

This section discuss the IEC 1131-3 addressing identifiers. This kind of identifiers are used in a PLC to address resources. A IEC address always starts with the '%' character.

We can classify two main classes of addressing

IEC variables that refers to a PHYSICAL resource of a PLC are expressed in the generic form

Where

'%' is the header of an IEC address

'Q' specify an OUTPUT resource

'I' specify an INPUT resource

The effective address is expressed in a hierarchical form that could consist of more numbers separated by dots. For example the address %QX1.2.3 could express the output bit 3 on the second word of the module 1

The IEC directive do not specify how deep a hierarchy can be, this is related to your particular PLC system

IEC variables that refers to an INTERNAL MEMORY resource of a PLC are expressed in the following form

Where

'%' is the header of an IEC address

'M' specify a MEMORY resource

The 'X', 'W', 'D' define, respectively, a BOOLEAN, a 16-bits WORD and a 32-bits double WORD

The address hierarchy, like explained before, follow

For example