Older PLC's and some new PLC's use an addressing scheme that is 'fixed'. I/O is addressed using physical hardware locations. This addressing scheme is called Rack/Slot addressing. The I/O addresses are derived using the rack number, the I/O module slot number and the screw terminal number where the I/O device is wired.

For detailed information of the Allen Bradley SLC500 series PLC visit the SLC500 Rack/Slot Addressing page on this site. (Under Construction)

For detailed information of the Automation Direct DL400 series PLC visit the Automation Direct DL400 Addressing page on this site. (Under Construction)

Most PLC/PAC today use a tag-based memory structure. Tag-based memory structures use variable names for input and output devices, internal relays (bits) and all other data types, structures and values. The variables are like the variable names used in languages such as C++, VB.Net or Java. PLC/PAC designers call these variables, Tags.

For detailed information of the Allen Bradley ControlLogix PAC visit the ControlLogix Memory Structure page on this site.