PMMI Programmable Logic Controllers (PLC) 1 Practice Test

The left-to-right tag order is a hierarchical path from broad location to the exact bit. You start by identifying where in the system the data lives (the location), then which slot in the rack contains the module (the slot number), then what type of module it is (the module type) so you know how its data is organized, then the specific data group or register inside that module (the member), and finally the exact bit within that group (the bit).

For example, if you have a digital input module in slot 3 at location 1 and you want bit 5 of the first data word, the tag would be 1.3.Input.0.5. This clearly locates the data from the system level down to the single bit.

Starting with a bit or placing the bit before locating the module would skip essential context and make it impossible to know which device or location the bit belongs to.