Addresses Like a Postal Code

Where a channel lives

This is rack 0 of an M580 station. Click any input or output channel — the panel below decodes its topological address into rack · module · channel, and shows the symbol a located variable gives it. Notice you address the position, not a memory cell: move the module to another slot and the address changes with it.

The format: %I (input) / %Q (output), then rack.module.channel. So %I0.2.3 = rack 0, the module in slot 2, channel 3. Give it a symbol like StartPB in the data editor and your logic reads the name — but the name is located, bolted to that physical position.

Pinned to hardware, or free-floating?

Every variable is one or the other. Toggle it and watch what the address field does.

Rule of thumb: located for anything wired to the field (it must map to a real terminal) and for data you expose over Modbus. Unlocated for internal logic — let the CPU manage the memory and you'll never fight an address conflict.

%MW is the Modbus map

Here's the Modicon trick a SCADA integrator must know: the %MW state-RAM words are directly the Modbus holding registers. %MW100 is register 400101. Locate a variable at a %MW and you've published it to any Modbus master — no extra config. Same memory, two names.

This is why Modicon is the SCADA workhorse: expose a value by parking it in %MW, and the whole Modbus world can read it. (More in Track 06 · Modbus.)

Decode it