PLC addressing explained

Every input terminal, output terminal and internal bit in a PLC has an address. The idea is the same in every brand, but the format is not: Siemens counts in bytes and bits, Mitsubishi in octal, TOYOPUC in hexadecimal, and modern Allen-Bradley uses tag names.

What an address is

An address tells the program which signal a contact or coil refers to. A push button wired to the first input terminal might be I0.0 on Siemens, X0 on Mitsubishi or X000 on TOYOPUC. When you program a normally open contact with that address, the contact closes when the button is pressed.

Addresses fall into a few areas in every PLC:

• Inputs: field signals coming in (buttons, sensors).
• Outputs: signals going out (contactor coils, valves, lamps).
• Internal bits: memory relays used inside the program, with no terminal.
• Timers and counters.
• Data registers / words: numbers such as setpoints and counts.
• System (special) bits: always-ON, first-scan, clock pulses and error flags.

Comparison table

Siemens: byte.bit

Siemens groups signals into bytes of 8 bits. I0.0 is byte 0, bit 0. After I0.7 comes I1.0, not I0.8. Outputs are Q (A in the German mnemonics, from Ausgang), memory bits are M, and a whole byte or word can be read as IB0, IW0 or MW10. In TIA Portal you usually give each address a tag name such as "Start_PB".

Mitsubishi: octal numbering

Mitsubishi FX and many MELSEC PLCs number inputs and outputs in octal: X0–X7, then X10–X17. There is no X8 or X9. Internal relays M, timers T, counters C and data registers D are decimal. Special relays start at M8000 on FX (M8000 = always ON while running, M8002 = first scan pulse). The iQ-R and Q series use hexadecimal X/Y numbers instead.

Allen-Bradley: file addresses and tags

Older SLC 500 and MicroLogix PLCs use file-based addresses: I:0/0 is input slot 0, bit 0; O:0/0 the first output; B3:0/0 a bit in binary file 3; T4:0 a timer in file 4 with .DN, .TT and .EN bits. ControlLogix and CompactLogix use tags: the hardware address looks like Local:1:I.Data.0 and you alias it to a readable tag name such as Start_PB.

TOYOPUC: hexadecimal

JTEKT TOYOPUC numbers bits in hexadecimal: X000–X00F, then X010. Besides X, Y, M, T and C it has K (keep relays), L (link relays), V (special relays such as the always-ON and clock bits) and P (edge relays used by rise/fall differential instructions).

Remote I/O addresses

When you add remote I/O stations over a network, their points get addresses that continue after the PLC's own I/O. On a TOYOPUC with X000–X007 on the CPU, the first 8-point station might be X008–X00F, the next X010–X017. On Siemens the station is assigned its own byte, for example I2.0–I2.7. Always check the I/O map in the configuration software.

Tips

• Write the address and a short comment on every wire label and in the program (device comments).
• Never use the same output address in two coils (a double coil); only the last one would win.
• Keep an I/O list: terminal, address, device tag and description.

In the PLC Sim Lab Playground you can switch the brand and watch every address in your program change format automatically.