Memory and addressing

Bits, bytes and words

All of a PLC’s memory is made of bits: cells that are 0 or 1. Bits are grouped into larger units:

Unit	Size	Example use
Bit	1 bit	A push button, an output, an internal marker
Byte	8 bits	A group of 8 inputs
Word	16 bits	An integer (INT), the value of an analog input
Double word	32 bits	A long integer (DINT) or a real number (REAL)

The data types of IEC 61131-3 give those bits a meaning. The most common are:

• BOOL: true or false (1 bit).
• INT: 16-bit integer, from −32,768 to 32,767.
• DINT: 32-bit integer.
• REAL: 32-bit number with decimals (floating point).
• TIME: a duration, such as T#5s or T#1m30s.

Memory areas

A PLC’s data memory is divided into areas with different jobs:

• Input image: a copy of the input states, taken at the start of every scan cycle.
• Output image: what the program writes; it is copied to the terminals at the end of the cycle.
• Internal markers (also called work bits or internal relays): bits and numbers the program uses as its own memory, not connected to the outside. For example, to remember that a cycle has finished.
• Timers and counters: each with its state and values.
• Data: areas for numbers, recipes and parameters (data blocks, D registers, DM memory, depending on the brand).

Some areas are retentive: they keep their value when the PLC is powered off. They are used, for example, for production counters or running hours of equipment.

How an address is built

In the style this platform uses (the same as Siemens with English mnemonics), the address I0.3 reads:

• I: inputs area.
• 0: byte number.
• .3: bit number inside the byte, from 0 to 7.

After I0.7 comes I1.0, because a byte has only 8 bits. With the same logic, MW10 is the marker word starting at byte 10.

Each brand its own way

Brand and family	Input	Output	Internal marker	Number
Siemens S7	I0.0	Q0.0	M0.0	MW10
IEC 61131-3 (direct addresses)	%IX0.0	%QX0.0	%MX0.0	%MW10
Schneider Electric (Modicon)	%I0.0	%Q0.0	%M0	%MW10
Mitsubishi Electric (MELSEC)	X0	Y0	M0	D10
Omron (CP, CJ)	0.00	100.00	W0.00	D10
Allen-Bradley (Logix)	tags	tags	tags	tags

Some important details:

• Mitsubishi numbers X inputs and Y outputs in octal: after X7 comes X10. There is no X8 or X9.
• Omron uses 16-bit channels: 0.00 to 0.15 are the bits of channel 0. On its compact PLCs, outputs usually start at channel 100.
• Allen-Bradley (Logix families) works with tags: named variables such as Pump_Motor. Physical I/O appears with names like Local:1:I.Data.0, and the program uses aliases with clear names.
• Modern PLCs from almost every brand, especially those based on IEC 61131-3, let you program with symbolic variables instead of addresses.

Addresses and symbolic names

A variable table (or tag table) links a name to each address: START → I0.0. The program becomes much easier to read:

  With addresses:   ┤ I0.0 ├──┤ I0.1 ├──( Q0.0 )
  With names:       ┤ START ├──┤ STOP ├──( MOTOR )

In the simulator you can type either the address or the name, and change the address style in the toolbar to see the same program in Siemens, Allen-Bradley, Mitsubishi or Omron notation.

Try it in the simulatorOpen a preloaded example and experiment yourself.

Summary

• Memory is measured in bits, bytes (8 bits) and words (16 bits).
• Data types (BOOL, INT, DINT, REAL, TIME) give the bits a meaning.
• There are input, output, internal marker, timer, counter and data areas; some are retentive.
• Each brand names addresses differently, but the idea is the same.
• Use symbolic names: a program with good names explains itself.