Memory word in PLC

So let first see what is memory address

in PLC there are two type of addressing

Physical input /output addressing

Logical memory addressing in Siemens PLC and binary address in Allen Bradley ( both are same but different name)- this is a logical address which square address and CPU memory card but you are not allowed to use it physically but you can use that address logically. Like in Siemens PLC M0.0,MB40,MW10,MD24 etc.


M= Memory ,
W= word,
D=Double Word,


I hope you know,

1 byte = 8 bits


1 word = 2 bytes

1 word= 16 bits

So , memory word is logical address which having 16 bits.


In PLC every data should store in memory locations.

Digital value (0 or 1) stored in Bit memory locations

Analog values stored in integers or real or word or double word etc,

Coming to your question word is equals to

1 word = 2 bytes

1 word = 16 bits

Maximum decimal value can stored in one word is -32768 to +32767 or 0 to 65535

Coming to field point of view, if a level transmitter connected to PLC, and current volume of tank is 70000 liters then we cannot store this large value in one word because it’s maximum decimal value is 65535. Then we have to use double word.