Word size?

Does word size vary with PLC? I've noticed that on AB and GE Fanuc programs a word is defined at 16 bits. However in computer language (say C++) a word is defined as 32 bits (4 bytes). Does word size vary on other controllers? Why is there a difference in the definition of the size of a word (16 vs 32 bit)?

Thanks.

The short answer is that it depends on the computer design. That's why standard data types (int, dint, sint, etc.) are more typically used in programming languages like C or C++.

Here's a bunch more info. on the definition of a word: http://en.wikipedia.org/wiki/Word_%28computing%29

AB ControlLogix define data types exactly like that (SINT, INT, DINT, REAL, etc). To me, 16 bits will always be a word, not an INT, but then that's just the way I've always worked.

Regards
Keith

It also appears to me that the IEC-61131 PLCs, like Beckhoff and Bosch Rexroth also define INT as being 8 bits, but my experience is still limited with those

One thing to bear in mind is the terms bit,byte and word are not really interchangeable Int, Dint and Float.

Thats because bit, byte and word relate to the size of the 'box' while Int, Dint and Float tell you what to expect in the 'box'.
A signed integer and an unsigned integer both use the same amount of bits, as do a Dint and Float but theyre not the same thing.

In all modern PLCs I know, word is 16-bit for description of PLC memory size or instruction length. It has nothing with data type, which may occupy from 7-bit (ASCII) to 64-bit.

I always considered a word to be the smallest addressable unit of memory - note that I said addressable, not accessible, as smallest accessable is always a bit.

A 16 bit processor (or a 32 bit processor on a 16 bit bus) fetches 16 bits from memory at a time, making an INT a word. To access a bit or byte it still fetches 16 bits and masks what it doesn't need.

A 32 bit processor on a 32 bit buss fetches 32 bits at a time, making a DINT a word for that processor. To access an INT or SINT or BOOL it still fetches the 32 bit word and then masks what it doesn't need.

A SLC500 word is 16 bits. A ControlLogix word is 32 bits.