Memory bits vs memory words

[mastertask]

Newbie looking for help, please. When should one use memory bits versus memory words? Am I right in thinking a memory bit can be used to simply store a TRUE or FALSE state, or a small value, whereas a memory word can store larger values?

 

[NetNathan]

Simply...Memory bits are for true/false and memory words store values.

 

[parky]

It depends on the PLC, many new ones have memory areas (files) that are bits words (16 bit) Double words (32 bit) float or real etc.
Some still only have 16 bit words, however, these can be doubled up as 32 or even 64 bit. The trend nowadays tends to be create memory areas required rather than have fixed areas.
Are you looking at a particular PLC ?

 

[RonJohn]

Welcome to the forum, mastertask.

While I concur with NetNathan, let's take it one step further. Looking at a Rockwell Logix based processor, for example, a single BOOL tag by itself takes up 4 bytes of memory, the same as a singe DINT tag. If you're concerned about memory, using the individual bits within a DINT gives 32x more True/False memory tags than a single BOOL.

Since you didn't mention a specific processor, it's impossible to give a specific answer. It's always helps others help you to give more info than less.

 

[parky]

While I agree with the last post it does depend on the plc they are probably all 16 bit words so a bit is a bit in an array of 16 bits or 32 bits or 64 bits.
Some plc's will allow even a word double word etc. to be addressed at bit level.
Just read your post again, doh. you would use bit memory as a single bool bit all they are is just like a relay so you energise it with the logic. then use the contacts to drive other bits etc. Words are generally used to store values these can be in integer, floating point etc.
Also look at this sites tutorials they should help you.

 

[mastertask]

Thanks for all the replies - that helps. Someone had told me to just use memory words throughout, but I couldn't figure why that was necessary for situations where the output is simply TRUE or FALSE. PLC is a Schneider TM221 series

 

[parky]

No problem, welcome to the forum.

 

[mass89]

Thanks for all the replies - that helps. Someone had told me to just use memory words throughout, but I couldn't figure why that was necessary for situations where the output is simply TRUE or FALSE. PLC is a Schneider TM221 series

Welcome to the forum, I'm familiar with the TM221, never heard of such thing as using memory words for bit instructions. Schneider's addressing is a bit different to other PLCs I've used, its based on Modbus addressing, its always helpful to read and understand Modbus addressing and the addressing architecture of the PLC you're working with. The only reason I could think they said to use memory word is so that all the memory is stored in the same register, but still .

 

[JaxGTO]

I have had programs where I want to turn on several bits in a word where each represents an operation being completed. Then when some other event is completed I can then turn them all off by just zeroing that word.


So a lot of the time it depends on how you are going to use them. Just have to think ahead sometimes.