bits or words

I am new at this PLC stuff. Can anyone explain to me how to know when a whole word is needed and or how many words is need to...say for instance....setting up a DCM card for Remote I/O for 1/4, 1/2, or full rack.
Also for sending and receiving speed references, how does using a word for this come about. Does a Speed ref need a whole word, and how does it use a whole word.
I understand the physical aspect of the DCM Card as far as setting the dip switches and the importance of what slots each end is in and how to send and receive but the whole "word" thing has been a slight mystery to me.
Any help would be greatly appreciated.Thanks

Can you be a tad more application specific?

I don't want to go into a dissertation about the difference between bits and words, (something you probably already know), but I'd like to help and I'm not sure I understand your question.

Dave

a litttle more specific

Say for instance...When having conveyor restart jam problems with an interfacing machine and when I asked about simply following the interfacing machine speed, and my answer was "We can't because we need a word to be able to do that and we are out of available words, at the DCM, we only have a few bits left".
That is all that was explained to me.
Thanks for the response.

A word is made up of 16 Bits.

16 Input Bits = 1 word in the Input image table.
16 Output bits= 1 word in the Output image table.

A Bit would be used for things such as a pushbutton, limit switch or a pilot light etc.(digital)

A word would be used for things such as a temperature or speed etc. (analog signals) Yes you need a Word for the speed ref. signal.

There is no need for me to attempt to give a full blown explaination here besides A-B can do it better.

Go to A-B manuals on line and study up, most PLC user manuals will answer your questions.

Roger

Re: a litttle more specific

SHERLOCKIN' said:

"We can't because we need a word to be able to do that and we are out of available words, at the DCM, we only have a few bits left".

Click to expand...

I think somebody's blowin' smoke up your a$$. The DCMs I've used add a communications port to the PLC. They don't have much memory of their own - their purpose is to communicate data that already resides in the CPU. In some cases this is transparent to the CPU, in other cases the CPU issues communications commands to the DCM, and in other cases the CPU uses block transfers to the DCM to send the data across. This is all dependent on the PLC platform and the DCM model.

You can send huge amounts of data using a DCM, and I don't think you can really be out of words at the DCM. What is probably going on is the existing block transfer logic in the CPU is set to send a specific number of words to your DCM, which in turn is communicating with your network. This data block may already be fully utilized. The solution is probably to modify the block transfer in the CPU to enlarge the data block, or to add another data block transfer to the programming. This is not simple, but is probably feasible if the programmer understands the equipment.

If you have specific model numbers of the hardware and the existing logic some here can surely get you started.

I did two project using DCMs to connect a SLC to a PLC3 network.

The 1747-DCM has 7 available input words, or 112 input bits, and 7 output words/112 output bits.

It actually has 8 words total, but one in and one out are used for status.

RT