Semper Vaporo
Pkgs.
If you mean DCC decoder Configuration Variables, about the simplest explanation you'll find is probably at Tony's Train Exchange and his "DCC For Beginners" article.
http://www.tonystrains.com
At the top of the home page, click on the DCC For Beginners item. You can either read it on-line or download the .PDF document.
In simplest terms, a Configuration Variable is a memory storage location in a DCC decoder that can be changed to make the decoder perform differently, such as keeping a headlight on all the time, or having it go off when the engine is backing up.
Each decoder comes with a list of the CVs that it supports and the values that need to be programmed into them to make changes in the way they perform.
Any further explanation than this is too technical to cover in this forum.
A configuration variable is an information packet that is sent to the decoder's brain telling it to do what you want the decoder to do via programming. In turn, the decoder is the brains for the engine in which it resides, telling lights, sound system if one is on board, and the motor what to do.
The decoder is designed to have so many CV's adjusted by the user, and some are not meant to be adjusted by the user. Each CV is meant to regulate a function, but there are levels of CV's in indices (at least the QSI ones have this feature). The QSI decoders require you to access an index first, and then to change the CV....it's a bit like reaching into the right bin to get a certain screw type. The bin might actually have two smaller bins nested in it, and you want a screw from the right hand bin. So, you have to instruct the decoder to allow access to the right bin. You must tell it to do that by first dialing in the correct bin code. The following step then gets you to pick up the correct screw size...except that you are adjusting the correct CV to get the right volume of horn, bell, the right chuff rate, whatever.
So, as simply as I can offer, a CV is an instruction to the decoder, but an instruction to a specific function that the decoder is meant to manage on your behalf. If you want the horn as loud as possible, then you input and programme in the value that the manual for the decoder says if max volume. So, if the range of values for a QSI steam decoder for volume for the whistle is from 1-15, you would programme the CV for "whistle volume" with a value of 15.
Or is there something else that is troubling you?
Also it is my understanding that CV's are based on Binary numbers.
I = on.
0 = OFF.
IIIIIIII = 255 the bits can never go beyond 255 without additional bits.
the numbers break down left to right for each bit:
128 64 32 16 8 4 2 1 = 255
Take away any bit, count the ones left and you know the number. You can count from zero to 255 this way.
If my choo choo had the number two on the side of it's cab and I wanted to set the CV address of that Choo choo to respond to it's new name "2" in dcc it will see this:
00000010 equals TWO coming down the track as a signal addressed ONLY to choo choo number TWO.
All other engines sit and listen to the signal but only engine number 2 should respond.
Your computer should have a calculator. Changing the display between bin, dec; you will quickly learn what bits make up what numbers.
Taking Selector's example:
ATTENTION ENGINE NUMBER Two.
Engine two: Im here go ahead.
ATTENTION ENGINE TWO. SET WHISTLE NUMBER AT 15 or 00001111
and the engine should turn up the whistle volume to the max.
Now.
It is best to have engine two alone with the command station on what is called the program track. So that the two can commune together without any other choo choo getting confused.
In a certain situation you might accidently tell ALL of your engines to put the whistle (Or whatever) to a value of 15. The result will be unsatisfactory because your whistles will be either louder or quieter away from values that they were set at previously.
On my road, any changes I make to any of my engines with CV's get written down in a notebook as they are input into my DCC system. That way if the situation gets out of control and the choo choo displays unwanted or error behavior I have the value written down and maybe can trace my mistake.