CV stands for Configuration Variable. Now, aren't you sorry you asked?
They are memory locations where configuration information for the particular decoder is stored.
They adjust the personality of a decoder to work best with the operator and the locomotive.
If you are not a computer geek, think of them as the way to set the personality of your loco.
This is NOT a comprehensive treatise on all CVs for all decoders. I am only going to talk about those with universally agreed usage. Not all decoders support all of these CVs.
Early on, decoders only had one byte of data (CV 1) for the address. It was decided that addresses from 00 to 127 would be defined by this byte of data. This leads to lots of confusion, like why is 120 a “two-digit" address? Because it is pseudo hexadecimal - now are you not sorry you asked? Do not worry about why, just know what. Best to call it a SHORT address.
Different manufacturers define 2-digit (short) and 4-digit (long) differently, all within the NMRA standard. Lenz defines any number larger than 99 as a long. Digitrax defines numbers larger than 127 as long. NCE lets you decide (0120 is a long address, while 120 is short).
So, if you are trying to address a loco with numbers between 100 and 127, you need to spend a bit of time thinking about it. If you only run on one system or only on NCE and Digitrax, fine - just program the loco address you want to use as a short address - CV01. If you are going with Lenz only, they handle it automatically. If you, like me, have locos to run on different systems which are numbered between 100 and 127, YIKES, it i’s a mess! For awhile, I addressed my Santa Fe FT Cat’s Whisker ABBA set as 27, instead of fighting with 127! Since then I've renumbered it 134 - a SAFE long address.
First off, this CV is misnamed. The NMRA standard calls it START VOLTAGE. The voltage on the motor is constant at about the track voltage. CV 02 adjusts the amount of time that the voltage is applied to the motor at speed step one. I like to set it so that the loco just creeps.
Here’s how to set CV 02
1. Set the loco to speed step 01
2. Set CV 02 = 001
3. See if the loco moves. If so, stop
4. If not, add 1 to the value of CV 02, then go to step 3.Then Bruce likes to set CV 05 and 06. See below.
First off, this CV is misnamed. The NMRA standard calls it MAX VOLTAGE. The voltage on the motor is constant at about the track voltage. CV 05 adjusts the amount of time that the voltage is applied to the motor at the fastest speed step. I like to set it so that the loco runs only as fast as it absolutely has to for my layout. Scale speeds of 40 to 90 MPH are reasonable. Frequently setting this to zero will have the decoder run the loco as fast as it is physically capable of running.
CV 06 sets the midpoint speed the loco can run. Again, it is misnamed (MID VOLTAGE). Once I have set the maximum speed (CV05), I may adjust this or I may not. Frequently setting this to zero will have the decoder calculate the midpoint for you. Some folks want the locomotive to come up to speed quickly and they will set this very close (75% or so) of the CV05 setting. Other want better low speed control and may set this at 20% of the value of CV05
When 4-digit addressing (actually any address above 127) was implemented, the NMRA standard included some very complex calculations. It is not simply putting two digits in one CV and the other two in another. Setting these CVs is very involved. Bruce recommends that you use some program, like DecoderPro to accomplish this task. If you don't have DecoderPro, you can use the calculator provided by The 2 mm Scale Association - CLICK HERE.
CV 29 sets the overall personality of the loco: whether it runs on DC or not, is the normal direction of travel forward or reversed, etc. Setting this CV is very involved. Bruce recommends that you use some program, like DecoderProto accomplish this task. If you don't have DecoderPro, you can use the calculator provided by The 2 mm Scale Association - CLICK HERE. If you want to roll your own, CLICK HERE for a little sheet I built for you to print and fill in personally. I dug into all the things that CV29 controls for you in my May 2016 column. CLICK HERE to read this column.