Youv'e just purchased your first Digital Command Control System and set it up according to the instructions.  Now you can run those decoder-equipped locomotives independently, right?  Not yet.  You first have to program the decoders to different addresses.  To do this, you'll have to learn a little about configuration variables, commonly known as CV's.

Configuration variables allow you to customize the operation of a decoder.  Each CV has a number that identifies its use and a value that controls how the CV behaves.  If you change the value of a CV in a decoder, you change how the decoder operates.  The necessary steps to program a CV vary from system to system, but each has a menue that you follow on a throttle or command station.  Some systems use plain English to program a decoder features without referring to a CV number.

Locomotive address.   The most basic Configuration Variable is a CV1, which is called the short or two-digit address.  By changing this CV, you can give your locomotives different addresses.  The initial value of CV1 on any decoder is 3.  Each locomotive must have a unuque address, otherwise locomotives with the same address will run at the same time.

There are many other CV's that control how a locomotive performs, but not all decoders use all CV's.  Often, different DCC manufacturers use different CV numbers for the more advanced decoder features.

Service Mode.     There are two different ways to program a CV.  The first using the programming  track, sometimes called "service mode" programming.  Most DCC systems support service mode programming.

To make a programming track, a special pair of terminals on the command station must be connected to an isolated section of track.    This track must not be electrically connected to the layout's main line, or you could damage your command station.

Some modelers use an isolated spur track as a programming track and rotary or double-pole double-throw switch to control whether that section of track is connected to the programming terminals. (programming track) or the DCC booster (main line).  Be sure the switch is a "beak-before-make"  type of switch or the tracks will short together while throwing the switch.

My programming track is away from the laypout on my work bench.  I use a rotary switch to select if the track is connected electrically to the programming track or the main line.

Ops Mode.    The second method for programming  a CV is on the layout's main line.  This is called programming on the main or "ops (operation) mode" programming.  Ops programming is especially useful when setting CV's that control motor operations.  The locomotive can be moving while the CV is reprogrammed, allowing you to instantly see how that change affected its performance.

The downside is that the value of the CV can't be read back in ops mode.  Since there may be locomotives on your main line when you are programming, your DCC system will ask you  to choose which decoder address you want to program.  If you have several new locomotives on the main line, all initially programmed to address 3, they'll all get programmed at the same time.  So, make sure you program new engines one at a time.

One other caution about using ops mode to program a new decoder is that the main line has the full power of the DCC booster on it.  If there is an error in your decoder wiring, you could destroy the decoder.  The programming track  current is limited; that may save your decoder if it's wired incorrectly.

Common CV's.    Besides the short address, there are other common CV's, which are listed below.   The more features a decoder has, the more CV's there will be.  Consult the decoder's instruction manual for information on advanced CV's.

CV2-Vstart (start voltage).     This CV sets the voltage that goes to the motor on throttle step one.  By increasing the voltage to a point where the locomotve just begins to move at step one, you increase the fine speed control over it.  Further, this helps match the speeds of differnt locomotives should they run a consist together.

CV3 - acceleration rate.    Configuration variable 3 is one of the two CV's that are collectively referred to as momentum.  This CV controls the rate of acceleration.  The higher the valeu the longer it takes to get your train up to speed.  This simulates starting a heavy train.

CV4 - deceleration rate.    This the second momentum CV.  The higher the value, the longer it takes to start the train.

CV5 - Vhigh(max speed).    Like CV2, CV5 helps match the speed of your locomotive.  This CV controls the voltage when the throttle is set to the maximum speed step.  The higher the value the more voltrage is applied to the motor.  If the value is 0 or 1, Vhigh isn't used.

Reading Back CV's.   Configuration Cv's can only be read on the programming track, and a motor or some other load must be connected to the decoder.

To read back a CV, the command station sends a message to the decoder through the track.  It asks, "Is CV1 a vaue of one?"  If the answer is "yes" the decoder sends a pulse to its motor.  If it's "no", no pulse is sent.

The command station senses the current change in the programming track and therefore knows the answer to the question.  The questioning continues until the command station figures out the what value is in the CV.

AT FIRST, the thought of programming a Configuration Variable may sound complicated, but really is not.

Other than the address, you don't have to program any CV to enjoy your DCC system.  However, as you get more comfortable with CV's, you can start customizing your decoders to get the optimal enjoyment from them.

(Model Railroader Magazine, April 2008)