CV's - Configurable Variables

How a decoder behaves can be adjusted using CV’s (Configurable Variables).  The key CV’s are specified by the NMRA DCC Recommended Practices however each manufacturer is then free to use it’s own CV’s to set up features that they have designed themselves.  Each decoder that you use will come with a manual containing a list of CV’s and their meanings. 

It is not mandatory to change CVs - it is nice to learn how to set addresses so that you can get on and enjoy running locomotives.  Below is an explanation of CVs that may be useful to you but all this can be done your supplier when you buy a loco with a DCC decoder fitted.  If you intend to fit decoders yourself then it would be useful to learn how to  set up a decoder or check for errors in the fitting.  Again speak to the supplier of your DCC system and they will show you how to programme a decoder with that system. 

Key Configurable Variables

The CV information below is a guideline - please refer to the manufacturers' instructions for your particular decoder settings - Link to NMRA recommended practices 9.2.2

CV1   Address – See section on locomotive addressing

CV2   Start Voltage.  This allows you to adjust the decoder to suit different models.  Some motors need a bigger kick to get them rotating, so increasing CV2 by a small amount may help to get a locomotive running at speed step 1. Adjust CV2 in small increments from the manufacturers’ setting if you have problems with a motor running too slowly.  CV2 has a range from 0-255. Setting a value of 255 would mean that the locomotive would start at maximum speed on the first speed step of your controller.  See section on speed curves

CV3 Starting delay.  Increasing CV3 has the effect of reducing the rate of acceleration of a locomotive giving it more of a sense of inertia.  For example a heavy freight locomotive, which would accelerate slowly, would have a high value set into CV3. A lightweight suburban train would be given a low value in CV3 and would have a sprightly rate of acceleration.  CV3 has a range of 0-255.  Feel free to experiment with a locomotive to get a feel for the effects of changing the starting delay.  Then use values that suit the size of your layout.

CV4 Braking Delay. Increasing CV4 has the effect of reducing the rate of deceleration of a locomotive giving it more of a sense of inertia.  Again a heavy freight train would slow down gradually so a higher value of CV4 would be used than for a light engine that would be able to stop fairly rapidly. CV4 has a range of 0-255.  Feel free to experiment with a locomotive to get a feel for the effects of changing the braking delay.  Then use values that suit the size of your layout. – remember that a loco that takes an eternity to stop will run out of rail!

CV5  Maximum speed.  This CV sets the maximum speed that the locomotive will run at.  On Lenz decoders CV5 has a range of 0-255.  On Uhlenbrock decoders the range is 0-63.

CV6 Mid Speed.  CV6 allows the decoder to calculate a steady, smooth speed curve using the values set in CV2 and CV5 as well.  As rule of thumb I use a value for CV6 of between a 1/3 and a1/2 of CV5 (maximum speed). See section on speed curves.

CV8  Manufacturer ID – Reading this CV will tell you who manufactured the decoder.  This is useful if you want to find a reset procedure for the decoder. 

CV29 General Settings – So far the CV’s that we have looked at have used decimal values that correspond the amount of effect the CV will have in operation.  CV29 is different and is easier to understand if you think of it as being eight separate switches (bits) each of which controls a different task of the decoder. (Remember that some systems count the bits from 0-7 and others count them 1-8) Please check decoder instructions for CV29 as everything listed below are not always supported.


A little technical detail on how CV's work - for information only

Some confusion can arise as CV’s can be expressed as a decimal value between 0 and 255 or as a sequence of Bits.  Each locomotive CV consists of eight switches that may be switched “on” or “off”.  These switches are known as “bits”.  If a bit is set (switched on) then it has a value of 1.  If a bit is deleted (switched off) then it has a value of 0.  Each of the eight bits is allocated a decimal value based on the binary scale. Binary is useful as you can make any value between 0 and 255 with just eight switches just by setting each of them on or off.  Importantly – no two different combinations of the eight switches can give the same result.

Further confusion can arise as different manufacturers have different ways of referring to the eight bits within each CV.  Some, for example Lenz, like to number them from 1 to 8, others, such as Uhlenbrock, choose the traditional computing method and number the bits from 0 to 7.  Just be aware of which counting method your system uses as this will make it easier to use the programming bit editor.

Calculating the decimal value from Binary bits. The decimal value is the sum of all 8 Bits.


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