Programming 101–CV29

CV29 is often referred to as the master CV because it controls 5 different features. These include:

  1. 2 digit addressing or 4 digit addressing

  2. Normal direction of travel

  3. Speed step control: 28/128 or 14 speed steps

  4. Analog mode conversion on or off

  5. Speed table on or off

So, let’s take a look at each. Two- vs four-digit addressing is pretty straightforward. While you can enter both a two- and four-digit address into the appropriate CVs this setting determines which your decoder will respond to. Most folks prefer using the locomotive number in the four-digt address but it can be useful to have a two-digit address available. For example you might want to run your locomotive 2238 at the club but find someone else is using that address already. By simply changing CV29 you could still operate that locomotive using address 38 instead.

The normal direction of travel setting determines which end of the locomotive is considered front and therefore which way the locomotive will go in relation to your throttle setting. However, be aware that if you install the motor leads backwards then simply changing this may not be a quick fix since it will also affect the directionality of your headlights.

Speed step control is important in that you don’t want to be operating on 14 speed steps. This is one of those legacy features from the early days of DCC. There still may be a few old command stations that only support 14 speed steps but in most cases this is one of those features you will likely never use.

The analog mode setting is very important, especially since in new decoders this is the default setting. This determines whether a decoder will automatically respond to DC power on the track. Some folks may have analog mode turned on if they still have a DC layout at home or the club and want to be able to run with either DC or DCC. However for most folks I recommend this feature be turned off. When intermittent shorts occur on the tracks, they can create a DC pulse which confuses the decoder in locos and they can take off at full speed. However if this feature is turned off the decoders will ignore these DC pulses.

The speed table setting determines whether user loadable or three step speed tables are used. Since in most situations the three step speed curves are adequate for speed matching locomotives this is the option of choice. You would only activate the user loadable 28 step speed curves if you desire to dramatically alter throttle response of the locomotive. The easiest way to create one of those is with DecoderPro. Most decoders now take the 28 speed steps and interpolate them to 128 speed steps so you don’t lose anything by using them.

OK, with that background let me say that actually determining a value for CV29 is a complex process, requiring counting bits for each of the five features and summing them into one value to enter into CV29. Digitrax has a free Digitrax Toolbox app for Apple and Android devices and they also have an online CV calculator on their website. To make this real easy I have included a table from the Digitrax literature. Just pick the features you desire and look up the value for CV29 in the first column–I commonly use a value of 34.

 

2 comments

  • I like your point about what issues will be faced if you reverse the wire connections on the motor and take the easy way out.

  • Thanks, Larry, I am old to the hobby but new to DCC. I appreciate this article — even though it may be like placing a first grader in with a bunch of high schoolers.