KB929: Speed Steps Explained - 14, 28, 128

This article was last updated on Oct. 19, 2010, 2:18 p.m. | Print Article | Leave Feedback

Most modern command stations and decoders are set up to run 28/128 speed steps.  If you are using older command stations or decoders you may need to adjust one or the other to get the trains to run.

Why are more speed steps better?


The reason you want to use a command station and decoders with 128 speed steps is for smoother speed control of your locomotives.   Digital command control signals are just that, digital.  When you send speed commands with your throttle, you are sending digital instructions telling the decoder to change from the current speed to a different speed.  When there are more speed steps available in the control range, the motion of the decoder equippd locomotive will be smoother.

Why do I need to know about speed steps?

DCC command stations send instructions to the decoders on the layout by using digital packets of information.  Over the course of the development of digital command control technology, 14, 28 and 128 speed step command stations have been produced.  On the other side of the equation, decoders have also been produced which will respond to 14, 28 and/or 128 speed step instructions.  The decoders in the locomotives must be capable of receiving and executing the instructions sent by the command stations.
 
How does this affect my operations?

All Digitrax command stations can send 14, 28 & 128 speed step instructions.  All Digitrax decoders can receive and execute 14, 28, & 128 step instructions. Digitrax command stations and decoders are shipped set up to run in 128 speed steps for the best performance.

Some non-Digitrax command stations can only send 14 or 28 speed step instructions.  If you are using Digitrax decoders with these less capable systems, you will need to program the decoders accordingly (see CV29).

14-Speed or 28-Speed Step Decoders

Some non-Digitrax decoders can only receive and execute 14 or 28 step instructions. If you are using these less capable decoders with your Digitrax command station, you have three options:

1. You can status edit the decoder so that the command station will send the correctly formatted instructions to the decoder.  The status editing process allows you to continue to have 128 speed step performance in your decoders that have this feature while running the less capable decoders at the same time.

2. You can change the option switches in your command station to treat all the decoders on the layout as 14 or 28 speed step decoders.  This will allow you to avoid status editing but you will sacrifice performance.

3. You can set up your individual throttle so that every new address that is selected will run as a 14 or 28 step decoder.  Again, you will avoid the need to status edit but you will sacrifice performance.
 
What do I need to program?

The factory default programming for all Digitrax decoders is set for 28/128 speed step operation and the factory default programming for all Digitrax command stations is set for 128 speed step operation.  For best performance, when using Digitrax decoders on Digitrax systems, set CV29 to a value that gives 28/128 step speed control and make sure the system is set to run in 128 speed step mode.

If you are using Digitrax decoders with command stations made by other DCC companies, you may need to program CV29 to a value that gives 14 speed step control if that system only sends 14 speed steps to the decoders.

            

 

Speed Steps

 
 

CV29 Where the light instruction  comes from

 
 

Status Edit where  the speed instructions comes from

 
 

14

 
 

14 Step Packet

 
 

3 byte speed packet

 
 

28

 
 

Function Packet

 
 

3 byte speed packet

 
 

128

 
 

Function Packet

 
 

4 byte speed packet

 



If the light on your loco is blinking when running, set CV29 to a value that gives 28/128 speed step control.

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