Mixing Up Speed Step Modes

Decoders and Command Stations typically support both 14, 28 and 128 Speed step modes.  When both the decoder speeds step mode and the command station speed step mode are matched, locomotive control works as expected.  However when they are not, funny things happen and that is what this section is about.

Want to learn more about 14, 28 and 128 speed steps? Go here:14, 28 and 128 Speed Steps

Mixing: 14 vs 28 Speed Steps

At no time do you lose control of the decoder/trains.  You just get funny control of it.

There is the reason why BOTH the command station and the decoder must be in perfect agreement when using 14 or 28 speed step mode.  Disagreement means the headlight will become improperly involved with the speed step operation of the decoder.

So what happens when they are not?

1) IF the decoder is set to 14 speed steps and the command station sends 28 speed steps commands, you get the headlight going on and off with each step in speed (Up or down).  You will also observe that you have to increment the speed steps by two to get the locomotive to increase speed.

2) IF the decoder is set to 28 speed steps and the command station sends 14 speed steps commands, you get only get half of the speed range (14 usable speeds) than you expect.  You will also observer that tuning on/off the headlight changes the speed of the engine.  That is because the headlight command is being interpreted by the decoder as a speed step command that account for the missing additional 14 speed step range you expect.

Mixing 14/28 speed steps VS 128 speed steps

Per the NMRA DCC Standards and RP's, 128 Speed Step mode is not a requirement for a given decoder to support.  This possible lack of support can result in loss of control of the engine.  Fortunately almost all decoder made today support it so this is NOT a common problem.

1) Sending a 128 speed step mode command to a Decoder that supports 128 Speed Step mode will have NO PROBLEMS PERIOD.  No confusion or loss of control.

2) Sending a 128 speed step mode command to a Decoder that does NOT support 128 speed step mode WILL RESULT IN LOSS OF CONTROL.  The decoder simply does not understand the command and continues to operated in 14 or 28 speed step mode.  So as long as you send 128 speed step command, you will not have any control of the engine.

The decoder does not need to know if the speed command is 128 speeds steps because the DCC 128 speed step command is completely different than the DCC 14/28 speed step command.  The decoder, if it is support 128 speeds step mode, switches from 14 or 28 speed step mode to 128 speed step mode "on the fly" and does not skip a beat.  Switch back from 128 speed step mode is simply done by sending a 14 or 28 speed step command.  The change back is again done on the fly without skipping a beat.