28 vs. 128 speed steps

[CSX Robert]

Most of what gngoatman88 said is right on, but I would like to make a few minor corrections. The decoder can not distinguish between a 14 speed step command and a 28 speed step command, that is why there is a bit in CV29 to tell the decoder whether to expect 14 speed step commands or 28 speed step cmmands. The original 14 speed step command used four bits for the speed, one bit for direction and one bit for the headlight. The four bits would allow for 16 steps. These 16 steps included "Stop" and "Emergency Stop, " which left 14 speed steps. For the 28 speed step command, the headlight control is left out and that bit is used to double the number of speed steps(headlight control is sent in a sperate instruction). Since it looks the same as the 14 speed step command, if the decoder is setup for 14 speed steps and the command station is sending 28 speed step commands, the headlight will turn on and off as you increase or decrease the throttle. If the decoder is setup for 28 speed step commands and the command station is sending 14 speed step commands, turning the headlight on and off will not control the headlight but will slightly adjust the speed. Using five bits gives the potential for 30 speed steps(32 - "Stop" and "Emergency Stop"), but they assigned two values to "Stop" and two values to "Emergency Stop." The reason for this is so that if you send 14 speed step commands to a decoder set for 28 speed steps, the decoder will recognize the "Stop" and "Emergency Stop" commands whether the command includes headlight on or headlight off(if they had used all 30 possible speed steps, if you sent a 14 speed step command with "Stop" or "Emergency Stop" and headlight on to a decoder set for 28 speed steps, it would not stop) .

The 128 speed step command is a completely different format from the 14 and 28 speed step commands, so you don't have to set a bit to tell the decoder to expect 128 speed step commands. It uses seven bits for speed which gives 128 possible values. Subtract out "Stop" and "Emergency Stop," and that gives 126 actual speed steps. I have no idea why they call it 128 speed steps instead of 126.

[Hamltnblue]

The bottom line is that there are 255 "Speed Selections" that are plotted in 14, 28, or 128 Speed Steps. In these speed steps you pick numbers between 0 and 255. If you want the engines half speed to be max you set the highest step to 128. If you use momentum you get all 255 steps over time. Go half throttle with max momentum and it'll take a couple of minutes to get to the voltage representing 128. When slowing you have to plan because a zero throttle is like idling a heavy train and will take plenty of time to slow. Some have an air brake and/or dynamic brake function to simulate lifelike braking.

[gngoatman88]

CSX Robert is absolutely right.  Thanks for adding that info.

[Calflash]

Phoebe Vet

I did not express doubt that it was true, I asked for someone to please explain how it works.  I don't understand how the digital instruction set in the decoder can be interpolated.

I never thought about it but it makes sense and I believe it. Interpolation is similar to what your computer does when you resize a graphic esp to make it larger - it fills between "hard" (original) values with ones in between based on a formula. I don't how it works as long as it does.

 Cal

[Phoebe Vet]

Cal:

I understand how a decoder can interpolate.  It was an early poster's claim that the interpolation was done by the command station that I could not comprehend.

Now that I know it is, in fact, done by the decoder, I am satisfied.