14, 28 and 128 Speed Steps
There are 3 speed step ranges supported by DCC. 14, 28, and 128. Here is both some facts and history about these different speed modes.
If you wondering about what happens when you mix up speed step modes, go here: Mixing Up Speed Step Modes
This section covers:
1) 1) KEY COMMAND POINTS TO REMEMBER WHEN WORKING WITH SPEED STEPS
2) 14 SPEED STEP MODE & HISTORY
3) 14 SPEED STEP FUTURE
4) 28 SPEED STEP MODE & HISTORY
5) 128 SPEED MODE & HISTORY
6) 128 SPEED MODE MYTH: DECODER MANUAL SHOWING CV29 "28/128" SPEED STEPS
7) 128 SPEED MODE PERFORMANCE DEGRADATION ON LARGE LAYOUTS.
1) KEY COMMAND POINTS TO REMEMBER WHEN WORKING WITH SPEED STEPS
1) COMMAND STATION: You must tell the command station what speed mode you want to be in for 14, 28 or 128 speed step modes. The command station typically has a default value. See your manual.
2) DECODER: When using 14/28 Speed Step mode, you must program CV 29 Bit to match the choice of 14 or 28 speeds step mode in the command station. (See Problem with mixing up 14/28 Speed step modes section below.)
3) DECODER: Use of 128 Speed steps DOES NOT REQUIRE ANY decoder programming and is not dependent on the 14/28 setting in CV29. (See 128 speed mode myth section below.)
2) 14 SPEED STEP MODE & HISTORY
14 speed step mode was the first speed step mode that was standardized with the original release of the DCC standards. The DCC 14 speed mode packet (command sent down the track) included a bit for the headlight function status to tell the decoder to turn the headlight on or off.
3) 14 SPEED STEP FUTURE
Decoder are dropping support for 14 speed step mode as the DCC standards no longer require It.
Why?
14 speed Step mode creates unnecessary operator frustration when the person is not aware of the rules and programming required.
To address this problem, the NMRA DCC standards have been change to make the old 14 speed steps mode from being a mandatory speed step range requirement to being optional. 28 speed step is now the requirement. The goal going forward is to phase out the 14 speed step option which will eliminate any need for ANY type of decoder speed step programming requirement.
Example: NCE decoders do not support 14 speed step mode so the CV29 14/28 speed step bit in the decoder is now gone. Your only speed choices now are 28 or 128 whose selection choice is ONLY done at the throttle/command station end.
4) 28 SPEED STEP MODE & HISTORY
Time moved and the DCC standard were evolving to add more features and control options. The need for more decoder functions drove the NMRA to provided a better way to control functions including the headlight. The NMRA created a special DCC packet to control nothing but decoder functions including the headlight. A consequence of this was the headlight control bit in the 14 speed step mode packet now became redundant.
With the old "headlight bit" technically now a free one could then double the number of speed steps from 14 to 28 giving the operator finer control over the speed of the train. For decoder manufactures, the software change to support 28 speed steps would be minimal since the basic DCC pack format is unchanged. A easy step forward to enhance the operation of the train.
Great, but whats the catch?
The only problem created with this option is how would the decoder know "which way" to interpret the speed packet. Is it a 14 speed step packet or a 28 speed step packet? There is nothing in the new combo 14/28 DCC speed packet to tell the decoder which way to read it.
To support the 28 speed step mode, the NMRA did the following:
1) Made the 28 speed step mode an option in it first release of the updated DCC standards. In other word, it is not a requirement that decoders must support it.
2) Required that CV29 add a bit to tell the decoder "which way" to read the combo 14/28 speed packet. In other words, it there a headlight bit or not?
To use 28 speed step mode, one had to do two things:
1) Tell the decoder decoder (Preprogram) to use 28 speed step mode.
2) Tell the command station (Set a parameter) to SEND 28 speed steps.
5) 128 SPEED MODE & HISTORY
A some point, there was a desire to increase the speed steps beyond 28. I believe Digitrax, working with the NMRA developed a 128 speed step DCC packet. The motivation were the same as the reasons behind the desire for 28 speeds steps. Finer speed control.
The advantage with the 128 Speed Step packet definition is that packet itself tells the decoder to switch to 128 speed step mode automatically by the simple fact the decoder gets one. If at anytime later a 14/28 Speed Step packet is sent, the decoder, again automatically switches over to that 14/28 speed mode. So the decoder does NOT need to know ahead of time what mode to use since it accepts both 14/28 or 128 speed step commands at the same time. In other words, there is no need to program a 128 Speed step setting in CV29. Only the command station needs to know what to send which is determined by what the USER wants to do.
128 Speed Mode a option. Sending a 128 Speed Step mode to a decoder that does not support 128 Speed Step mode does nothing to the decoder. It will remain in 14/28 speed step mode. However you will lose control of the locomotive until you switch back to 14/28 speed mode.
To use 128 speed step mode, one had to do two things:
1) Make sure the decoder in the engine supports 128 Speed Step mode.
2) Tell the command station (Set a parameter) to SEND 128 speed steps.
Digitrax became a big promotor of using 128 speed steps and every decoder they offered after the NMRA approved it supported it. Today almost all decoders support 128 speed steps. All the big DCC players support it. There are very few limited low cost decoders that do not.
6) 128 SPEED MODE MYTH: DECODER MANUAL SHOWING CV29 "28/128" SPEED STEPS
Some decoder manuals show that CV29 has as a "14 or 28/128" settings. The implication is that if you choose 14 speed step mode, 128 speed step mode is disabled. There is nothing further from the truth. 128 Speed step mode does NOT require any settings in CV29 period. 128 speed step DCC commands are always recognized anytime a 128 speed step supporting decoder receives the DCC command. See section above for more information about HOW 128 speed mode works.
Why did they do this? I suspect is was to reduce customer questions who:
1) believe that if there is a 14/28 speed step setting, the decoder should also have a 128 speed step setting too.
2) want to know if the decoder supports 128 speed step mode by reading decoder CV technical specifications in the manual.
I have to admit this simple good intention documentation deception works. KISS principle at the expense of technical accuracy.
7) 128 SPEED MODE PERFORMANCE DEGRADATION ON LARGE LAYOUTS
Unlike 14 and 28 speed step DCC command, the DCC command to send a 128 speed step command to a given locomotive requires a lot more data to be sent to the locomotive. Simplistically, It takes twice a long for the DCC command station to talk to a 128 speed locomotive on the track as it does talking to 14 or 28 speed step locomotive. That means 14/28 speed step locomotive must WAIT for the 128 speed step locomotives before they can get there commands.
When with this be visible to the operators?
On large layout with lots of running locomotives. It can be measured.
But unless you paying attention
.
The DCC standard define a maximum bit data rate.
7/7/20