Summary: Speed steps are the divisions between Stop and Maximum Speed. The command station receives a throttle input, computes a value based on the number of speed steps, and transmits that value to the track. Multifunction decoders can use 28 or 128 distinct values, older decoders may only accept 14 values. Larger values result in finer control of the vehicle's speed.
See the Video.
Speed steps can be thought of as how many notches there are between idle (stopped) and full speed. That is, as you turn the knob on your DCC throttle, how much control can you achieve. For example, if you had 4 speed steps (stopped, slow, medium, and full), you wouldn't have much fine control. In our example, you would turn the knob and the train would lurch to its slowest speed. The next step up, it lurches again to its medium speed, and then to full speed. This isn't very realistic, and doesn't allow for very fine control. Think of your lawnmower's throttle compared to your car.
Most diesels have eight notches on the throttle. There may not be an increase in RPM when the throttle is moved from the idle stop to the first notch, but the locomotive can begin to move. The throttle does not control the prime mover, it controls power to the traction motors. Many sound decoders ignore that because the user expects an increase in RPM will happen on the first notch.
Steam is a little different, because the engineer controls not only the throttle valve in the boiler, but also the valve action at the cylinders. It would be the same as being able to control the valve lift and duration in your car, to optimize the engine's operation under different loads and conditions. Opening the throttle only admits steam into the steam chest, adjusting the cutoff (value action) controls the amount and duration of steam admitted to the cylinder. Adjustments are always being made to maintain speed or power while economizing on steam (and fuel) consumption by maximizing utilization of the energy released by the expanding steam. Even when the engine is drifting downhill, some steam is still admitted to the cylinders to cool them and provide lubrication. The reverser, which controls the valve motion, can also shift the valves to provide forward or reverse operation.
While steam and diesels have different approaches to the same issue, the number of speed steps determines the level of control afforded, just as the engineer has various controls available to him which determine how the locomotive will react. The engineer does not want to melt the traction motors when starting a heavy train.
Speed Step Choices
With DCC, there are three speed step choices: 14, 28, and 128. Lenz also had 27 and 55 speed steps that were a simulation of some kind - it is not covered in the NMRA Standards and RPs. It's unclear if they still offer these odd speed steps now that they offer 128 speed steps.
The command station will calculate a value which is transmitted to the multifunction decoder within a vehicle. With the 14-step mode in use, each increment of the throttle is equal to 1/14th, or approximately 7% of full throttle.
For more information refer to the page about DCC Packets.
The NMRA DCC Standards structure the baseline packet as follows:
|Speed and Direction Packet|
|111111111111 0||0AAAAAAA 0||01DCSSSS 0||EEEEEEEE 1|
|Preamble||Byte One||Byte Two||Error Correction|
Byte One is the Address Byte. Byte Two is the Instruction Byte.
Byte Two can contain speed and direction instructions. Bits 0 – 3 (SSSS) are 4 bits for speed, bit 0 being the least significant bit. These provide 14 speed steps.
Bit 4, C, shall contain one additional speed bit, which may also be used to control the headlight for reasons of backwards compatibility. This bit is used to create 28 speed steps.
Bit 5 controls Direction. Bits 6 and 7 are 01, indicating the byte contains speed and direction instructions.
Speed Step Structure for 28 Speed Steps
This table shows the speed step structure for 28 step mode. Note how the bit C alternates between On and Off. This causes the headlight to switch state every time a change in throttle position is made when the command station is transmitting 14 steps to a 28-step decoder.
|SPEED||C S3 S2 S1 S0||SPEED||C S3 S2 S1 S0|
- † Multifunction decoders may ignore the the direction bit (optional)
- ‡ Multifunction decoders must immediately cease power delivery to the motor
Table adapted from NMRA Standard S-9.2. Also refer to S-9.2.1.
If CV29 Bit 1 is set to 0, Bit 4 controls FL. In this mode there are 14 speed steps defined. If CV 29 Bit 1 is set to 0, bit 4 is used as an intermediate speed step.
Fourteen Speed Steps (Optional)
- The 14-speed step mode is now optional.
It requires one byte to transmit, within that byte is the status of the lights and direction of travel.
Twenty-Eight Speed Steps (Mandatory)
The 28 speed steps also fit within one byte. This mode is required by the NMRA DCC Standards. This is controlled by CV29, Bit 2, FL Location, which defaults to a value of 1. If that bit is set to Zero the 14 Speed Step mode is enabled, the headlight controlled by bit 4 of the speed instruction.
Be aware that when a multifunction decoder which only supports the 28-step mode (not 28/128) receives a 128-step speed command, loss of control occurs. The command station must be set for the correct speed step mode for that decoder.
One Hundred and Twenty-Eight Speed Steps (Optional)
This mode requires two bytes of data, unlike the other modes which use only one byte. One byte is the equivalent speed in 28 step modes, the other contains the additional speed step value.
The optional NMRA 128 speed step mode is available for both multifunction decoders and command stations which support it. Both the decoder and the command station controlling it must support this feature.
The multifunction decoder switches to 128 speed step mode automatically at the track level when it receives a 128-speed step command from the command station. The decoder will revert back to 14 or 28 speed step mode when it receives a command in that speed step format. Unlike 14 and 28 speed step commands, the multifunction decoder does not need to pre-programmed to enable 128 speed step mode operation. It is always on, ready to be used at any time.
One Hundred and Twenty-Eight Steps is Really One Hundred and Twenty-Six Steps
The term "128 speed steps" is a bit of a misnomer. It should be called 126 speed step mode for consistency, much like the 14 and 28 speed steps are accurately called.
You would normally expect the maximum to be 127 since that is the largest value that can be encoded in the 7-bit binary field transmitted in a 128-step speed control packet. The lowest speed step (step 0) is Stop. The missing speed step (Step 1) means Emergency Stop (i.e. Stop NOW and ignore deceleration settings). So, your command station's speed steps 1 to 126 actually correspond to 2 to 127 as encoded in the DCC packet. There are 254 valid steps available, half in either direction. An additional bit controls direction.
It is possible to accidentally alter the speed step parameter in the command station. It may be a global change, or specific to one locomotive address. Should that happen, consult the manual for your system on how to set the default number of speed steps. Digitrax uses the term Status Editing for parameters related to a specific slot in the command station. Other manufacturers may have similar terms.
Fourteen Step Vs. Twenty-Eight Step
- Fourteen Speed Step Mode is OPTIONAL for a multifunction decoder. Few decoders still support this mode. Twenty-Eight steps are mandatory.
Sending 28 step commands to a decoder which can only accept 14 results in the following:
- The headlight may alternate between on and off as the throttle is changed. It may be on, yet turns off when the throttle is changed to the next step. A change to CV29's Forward Lamp Location (Bit 1) may be required. A value of "0" means 14 Speed Step operation.
If a 28-step decoder receives packets in 14 step mode, the maximum speed will be limited to half. Turning the headlight on or off may also have a definite effect on the speed.
For compatibility reasons, some decoders may not use the address byte to control the headlight operation. The headlight is controlled by a bit in the instruction byte instead, which will change with the throttle settings. This bit will alternate between On and Off as the throttle changes.
See NMRA Std. S-9.2 for more details.
Speed Step Mis-match Demonstration
Issues Involving 128 Speed Step Mode
The 128-speed step mode is OPTIONAL. Most decoders support it, and will seamlessly switch between 28 and 128 step modes.
If a multifunction decoder does not support or understand the 128-step mode, it will cause issues as it will remain in its normal mode. Loss of control will be the result. The command station needs to be changed to 14 or 28 speed step mode for that decoder, so that it can format the digital packets correctly. A packet with 128 speed steps will have extra data which can cause control issues.
Some command control systems (previous to NMRA DCC) advertised forward and reverse speed steps combined - 128 speed steps would be 64 forward and 64 backward. But with DCC, when you're using the 128-speed step mode, you have 128 speed steps forward and 128 speed steps in reverse (for a total of 257 speeds - 256 moving speeds, and one stopped). 14 and 28 speed step modes work the same way.
Almost all systems now support 14, 28, and 128 speed steps - simultaneously. But don't assume, check the DCC Systems comparison guide for more information. Some systems are still limited to 14 or 28 speed steps, not both at the same time. A few manufacturers still make decoders that operate only in 28 speed step mode - for those, you may find it necessary to status edit the slot this locomotive uses in the command station.
Which Mode is Better?
There have been discussions over the years as to whether 128 speed steps are necessary or not. The nay-sayers would always say, "The real ones don't have 128 speed steps". Just keep in mind, these are not real trains, these are models. And like everything that gets scaled, all things can't be scaled exactly the same and still appear correct. With model railroad control, the smaller the scale the more speed steps you need to have good control. You also lack an exact replica of the control systems found on the prototypes.
It could be argued that those who disagreed had systems lacking 128 speed steps capability. Being loyal to their system, those with systems lacking 128 speed steps would argue that 128 speed steps were unnecessary. However, those that had systems capable of 128 speed step systems argued that mode is the greatest thing since sl__(you get the idea).
As a general rule, the smaller the scale, the finer the control needed to achieve realistic switching and other operations. While 28 speed steps may be OK for G scale, 128 is certainly a whole lot better for HO and N scales. Of course, if all you want to do is turn your trains on and watch them go, any amount of speed steps is fine.