Speed Steps

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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.

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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.

UP Centennial control stand.jpg

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.

Some multifunction decoders may internally use 1024, 2048 or even 4096 speed steps, using the CVs which control acceleration/deceleration to calculate the rate at which the internal steps are incremented/decremented for smoother motor control.

Available Modes

Fourteen Speed Steps

The 14-speed step mode has been deprecated and is now optional.

The 14-speed step mode has been retained for backwards compatibility with older multifunction decoders. It requires one byte to transmit, within that byte is the status of the lights and direction of travel. It shares the same space with the 28-speed step mode. If a decoder which supports 14 speed step mode is controlled using 28 speed step packets, the headlight will toggle on and off for each throttle change. In this scenario, the bit that controls the headlight is now the Least Significant Bit of the speed value.

If CV29 Bit 1 is set to 0, Bit 4 controls FL. In this mode there are 14 speed steps defined. When CV 29 Bit 1 is set to 0, bit 4 is used as an intermediate speed step.

See the table below. Bit 5 is defined as C.

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 5 of the speed instruction. In this instance, the 14 speed step mode data occupies the same instruction space as the 28-step mode.

When a multifunction decoder which supports 28–step mode only (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. See the instruction manual for your system.

This is explained by the data structure for the 128-speed step mode, which uses an Extended Format Packet. The sub instruction which activates the 128-speed step mode occupies the same data space used for speed values in the 28-step mode.

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 28 steps to a 14-step decoder.

SPEED STEPS
DCCWIKI.COM
SPEED C S3 S2 S1 S0 SPEED C S3 S2 S1 S0 SPEED C S3 S2 S1 S0 SPEED C S3 S2 S1 S0
STOP 00000 5 00100 13 01000 21 01100
STOP [1] 10000 6 10100 14 11000 22 11100
E-STOP [2] 00001 7 00101 15 01001 23 01101
E-STOP [1][2] 10001 8 10101 16 11001 24 11101
1 00010 9 00110 17 01010 25 01110
2 10010 10 10110 18 11010 26 11110
3 00011 11 00111 19 01011 27 01111
4 10011 12 10111 20 11011 28 11111

Table adapted from NMRA Standard S-9.2. Also refer to S-9.2.1.

One Hundred and Twenty-Eight Speed Steps (Optional)

This mode requires two bytes of data, unlike the other modes which use one byte. The first byte contains the Advanced Operation Command and the sub instruction for 128 step mode, the other contains the direction and speed value. [3]

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.

A compatible 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 to 28 speed step mode when it receives a command in that speed step format. Unlike 14 and 28 speed step commands, it 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. Bit 7 controls direction.

Comparison of Speed Step Modes

CV29 determines which mode is in use. Bit 1, FL, is by default set to 1, activating the 28/128 speed steep modes. To use 14 speed steps, the status of this bit must be changed.

Speed Step Comparison
Mode Instruction Byte One Instruction Byte Two
14 Speed Steps UUDCSSSS
28 Speed Steps 01DCSSSS
128 Speed Steps CCCGGGGG DDDDDDDDD

14 Step Mode:

  1. D = Direction, 0 = Reverse, 1= Forward [4]
  2. C = FL on/off
  3. S0 is the Most Significant Bit

28 Step Mode [5]

  1. Bits 6 and 7 are 01, indicating this data packet contains speed and direction
  2. D = Direction, 0 = Reverse, 1= Forward [4]
  3. C = Bit 5 is an additional speed bit, which is the Least Significant Bit
  4. S0 is the Most Significant Bit

128 Step Mode [6]

  1. CCC= 001, Advanced Operation Instruction
  2. GGGGG = 11111, 128 Speed Step Control
  3. D = Data Byte, D7 controls direction, D0 – 6 are the speed value
  • 28/128 Speed Step Modes are active by default, see CV29.

Packet Structure

Baseline Packet

The NMRA DCC Standards structure the baseline packet as follows:

Speed and Direction Packet
111111111111 0 0AAAAAAA 0 UUDCSSSS 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.
  • U = Undefined

Packet Format for 28 Speed Steps

The 28-speed step mode uses one Instruction byte:

01DCSSS

Where

  • 01 indicates this is a data byte with speed and direction indication.
  • Bit 5, D, is the direction bit, 1 = Forward
  • C, Bit 4, is an additional speed bit, also the least significant bit.
  • S, Bits 0 – 3, are speed bits

Bits 0 – 3 indicate the speed as per S–9.2, if CV29 Bit 1 is equal to 1, Bit 4 is used as an intermediate speed step. If CV29 Bit 1 is set for 14 speed steps, Bit 4 is FL status.

Extended Packet Format

Parent article: Digital_Packet/DCC_Extended_Packet

The Extended Packet Format [6] is used for 128 speed steps:

{preamble} 0 [Address Byte] 0 {Instruction Bytes} 0 [Error Byte] 1

In this example the instruction Bytes are formatted as follows:

CCCGGGGG DDDDDDDD

  • CCC=001 (Advanced Operation Instruction) and
  • GGGGG (Sub-instructions) = 11111 which activates the 128-speed step mode.
  • The Second Instruction byte uses Bit 7 to determine direction (1=Forward) with Bits 0 – 6 indicating speed (MSB is bit 6).
  • The value of U0000000 is Stop, and U0000001 Emergency Stop.

CV29 activates this mode.

Mode Changes

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.

Digitrax

Digitrax Status Codes
First Digit Second Digit
Status Locomtive In Consist
New 0 0 28 Speed Steps
Common 1 5 2 14 Speed Steps
Idle 2 3 128 Speed Steps
In Use 3 7 4 28 Speed Steps FX/LX with Advanced Consisting Enabled
7 128 Speed Steps
F 128 Speed Steps FX/LX with Advanced Consisting Enabled
and Top Locomotive in Consist

NCE

NCE's Power Cab defaults to 28 speed steps, pressing a button on the cab/throttle changes that mode to 128 for all locomotives. Boosters added later to a Power Cab may store the mode used and restore those settings during power up. The Power Pro can store that setting in the command station memory for each locomotive, for up to 512 locomotive addresses.

Troubleshooting

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
MRH DCC columnist Bruce Petrarca shows how a loco headlight acts strangely when you have the speed step setting wrong in CV29.
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, loss of control will result. The command station needs to change to 14 or 28 speed step mode for that decoder, to format the digital packets correctly. A packet with 128 speed steps will have extra data which can cause control issues for a 28 step only decoder.

Which Systems?

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).

Scale Matters

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As a general rule, the smaller the scale, the finer the speed control needed to achieve realistic switching and other operations. While 28 speed steps may be OK for G scale, 128 is more realistic 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.

Further Reading

  1. 1.0 1.1 Multifunction decoders may ignore the direction bit (optional)
  2. 2.0 2.1 Multifunction decoders must immediately cease power delivery to the motor
  3. S-9.2.1, section 2.3.2.1
  4. 4.0 4.1 Forward is the front of the locomotive, as referenced to the engineer's position in the cab.
  5. See S-9.2, B:Baseline Packets
  6. 6.0 6.1 See S-9.2.1, Secton 2.32