What's all this 128 speed step stuff, anyway?

The basic standard for DCC defines a speed/direction packet for 14 speed steps. These are encoded as a 4-bit binary value within the command. The remaining two steps (since a 4 bits allows 16 different values) are stop (0000) and emergency stop (0001). In response to the stop speed step a decoder should come to a stop according to the programmed inertia. Emergency stop means stop NOW.

To allow finer control, an extra bit was stolen from the directional lighting control in the basic speed/direction packet giving a 5 bit binary value, allowing 32 speed steps. The extra bit is treated as the least significant so that new speed steps are inserted between the original 14. There are now two stop and two emergency stop commands, leaving 28 actual speed steps available. Hence "28 speed steps" which is now the default mode of operation.

The term "128 speed steps" is a bit of a misnomer.

In the same way that you can have 14 or 28 speed steps rather than 16 or 32 in the basic speed/direction packet, this mode should be called 126 speed step mode for consistency since two steps are again reserved for stop and emergency stop.

Your command station may display speed steps in this mode as 1 to 126. These correspond to 2 to 127 in the command to the decoder with 0 being used for stop and 1 for emergency stop.

Some command stations can alternate rapidly between two adjacent speed steps so that you actually get 252 sub-divisions. Decoders are designed to understand this and generate the appropriate drive to the motor.

The three speed step modes give you a linear progression in speed as you adjust the throttle. What you may actually want is a non-linear relationship between the throttle setting and the speed of the loco. This could be to compensate for the behaviour of the motor in a particular loco, to match two locos for double-heading (consisting in DCC speak) or to give finer control at low speed for shunting. This is where the speed table comes but I'll leave that for part two as this is getting quite long.

Using my favourite TV analogy, consider the volume control. When you press one of the volume buttons on the remote control you often get a display showing the volume as a number. if completely mute is 0 and full volume is 28 then we can say we have 28 volume steps (ignoring the mute state just as DCC gnores the stop state). If another TV ranges from 0 to 127 then it doesn't neccessarily mean that it can be any louder , just that it has finer control, ie each step is smaller. The same applies to 14, 28 and 128 speed step operation in DCC.