The NMRA DCC standards defined CV to control momentum. In prototype terms, it represents the mass of the train connected to the given engine(s). Locomotive speed response is very sensitive to the mass/load of the train. Small trains can move quickly but big heavy trains take a long time to start and stop. In model railroading terms, we can use these CV to model or emulate the mass of the train connect to the drawbar of the locomotive(s).
Momentum has also taken on other responsibilities when used with sound decoders. It allows the sound effects to be implemented realistically with the speed change of the engine especially when starting from a stop condition.
However, many decoders manufactures for various reasons, good/bad, have implemented variations of decoder momentum control that does not conform to NMRA Standards. This has lead to differences in expectations in both operation and programming when you mix and match decoder brands in consisting situations. Simply put, incompatibilities.
1) NMRA CV DEFINTIONS.
CV3: Acceleration (Power) Rate. The value in this CV is used as a multiplier of time in a momentum equation defined by the NMRA DCC standards. It defines the acceleration "rate of change" between train speed and time from a given throttle speed increase command. With a value of zero, there is no acceleration momentum. Throttle speed increases are instantly relayed to the motor. No delay. Low non zero value adds a little delay in how fast the speed changes to the new higher speed setting on the throttle. Low train mass High values take a lot longer. High train mass. CV3 is something the user typically adjust as part of setting up the locomotive for the first time.
The equation is: (CV3 * 0.896) / (number of speed steps in use). Speed steps being 14/28/128.
CV4: Deceleration (braking) Rate. The value in this CV is used as a multiplier of time in a momentum equation defined by the NMRA DCC standards. It defines the deceleration "rate of change" between train speed and time from a given throttle speed decrease command. With a value of zero, there is no deceleration momentum. Throttle speed decreases are instantly relayed to the motor. No delay. Low non zero value adds a little delay in how fast the speed changes to the new lower speed setting on the throttle. Low train mass High values take a lot longer. High train mass. CV4 is something the user typically adjust as part of setting up the locomotive for the first time.
The equation is: (CV4 * 0.896) / (number of speed steps in use). Speed steps being 14/28/128.
CV23: Acceleration Momentum Adjustment. Allows one to modify the value in CV3 by add or subtracting momentum using the same rate equation. This CV23 is used to modify the mass of the train relative to the default or user preprogrammed CV4 value on the fly. The value in CV3 NEVER changes. CV23 purpose is to be a temporary adjustment for use with consisting and on other operational modes as opposed to CV3 which is for permanent locomotive setup.
CV24: Deceleration Momentum Adjustment. Allows one to modify the value in CV4 by add or subtracting momentum using the same rate equation. This CV24 is used to modify the mass of the train relative to the default or user preprogrammed CV4 value on the fly. The value in CV4 NEVER changes. CV24 purpose is to be a temporary adjustment for use with consisting and on other operational modes as opposed to CV4 which is for permanent locomotive setup.
2) COMPARISON OF DECODER BRAND MOMENTUM COMPATIBILITY AND SUPPORT
Blank Space means it is not supported.
A blank space means the given CV is not supported by the decoder.
Exception Notes:
1) TCS supports CV3 and CV4 but have chosen to divide the range up into 3 speed ranges using 8 more CV's. CV3 and CV4 are only used for the low speed momentum range including stop. For each range, they do follow the NMRA formula. Since the slow speed range is the most critical range for momentum matching, in practice this has not been a problem.
2) ESU V4.0 decoders use a NON standard momentum scaling factor that is not compatible with the NMRA standard. It is 3.6 times less sensitive. In other words, you must multiple the NMRA CV3/4 value for a given momentum level by a factor of 3.6 to get the SAME momentum effect with ESU V4.0 decoder.
The equation is: (CV3 * 0.25) / (number of speed steps in use). Speed steps being 14/28/128.
The equation is: (CV4 * 0.25) / (number of speed steps in use). Speed steps being 14/28/128.
That said you can adjust it to work as part of a locomotive setup.
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