NMRA CV 5 - Vhigh (Top Volts)
CV 5 is used to specify the motor voltage level at maximum speed. This value is a fraction of available voltage. When CV 5 equals 255 or 0, the full available voltage will be applied at full speed, a value of 128 will apply half of the available voltage at full speed, and so on. Changes to CV5 will automatically adjust the entire speed curve to linearly scale from 0 speed to the new full speed unless otherwise affected by CV 2 and/or CV 6 .
CV 5 should be the first CV programmed when setting up "speed limits" in locomotives. In a TCS decoder, CV 2 and/or CV 6 do not need to be modified before modifying CV 5; however, if you have already set values in CV 2 and CV 6, this must be taken into account first before programming CV 5.
NOTE: CV 5 will not be "in use" as part of the speed curve calculation if programmed with a value of 0 or 1.
From S-9.2.2 of the NMRA Standards and Recommended Practices:
"Vhigh is used to specify the motor voltage level at maximum speed. This value is a fraction of available voltage. When CV 5 equals 255, the full available voltage will be applied. Values of 0, 1, or 255 indicate that Vhigh is not used in the calculation of the speed table. Vhigh requires no other CV programming to be activated."