KB928: Three Step Speed Table-CV02, CV06, CV05

Three Step Speed Table CV02 V-Start (Start Voltage) CV06 V-Mid (Mid Point Voltage) CV05 V-Max (Maximum Voltage)

When you are using a throttle to control a locomotive, you will notice that as you increase and decrease the speed, the loco responds to the change in throttle settings according to the relationship between motor voltage applied and the throttle setting. This is called the throttle response curve. Decoders are shipped from the factory with a linear throttle response curve. This means that as you increase the throttle setting from 0 to full speed, the loco will look like the default curve in this diagram.



If you want to create a more realistic throttle response curve, you can set up a 28 step loadable speed table. Loadable speed tables define the percentage of motor voltage applied at each of 28 digital speed steps. By programming a value for each of the 28 steps, you can set up a customized throttle response curve for each individual decoder equipped locomotive. The shape of this curve defines the "feel" of the decoder's throttle response.

You can use the 3 step method for some locos and the 28 step method for other locos on your layout, you do not have to choose only one method or the other.

Simple 3 Step Speed Tables with V-max, V-mid & V-start

In most cases, a simple 3 step speed table will give you the throttle response curve you want. Try this simple method and if you still want more resolution in your throttle response curve, click here to learn about 28 step speed tables.

You can set up a simple 3 step throttle response curve by setting V-start (CV02), V-mid (CV06) and V-max (CV05). This method requires programming just 3 CVs. Once these three CVs are set, you are ready to run the loco, no programming of CV29 is needed for this type of speed table.

The following are examples of 3 step speed tables you might want to try. Keep in mind that these are average examples for average locos and you may need to adjust the values for your particular loco to achieve the results you desire.

CV02 V-Start (Start Voltage)

The start voltage, V-start, is the extra voltage added to the motor drive voltage at the first speed step. This adjustment allows you to compensate for the loco motor's efficiency. The range you can program for this CV value is from 000 to 255 (x00 -xFF Hex). Each value increment represents an increase of approximately 1/2% of the total motor drive voltage, when a "straight-line" throttle response curve is used. The value of 255 (xFF Hex) represents 100% motor voltage. In advanced 28/128 speed step mode, the V-start value is interpolated from the first speed step to the middle speed step or "mid" step, 15.

NOTE: Digitrax FX and later model decoders use V-start in 128 speed step mode and run loadable speed tables in 128 speed step mode. Other standard and non-FX decoders disregard V-start in 128 speed step mode and run loadable speed tables in 14 or 28 speed step mode.

CV05 V-Max (Maximum Voltage)

Setting CV05, V-max or maximum voltage, specifies an exact voltage that is applied to the motor at the highest speed step Setting V-max to a lower value than 255 (xFF Hex) allows you to limit the top speed of a locomotive. The range of available V-max CV values is 000 to 255 (x00 -xFF Hex). A value of 128 applies 50% of total voltage to the motor at the highest speed step. A value of 255 (xFF Hex) applies 100% voltage at the highest speed step.

For backward compatibility, CV05 values of 000, 001 & 255 all mean 100% voltage at step 28. If V-Max is accidentally set below V-mid, the decoder will use the V-mid setting as V-max.

V-max is not available when loadable speed tables are in use. In this case, set the maximum voltage by programming speed step 28 as the max voltage.

Note: Some Digitrax decoders are not able to set V-max in 128 speed step mode. If you encounter this problem, status edit the decoder to run in 28 speed step mode and you will be able to set up v-max.

CV06 V-Mid (Mid Point Voltage)

Setting CV06, V-mid or mid point voltage specifies an exact voltage that is applied to the motor at speed step 15 ( or speed step 7 in a 14 step system). The range of available V-mid CV values is from 000 to 255 (x00 -xFF Hex. A value of 128 applies 50% of the total voltage to the motor at step 15 (28 speed system). A value of 255 applies 100% voltage at the middle speed step.

If V-start (CV02) is accidentally programmed to a CV value greater than that programmed for V-mid (CV06), the decoder will force the output voltage for all steps below the V-mid value to be fixed at the V-mid value. This is done to prevent undesirable operational effects.

If a value of 00 or 01 is programmed into CV06 (V-mid), the decoder assumes a "straight-line" throttle response curve is desired. In this case, the decoder will run as though V-mid were set at a value of 50% of total motor voltage.

Note: Some Digitrax decoders are not able to set V-mid in 128 speed step mode. If you encounter this problem, status edit the decoder to run in 28 speed step mode and you will be able to set up v-mid.

V-mid is not available when loadable speed tables are in use.