Configuration Variable/Configuration Register CV29

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Configuration Register

Referred to as CV 29

Configuration Register Bit Switches

CV29-chart.png


FL Location

This bit controls the location of the Forward Headlight.

If Bit 1 has a value of 0 (zero), then bit 4 of the speed instruction is used to control FL. This provides 14 discrete speed steps. If the decoder is configured for 14 speed steps, and is operated using 28 speed steps, the headlight will toggle on or off each time the throttle is changed an increment.

If Bit 1 is set to a value of 1 (default value), the decoder will operate in 28 speed step mode

Acknowledge (Bit 3)

This bit is used to activate Bidirectional Communications, such as RailCom. Normally this bit would not be used, and some online calculators for CV29 may not allow this bit to be set. If the decoder lacks this capability the bit would default to OFF and cannot be changed.

Setting the Configuration Register

This is a more advanced subject, and is for information only. Other bits may need to be set manually. For example, subtracting 4 from the default value will set the decoder for NMRA DCC operation only.

You DCC system may automatically set some bits, such as enabling the Extended Address Mode, when programming a multifunction decoder. Other bits may need to be set manually. For example, subtracting 4 from the default value will set the decoder for NMRA DCC operation only.

Since this CV has a large impact on other CVs and the operation of the decoder, it is advisable not to blindly write values to it. Many will tell you to "write this value to CV29 and all will be well!" If you wish to alter this CV, first read the value in it, determine the changes needed, calculate the required value, then write that value. You can also perform this process with JMRI. Making changes without regard for the current value may lead to more problems than solutions.

Software such as JMRI can set CV29 to the correct values without dealing with any calculations by the user.


Example: Default Value = 6

CV29 Default
Bit Position Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value 27 26 25 24 23 22 21 20
Decimal Value 128 64 32 16 8 4 2 1
Default Binary Value 0 0 0 0 0 1 1 0
Result (Decimal) 0 0 0 0 0 4 2 0
Sum 6

Example: Extended Address Mode Enabled

CV29 Extended Address Mode Enabled
Bit Position Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value 27 26 25 24 23 22 21 20
Decimal Value 128 64 32 16 8 4 2 1
Default Binary Value 0 0 1 0 0 1 1 0
Result (Decimal) 0 0 32 0 0 4 2 0
Sum 38
Sum, Hex $26

The default value for CV29 is 6, in both decimal and hexadecimal numbers. As shown in the examples, adding a value of $20 to the default value in CV29 allows extended addresses. Subtracting $20 from the value in CV29 turns it off. In many cases this will be unneeded, as the command station software will set CV29 for a Primary or Extended address automatically.

If NMRA DCC Mode only, with extended address mode is desired, the required value would be 6 - 4 + 32 = 34.

NDOT: If the value in CV29 is an even value, NDOT has not be enabled. If odd, NDOT is enabled. To reverse the direction of the locomotive using NDOT, you just add or subtract 1, depending on the CV's value being even or odd.

Using the NDOT to rectify an incorrectly wired motor is considered bad practice.

Further Reading

NMRA/NMRA_Standards

Main article: NMRA/NMRA_Standards

Specifically S-9.2.1 and S-9.2.2

Videos Regarding Programming

CV29 and Extended Addresses