The NMRA Standard CV Definitions for Mobile Decoders

Intro to DCC Signals

DCC uses a method similar to IP (internet protocol) over AC. The DCC wave form is created by a DCC controller (command station). It is sent to a booster (or a couple of boosters) where AC is added to the DCC Commands. That is sent from the booster to the rails. The DCC decoder in the loco picks up the AC with DCC codes from the rails, converts the AC to DC and then applies the DC to the DC motors in the train. The amount of DC current (amp) applied is controlled by the DCC Codes.

Wireless DCC controllers have an RF transmitter in the hand (throttle), and an RF receiver that controls the DCC controller (command station). But the DCC Signal and power still goes through the booster and the rails.

DCC Power

With DCC, you put a 'fixed' electrical power on the track. This means that all locomotives have power to their wheels, all the time. Instead of the power controlling the trains, a receiver (decoder) inside each locomotive listens to commands sent out over the rails from the command station. These commands tell the decoder to make the train for go forward, reverse, fast, slow, or turn on/off lights or sounds.

With this setup, you control the trains and not (like with DC) the track. Because of this, and the whole point of DCC as a whole, it is possible to control multiple trains on the same track without having to deal with complex wiring to isolate each section of track to control each train.

DCC Wiring

You need to be aware that, since all locomotive power comes from the track and more than one locomotive may be running at once, DCC boosters (and their power supplies) are designed to have current ratings of 5 to 10 amps. Because DCC locos and their ampere demand may be located anywhere on the layout, wiring to the track for DCC needs to be designed to handle a higher number of amps.

For all locomotives to run properly, you need to be sure that there are no voltage sags. That is, you need to be sure that all sections of track have sufficent power to handle the locomotives. The best way to do this is to scatter feeder wire connections around the layout. A second reason for a robust wiring system is to ensure that the over current protective devices built into the DCC booster will operate correctly. This is necessary to protect your railroad equipment from damage caused by an accidental electrical problem, such as derailments.

With DCC, a booster can supply 12 volts (or as high as 18 volts for larger scales) at 5, 8 or even 10 amps into a track short-circuit without becoming overloaded. Such a short-circuit represents perhaps 60+ continuous watts. This current can quickly melt trucks, engines, or let the magic smoke out of decoders. To help prevent this, DCC systems have short-circuit protection built into the booster. When a short-circuit is detected, the booster will shut down. Once the short-circuit has been removed from the track, the booster will automatically turn the power back on.

For this to work, any point on the track needs to wired so that the full load of the booster can pass through it so that the booster's protection can function properly. To test this, simply place a metal coin, or some other metal object, across the rails. The booster should shut off the power automatically, and turn the power back on once the object has been removed. This usually means that heavier, larger wire, better track connections, and more track feeders are needed as compared to a traditional DC-powered electrical circuit.

 

Mobile Decoder CVs
Thanks to Dayton N-Track for permission to reprint this table.

CV

Name

Default
Value
(decimal)

Read Only

1

Primary Address (2 digit) If value = "0", decoder will convert to Alternate Power Source (see CV 12)

3

 

2

Vstart (minimum starting voltage at speed step 1)

7

 

3

Acceleration Rate (starting momentum)

0

 

4

Deceleration Rate (stopping momentum)

0

 

5

Vhigh (maximum voltage at top speed step)

1

 

6

Vmid (maximum voltage at middle speed step) Together Vmin, Vmid, and Vmax give a 3 point speed curve.

1

 

7

Manufacturer Version Number

-

Yes

8

Manufacturer ID (assigned by NMRA) SOME decoders reset when a specified value is written to this CV - check your documentation! List of IDs.

-

Yes

9

Total PWM Period (optional)

Reccommended formula is: PWM period in microseconds = (131 + MANTISSA x 4) x 2 EXP

MANTISSA is stored in bits 0-4, EXP in bits 5-7.

0

 

10

EMF Feedback Cutout (optional) Sets the speed step above which back EMF is no longer applied.

-

 

11

Packet Time-Out Value Sets the maximum time that the docder will maintain speed without receiving a valid data packet from the command station.

-

 

12

Power Source Conversion (optional)

Determines the secondary power source for the decoder. Available are:

  • 0 0 0 0 0 0 0 1 = Analog Conversion (1)
  • 0 0 0 0 0 0 1 0 = Radio (?) (2)
  • 0 0 0 0 0 1 0 0 = Zero-1 (4)
  • 0 0 0 0 1 0 0 0 = TRIX (8)
  • 0 0 0 1 0 0 0 0 = CTC16/Railcommand (16)

-

 

13

Analog Mode Function Status (optional) Selectively sets decoder functions 1-8 to operate in analog mode with bitswitches (bit 0=F1, bit 7=F8)

-

 

14-16

Reserved for expansion

-

 

17-18

Extended Address (4 digit - optional)

-

 

19

Consist Address (2 digit - optional) Bits 0-6 sets 2 digit consist address and bit 7 sets unit direction relative to the normal direction of travel in CV29.

-

 

20

Reserved for expansion

-

 

21

Consist Address Active for F1-F8 (optional) Sets whether F1-F8 respond to commands at Consist Address in addition to the locomotive address.

-

 

22

Consist Address Active for FL (optional) Sets whether F0 (headlight control) responds directionally to commands at Consist Address

-

 

23

Acceleration Adjustment (Acceleration trim - optional) Modifies CV3. Values below 127 subtract from CV3, above 127 add to CV3. 0=off.

-

 

24

Deceleration Adjustment (Deceleration trim - optional) Modifies CV4. Values below 127 subtract from CV4, above 127 add to CV4. 0=off.

-

 

25

Speed Table/Mid-range (optional) Allows selection of factory preset speed tables, and cab throttle position for the middle of the speed table. 0 or 1=off.

-

 

26-28

Reserved for expansion

-

 

29

Configuration Data #1

Bitswitches as follows:

  • Bit 0 - Locomotive Direction
  • Bit 1 - Control of F0
  • Bit 2 - Power Source Conversion
  • Bit 3 - Advanced Decoder Acknowledgement
  • Bit 4 - Speed table off/on
  • Bit 5 - 2/4 digit addressing
  • Bit 6 - Reserved
  • Bit 7 - Multifunction/accessory decoder