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Very few modellers will ever need to modify or work with DCC Protocol / Decoder data directly, but when the need arises, finding any form of useful technical data on the subject will quickly lead to frustration.

This Document has been carefully prepared by cross referencing and verifying actual DCC Protocol Data against numerous technical specifications as well as restricted-access / confidential manufacturer manuals that are not typically made available to the public.

If you have no idea what “DCC Protocol” is, then start by reading this Technical Introduction to DCC, that forms part of the “DCC for Engineers” series.

Edited & Reviewed: Stéfan Stoltz

The term DCC is trademarked the National Model Railroad Association (NMRA)
So while the term Digital Command Control is generally used to describe any digital model railway control system, strictly speaking it refers to NMRA DCC.
Digital Command Control (DCC), was developed by Lenz, and is the original and most widely adapted standard for a system to operate model railways digitally.
The DCC protocol is defined by two NMRA Standards:
  • S-9.1 specifies the electrical standard
  • S-9.2 specifies the communications standard
It is important to note that the DCC protocol defines signal levels and timings on the track. DCC does not specify the protocol used between the DCC command station and other components such as additional throttles. A variety of proprietary standards exist, and in general, command stations from one vendor are not compatible with throttles from another vendor.


1.  The Decoder

Lets review the function of a Decoder:

  • Each decoder is programmed with a unique address (duplicates addresses is a common problem)
  • The decoder monitors the DCC data and listens for “commands”, embed in the DCC signal
  • The decoders are programmable, allowing you to change the address, motor performance, and lighting and sound effects
  • The decoder programming is stored in semi-permanent (non-volatile) memory that saves the information when there’s no power to the decoder
  • Decoders can be re-programmed with new information as required

2.   Decoders and Configuration Variables (CV)

“A Configuration Variable is a memory location in the decoder that controls the behavior of the decoder”

Just like any other computer, decoders must be programmed by the user to reach their full potential.

Decoders are commercially supplied with basic “factory default” settings, that determine the decoder address, motor control, lights, sound, and other functions.

In order to change/customize the “settings” of a decoder you need to edit the CVs, or Configuration Variables, in the decoder.

Some CVs use values ranging from 0 to 255, others use their space in the decoder’s memory as a bank of eight on/off switches.

  • A CV can store raw data.
  • A CV can store data as a binary number with a value between “00000000” (decimal 0) and “11111111” (decimal 255), as two binary numbers in high bits/low bits format “HHHHLLLL”, or some variant of these.
  • Data can also be spread across multiple CVs if the necessary maximum value exceeds decimal 255.
  • A CV can store information as a series of “bit switches”, each bit controlling the state of a specific function, turning it on or off.


3. Configuration Variables (CV) and their meaning

Keep in mind that the NMRA defines standards for specific functions and the associated CVs, but has made provision for manufacturers to add their own characteristic or proprietary functions.

The current NMRA standards for configuration variables cover only the NMRA specified CVs


 3.1  NMRA Basic Configuration Variable Set

The NMRA has defined a basic set of CVs that all decoders support in order to ensure that interchange between various manufacturers is possible.

You can reference the “complete “NMRA Standard CV Definitions for Mobile Decoders” for additional info


CV 1 Mandatory


  • The Early on, decoders only had one byte of data (CV 1) for the address. It was decided that addresses from 00 to 127 would be defined by this byte of data. This leads to lots of confusion, like why is 120 a “two-digit” address? Because it is pseudo hexadecimal – now are you not sorry you asked? Do not worry about why, just know what. Best to call it a SHORT address.
  • Different manufacturers define 2-digit (short) and 4-digit (long) differently, all within the NMRA standard. Lenz defines any number larger than 99 as a long. Digitrax defines numbers larger than 127 as long. NCE lets you decide (0120 is a long address, while 120 is short).
  • So, if you are trying to address a loco with numbers between 100 and 127, you need to spend a bit of time thinking about it.
  • If you only run on one system or only on NCE and Digitrax, fine – just program the loco address you want to use as a short address – CV01.
  • If you are going with Lenz only, they handle it automatically.
  • If however, you need to run on different systems which are numbered between 100 and 127, it quickly becomes messy! The best advise is to use the Long Address

CV 7 Mandatory: Manufacturers Software revision code
CV 18 Mandatory: NMRA Assigned Manufacturer code


  • Reading the contents of these registers will let you determine what decoder is in the locomotive without having to open it up.
  • Writing a value of 8 to CV8 will reset the entire decoder to factory defaults
  • When 4-digit addressing (actually any address above 127) was implemented, the NMRA standard included some very complex calculations.
  • It is not simply putting two digits in one CV and the other two in another. Setting these CVs manually can be very complex, so it is recommends that you use some program

CV 02  Mandatory
CV 03
CV 04
CV 23
CV 24
CV 65

Notes:  (CV 2)

  • CV 2 is misnamed – it is frequently called “START VOLTAGE”, yet technically, the voltage on the motor is constant at about the track voltage. CV 02 adjusts the amount of time that the voltage is applied to the motor at speed step one.
  • A Good Value for CV 2 it to set the value so that the loco just creeps (doesn’t apply to BEMF)
  • BEMF decoders should auto adjust for this, so CV02 should be set to zero
  • Tsunami decoders should have this CV set to zero unless BEMF is totally disabled


Here’s how to set CV 02

1. Set the loco to speed step 01
2. Set CV 02 = 001
3. See if the loco moves. If so, stop
4. If not, add 1 to the value of CV 02, then go to step 3.


Notes: (CV 5)

  • CV 05 sets the top speed the loco can run
  • You can adjust this so that the loco runs no faster than your desired top speed.
  • Scale speeds of 40 to 90 MPH are reasonable.
  • Frequently setting this to zero will have the decoder run the loco as fast as it is physically capable of running.

Notes:  (CV 6)

  • CV 06 sets the midpoint speed the loco can run
  • Once the maximum speed (CV05) has been set, set the speed of the loco at a midpoint throttle setting
  • Frequently setting this to zero will have the decoder calculate the midpoint for you.


CV 29 Mandatory


 CV 29 Controls the Following:

Bit 7
Multifunction decoder, always reads 0
Bit 6
Reserved for future use
Bit 5
EAM, Extended Address Mode enable,0 = Primary address in CV 1, 1 = Extended Address in CV 17/18To use extended addresses (14 bit), Bit 5 is set to “1”. When this bit is cleared (set to zero), the decoder reverts to the primary address in CV 1.
Bit 4
STE, Speed Table Enable, 0 = values in CVs 2, 4 and 6, 1 = Custom table selected by CV 25
Bit 3
ACK, Advanced Acknowledge mode enabled if 1, disabled if 0
Bit 2
APS, Alternate Power Source enable, 0 = NMRA Digital only, 1 = Alternate power source set by CV12
Bit 1
F0 location: 0 = F0 controlled by bit 4 of speed direction instruction (14 speed steps). 1 = F0 controlled by bit4 of function group 1 (28 and 128 speed step modes).
Bit 0
DIR: 0 = normal, if 1 the direction bit in speed/direction instruction is inverted.


  • When the DIR bit (o) is set, Locomotive and headlight will run in opposite direction, useful for long hood forward operation.
  • The DIR bit (0) controls the NDOT (Normal Direction of Travel)
  • With APS bit set, CV 12 must also be set to 1.
  • CV 29 sets the overall personality of the loco: whether it runs on DC or not, is the normal direction of travel forward or reversed, etc.
  • Setting this CV is very involved and it is recommended that you use some program to accomplish this task.

CV 66 Speed Table CV 81 Speed Table
CV 67 Speed Table CV 82 Speed Table
CV 68 Speed Table CV 83 Speed Table
CV 69 Speed Table CV 84 Speed Table
CV 70 Speed Table CV 85 Speed Table
CV 71 Speed Table CV 86 Speed Table
CV 72 Speed Table CV 87 Speed Table
CV 73 Speed Table CV 88 Speed Table
CV 74 Speed Table CV 89 Speed Table
CV 75 Speed Table CV 90 Speed Table
CV 76 Speed Table CV 91 Speed Table
CV 77 Speed Table CV 92 Speed Table
CV 78 Speed Table CV 93 Speed Table
CV 79 Speed Table CV 94 Speed Table
CV 80 Speed Table CV 95 Speed Table

CV 17 Mandatory
CV 18 Paired with CV 17 for “Long Address”


  • The standard “Short Address” stored in CV 01 can be changed to a “Long Address”
  • CV 17 and 18 are paired together, the resulting in each register holding one piece of data, which when combined results in the 14 bit extended address, or four digit address.
  • CV 17 holds the MSB, the LSB is in CV 18. (Most Significant Bit and Least Significant Bit).
  • CV 29 must have bit 5 set to a value of “1”. If this bit is not set, the four digit address will not work.
  • Many systems automatically calculate the values needed for CV 17 and 18, and set CV 29 correctly to enable the four digit address.

CV 19  Consist Address and Direction


  • Consist address is stored in bits 0 to 6
  • Bit 7 is the Consist Direction
  • Consist address run from 1 to 127
  • If set to 0, consist instructions are ignored then Instructions set to the primary or extended addresses are ignored
  • Addresses set from 1 to 127 will assign the locomotive to that address, facing forward.
  • Addresses from 129 to 255 result in the locomotive running backwards.

4.   Programming Mode

Decoders support different modes of programming:

  • reg……..NMRA Physical Register Mode
  • pag…….NMRA Page Mode
  • dir……..NMRA Direct Mode
  • roc……..A non NMRA Register Mode version, used by ROCO
  • rpg……..A non NMRA Page Mode version to use if Page Mode fails, but the decoder manual says Page Mode should work.
  • old……..An old, obsolete NMRA Direct Mode version, used by ZTC

5.   Service Mode


5.   Binary numbers, decimal conversion and bit switches

There are several ways to store data in CVs.

The one that causes the most confusion is the “bitswitch”.

This system is far simpler that it might appear. Consider the table below:

Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Least
128 64 32 16 8 4 2 1
0 0 0 0 0 0 0 0

Think of the eight bits of the CV as a bank of eight simple on/off switches. Each switch is identified by the bold number above it. A value of “0” means the switch is off or in its default mode, a value of “1” turns it on. A decimal number between 0 and 255 identifies a unique setting of every switch in the bank of 8. This allows you to turn on or off any possible combination of these 8 switches by entering the appropriate decimal number into the CV. These software switches are the digital equivalent of the DIP switches, selector switches, or the jumpers that were often found on computer hardware.

Here is an example of the best-known CV that uses this technique, CV29:

Reserved 2/4 Digit
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
128 64 32 16 8 4 2 1

In the above example, 2 bits are on – bits 32 and 4. Add 32+4 and you get 36, the decimal value of the bitswitch settings shown. If you wanted to turn on the “speed tables”, then you would add 16 to that value, for a decimal value of 52.

Some times you need to work in reverse– you read the value of the CV from the decoder, then have to figure out what switches are on and which are off.

It’s simple, but tedious.

The binary equivalents of the switch positions (as shown in the table above) are 128 – 64 – 32 – 16 – 8 – 4 – 2 – 1.

  • Start with the value you read from the CV, in this case 52.
  • 52 is less than 64, so there are no 128s or 64s in the CV (ie those switches are off).
  • 52 is greater than 32, so there is a 32 in the CV; that switch is on.
  • Subtract 32 from 52 giving the remainder of 20.
  • 20 is greater than 16, so there is a 16 in the CV; that switch is on.
  • Subtract 16 from 20 giving the remainder of 4.
  • 4 is less than 8, so that switch is off.
  • 4 = 4, so that switch is on.
  • Subtract 4 from 4 giving 0; all remaining switches (2 and 1) are off.
  • Switches 32, 16, and 4 are on, the rest are off.

5.   Service Mode


6. CV Numbers


CV 1 Loco address CV 51 Manufacturer unique
CV 2 Vstart CV 52 Manufacturer unique
CV 3 Acceleration rate CV 53 Manufacturer unique
CV 4 Deceleration rate CV 54 Manufacturer unique
CV 5 Vhigh CV 55 Manufacturer unique
CV 6 Vmid CV 56 Manufacturer unique
CV 7 Manufacturer version CV 57 Manufacturer unique
CV 8 Manufacturer ID CV 58 Manufacturer unique
CV 9 Total PWM period CV 59 Manufacturer unique
CV 10 EMF feedback cutout CV 60 Manufacturer unique
CV 11 Packet timeout value CV 61 Manufacturer unique
CV 12 Power source conversion CV 62 Manufacturer unique
CV 13  Analog function status CV 63 Manufacturer unique
CV 14 CV 64 Manufacturer unique
CV 15 CV 65 Kick start pulse time
CV 16 CV 66 Forward Trim
CV 17  Extended Address CV 67  Speed Table
CV 18  Extended Address CV 68  Speed Table
CV 19  Consist Address CV 69  Speed Table
CV 20 CV 70  Speed Table
CV 21  Addr Active for F1 – F8 CV 71  Speed Table
CV 22  Addr Active for FL CV 72  Speed Table
CV 23  Acceleration adjustment CV 73  Speed Table
CV 24  Deceleration adjustment CV 74  Speed Table
CV 25  Mid-range cab speed step CV 75  Speed Table
CV 26 CV 76  Speed Table
CV 27 CV 77  Speed Table
CV 28 CV 78  Speed Table
CV 29  Configuration data #1 CV 79  Speed Table
CV 30  Error information CV 80  Speed Table
CV 31  Configuration Data #2 CV 81  Speed Table
CV 32 CV 82  Speed Table
CV 33  Output Loc FL(f) CV 83  Speed Table
CV 34  Output Loc FL(r) CV 84  Speed Table
CV 35  Addr Active for F1 CV 85  Speed Table
CV 36  Addr Active for F2 CV 86  Speed Table
CV 37  Addr Active for F3 CV 87
CV 38  Addr Active for F4 CV 88
CV 39  Addr Active for F5 CV 89
CV 40  Addr Active for F6 CV 90
CV 41  Addr Active for F7 CV 91
CV 42  Addr Active for F8 CV 92
CV 43 CV 93
CV 44 CV 94
CV 45 CV 95  Reverse Trim
CV 46 CV 96
CV 47 CV 97
CV 48 CV 98
CV 49  Manufacturer unique CV 99
CV 50  Manufacturer unique CV 100
CV 101 CV 102
CV 103 CV 104
CV 105  User Identifier #1 CV 106  User Identifier #2
CV 107 CV 108
CV 109 CV 110


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