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Just like any other computer, decoders must be programmed by the user to reach their full potential. While they come with basic "default" programs, most users will want to customize the decoder address, motor control, lights, sound, and other functions to meet their specific needs. You do so by editing 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 8 on/off switches. While this lets you do a lot with very little memory, it can get very complex for those of us that aren't on speaking terms with binary code.

There is Software available which simplifies the programming of decoders. Many people agree that JMRI is the best, free, open source software available - often rivals commercially available software.

[edit] Configuration variables

Configuration variables or CVs are memory locations within a decoder. They are the means for controlling the performance of a DCC decoder. They can be used in a couple of different ways.

• A CV can store raw data. It can do this 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 "bitswitches", each bit controlling the state of a specific function, turning it on or off. For example, CV 29 works in this way.

• A combination of the above. For example, CV 19 works in this way.

There are NMRA standards for the functions specific CVs are to control, but there is plenty of room left for manufacturers to add their own characteristic or proprietary functions, as well. The current NMRA standards for configuration variables cover only the NMRA specified CVs. See your specific decoder documentation for other capabilities beyond the basic capabilities. It is beyond the realm of this document to list the manufacturer-specific CVs for all available decoders.

[edit] Explanation of binary number/decimal conversion and bitswitches

There are several ways to store data in CVs. The one that causes the most confusion for those who are not acquainted with the inner workings of a computer is the "bitswitch". This system is far simpler that it might appear. Consider the table below:

Think of the 8 bits of the CV as a bank of 8 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. Here is an example of the best-known CV that uses this technique, CV29:

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 too, 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.

[edit] The NMRA standard CV definitions for mobile decoders

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

[edit] See Also

• Decoder - All aboout various decoders.

[edit] External Links

• NMRA 9.2.2 - NMRA CV Specs and further details.