Trackside Prints

DCC CV29 calculator

Tick the settings you want and get the number to put in CV29. You can also decode a CV29 value read off a decoder, and work out CV17 and CV18 for a long address.

Set the bits

Tick each setting you want on

CV29 = 6

In binary, bit 7 first: 00000110. Bit 5 is clear, so the decoder answers to its short address in CV1 (1 to 127).

  • Bit 0 (1), Locomotive direction: normal
  • Bit 1 (2), FL location: bit 4 in function group one instruction controls FL
  • Bit 2 (4), Power source conversion: power source conversion enabled (see CV12)
  • Bit 3 (8), Bi-directional communications: disabled
  • Bit 4 (16), Speed table: set by CVs 2, 5 and 6
  • Bit 5 (32), Addressing: one byte (short address in CV1)
  • Bit 6 (64), Reserved: reserved for future use
  • Bit 7 (128), Decoder type: multifunction decoder
A red parcel tied with ribbon under a Christmas tree, a green model steam engine on a short length of track beside it
New engine, new decoder: set CV29 before its first run.

Nothing you type is saved. The boxes you tick are kept in the page link, not by us.

Got a railway photograph you love? Print it as it is, or turn it into railway art.

How to use it

Tick each setting you want switched on. The CV29 value updates as you go, with the same number in binary and a line for each bit.

If your decoder already has a value, type it in the decode box to see what it switches on. For a long address, type the number and the tool gives you what goes in CV17 and CV18. Or type those two to read an address back.

The inside of a steam locomotive cab: brass and copper valves, round pressure gauges and pipes above the glow of the firebox
A real cab has valves and gauges. A decoder has CVs. Photo: Andrew Bowden from London, United Kingdon, CC BY-SA 2.0

How it is worked out

CV29 is one byte, eight on-off bits, and the value is the sum of the bits that are on: 1, 2, 4, 8, 16, 32, 64 and 128. The meaning of each bit is from the NMRA's standard S-9.2.2, Configuration Variables for DCC, the July 2012 version linked from the NMRA standards index. Bit 6 is "Reserved for future use", so it has no box.

Bit 5 picks the address. Clear, and the decoder answers to the short address in CV1, which the standard gives as "a value between 1 and 127". Set, and it answers to the two-byte address in CV17 and CV18. CV17 must hold 192 to 231 and CV18 any value, so the address is (CV17 minus 192) × 256 + CV18. With CV17 at 231 and CV18 at 255, that comes to 10239.

The standard says a decoder that does not support a feature must not let its bit be changed. So if a value will not stick, check your decoder's manual.

Source: NMRA Standard S-9.2.2 (July 2012), read 30 October 2026.

Written by Craig Fearn, Trackside Prints. Last updated .

Questions, answered

What CV29 value gives a long address?
Set bit 5, which adds 32. With bits 1 and 2 already on, 6 becomes 38.
What do CV17 and CV18 hold?
The long address in two parts. CV17 is 192 plus the address divided by 256, rounded down. CV18 is what is left over.
Why does my decoder ignore a bit?
S-9.2.2 says a decoder must not let a bit change for a feature it does not support. Check its manual.