DCC decoder CV reference

The configuration variables you actually use, what each one does, and the sensible starting values — in one table for quick lookup.

Updated 9 August 2026

Every DCC decoder implements the NMRA-standard CVs below. Manufacturers add their own on top, so always check the sheet that came with the decoder before writing anything above CV100.

The essentials

CV	Name	Range	What it does
1	Primary address	1–127	The short address. Ignored if CV29 bit 5 is set.
2	Vstart	0–255	Motor power at the lowest throttle step. Raise until it creeps reliably.
3	Acceleration	0–255	How gradually it gets up to speed. Higher is slower.
4	Deceleration	0–255	How gradually it stops. Higher is slower.
5	Vhigh	0–255	Top speed. Lower it to tame a fast loco or match a pair.
6	Vmid	0–255	Mid-point of the three-point speed curve.
7	Version	read-only	Decoder firmware version.
8	Manufacturer	read-only	Manufacturer ID. Writing 8 here resets most decoders to factory defaults.
17	Long address (high)	192–231	High byte of the long address.
18	Long address (low)	0–255	Low byte of the long address.
19	Consist address	0–127	Address when running in a consist. 0 disables.
29	Configuration	0–255	Bit field — see below.

CV29 bits

CV29 is eight settings in one byte. Add the values of the bits you want.

Bit	Value	Off	On
0	1	Normal direction	Reversed direction
1	2	14 speed steps	28/128 speed steps
2	4	DCC only	Analogue DC conversion allowed
3	8	RailCom off	RailCom on
4	16	Use CV2/5/6 curve	Use speed table CV67–94
5	32	Short address (CV1)	Long address (CV17/18)

Common combinations:

Value	Meaning
2	Short address, 28/128 steps. The usual default.
3	As above, reversed direction.
6	Short address, 28/128 steps, DC running allowed.
34	Long address, 28/128 steps.
35	Long address, 28/128 steps, reversed.
50	Long address, 28/128 steps, custom speed table.

Tick the boxes instead of adding it up: CV29 calculator.

Working out a long address

CV17 = (address / 256) + 192      # integer division, discard remainder
CV18 = address % 256              # remainder

Worked examples:

Address	CV17	CV18
128	192	128
1000	195	232
2451	201	147
4468	209	116
9999	231	15

Remember to set CV29 bit 5 as well, or the decoder will carry on answering to CV1.

Or skip the arithmetic: the long address calculator converts both ways and checks the ranges for you.

Speed tables and trim

CV	Name	What it does
65	Kick start	A brief power pulse to overcome stiction at low speed.
66	Forward trim	Scales forward speed. 128 is neutral.
67–94	Speed table	28 individual step values, used when CV29 bit 4 is set.
95	Reverse trim	Scales reverse speed. 128 is neutral.

CV66 and CV95 are the neat way to make a loco run at the same speed in both directions without rebuilding the whole speed table.

Sound decoders

Sound-specific CVs are almost entirely manufacturer-defined, but two are near-universal:

CV	Typical use
63	Master sound volume
64	Brake sound / squeal volume

Check the manufacturer’s sheet. Values above CV100 vary enormously between makers, and writing a guessed value into a sound decoder is a good way to spend an evening resetting it.

Recovery

Situation	Do this
Lost the address	Read CV1, CV17, CV18 and CV29 on the programming track.
Decoder unresponsive	Write 8 to CV8 to factory reset. Address returns to 3.
Writes work, reads fail	Not the decoder — see EX-MotorShield8874 vs the L298N.
Loco runs backwards	Toggle CV29 bit 0 (add or subtract 1).
Jerky, lights flicker	Speed step mismatch — see 128 vs 28 speed steps.