Programming DCC Decoders by Stan Ames

NMRA DCC is unique in that it specifies a method for customizing your decoders to individual locomotives. In the most basic form this allows you to set the decoders address. For many decoders this technique can also be used to either set or read back such things as speed matching, customizing special effects and optimizing motor performance. Once set these values stay with the decoder until the user wishes to change them again. We call this concept if setting or reading decoder parameters as programming.

The NMRA has specified a Recommended Practice that defines how to program decoders (NMRA RP-9.2.3). While this recommended practice provides the precise packet formats a command station must transmit to program decoders, it does not specify the user interface provided to the model railroad operator who is using the system. This is left up to each manufacturer to provide.

To program a value into the decoder each system asks the user a value. The problem is there are several methods to represent this value, (Decimal, Hex, Binary and English explanation) and not all systems support all forms. Thus the user must often translate the numbering scheme provided by the decoder manufacturer to the numbering scheme requested by the Command Station/Programmer. The purpose of this page is to assist users in this conversion process.

Do I need to Know any of this?

In most cases no. For the most part you can use the native instructions provided by the command station manufacturer to program any decoder (for exceptions to this rule see the DCC problems page). This is because for the most part the NMRA has defined and the Manufacturers have agreed to use common CVs (Configuration Variables) with common values. However, there are differences in preferred values and there are several manufacturer unique CVs. In this case an understanding on how to convert between the various styles makes programming much easier.

Return to Index | Return to The World of DCC

Numbering Schemes

Lets first start with the popular forms of representing values

Decimal

Decimal number has 10 numbers (0-9). This is perhaps the easiest form as we all know how to count using Decimal. Systems such as Lenz and the System One family commonly use Decimal for all values.

Hex

Hex is a computer form of counting which has 16 numbers (0-9 and A-F). Using Hex can be useful to the command station developer as 256 values can be expressed in just 2 positions (FF Hex = 255 Decimal). Digitrax uses hex for many values.

Binary

Binary uses just 2 numbers 1, and 0. This form may be the easiest from a DCC standpoint as DCC is a binary format. A DCC word has 8 binary bits or 255 different values in a data word. (11111111 = 255 Decimal). The NMRA RPs are written using the binary representation. Now just to confuse you a little more there are two common methods for numbering the bits in binary. Consider the Decimal number 3 which is represented by 00000011 in binary. Lenz counts the bits from 1 to 8.

bit 8	bit 7	bit 6	bit 5	bit 4	bit 3	bit 2	bit 1
0	0	0	0	0	0	1	1

The NMRA (which developed its numbering scheme after the Lenz system was on the market) counts the bits using the more common computer notation from 0 to 7

bit 7	bit 6	bit 5	bit 4	bit 3	bit 2	bit 1	bit 0
0	0	0	0	0	0	1	1

Notice that the NMRA numbering starts with 0 while the Lenz numbering starts with a 1. Translation is easy. Just add one to the NMRA bit positions to achieve the Lenz notation or subtract one from the Lenz bit numbers to achieve the NMRA notation.

English

Some systems use buttons, sliders, knobs, or menus to enter values. The positions of these buttons, sliders, or knobs are converted by the command station into the proper binary representation. To program other decoders on these systems it is best to understand what these various buttons, sliders, or Knobs actually do. Most entry systems (such as the Digitrax Challenger, and MRC Command 2000) use this form of programming. The System One family as well as most computer based programmers use menus to help guide the user in selecting the appropriate value.

Return to Index | Return to The World of DCC

Understanding CV 29

CV29 (The Basic Decoder Configuration), is used to set up the decoder's basic properties. This may be the hardest CV to understand as each bit means something completely different and many manufacturers try using words while others prefer HEX or Decimal or Binary.

The following table provides an explanation of this most basic decoder configuration CV as well as provides the translations into the various numbering schemes.

Function	English Description	NMRA Bit meaning	Lenz Bit meaning	Decimal	Hex	Binary
Locomotive Direction	Normal Direction	NMRA Bit-0=0	Lenz Bit-1=0	0	00	00000000
	Reverse Direction	NMRA Bit-0=1	Lenz Bit-1=1	1	01	00000001
Headlight Location	14 Speed Step mode	NMRA Bit-1=0	Lenz Bit-2=0	0	00	00000000
	28 Speed Step Mode	NMRA Bit-1=1	Lenz Bit-2=1	2	02	00000010
Power Source Conversion	NMRA Digital Mode Only	NMRA Bit-2=0	Lenz Bit-3=0	0	00	00000000
	Analog Conversion Enabled	NMRA Bit-2=1	Lenz Bit-3=1	4	04	00000100
Speed Table	Use Decoder Defined Speed Curve	NMRA Bit-4=0	Lenz Bit-5=0	0	00	00000000
	Use User Defined Speed Table	NMRA Bit-4=1	Lenz Bit-5=1	16	10	00010000
Extended Addressing	127 address mode	NMRA Bit-5=0	Lenz Bit-6=0	0	00	00000000
	10,000 address mode	NMRA Bit-5=1	Lenz Bit-6=1	32	20	00100000

To achieve multiple effects just add the various properties desired. For example if you want the decoder to allow for analog conversion and operate in 28 speed step mode you provide the Decimal 6 (4+2). I suggest adding the decimal values and then looking up the appropriate value in the conversion table below as adding binary or Hex can be somewhat more complicated.

Return to Index | Return to The World of DCC

Conversion Table

Following is a table that converts the 256 DCC numbers between Decimal, Hex and Binary. This table is useful in converting between the different manufacturers preferred different programming methods.

Decimal	Hex	Binary
0	00	00000000
1	01	00000001
2	02	00000010
3	03	00000011
4	04	00000100
5	05	00000101
6	06	00000110
7	07	00000111
8	08	00001000
9	09	00001001
10	0A	00001010
11	0B	00001011
12	0C	00001100
13	0D	00000101
14	0E	00001110
15	0F	00001111
16	10	00010000
17	11	00010001
18	12	00010010
19	13	00010011
20	14	00010100
21	15	00010101
22	16	00010110
23	17	00010111
24	18	00011000
25	19	00011001
26	1A	00011010
27	1B	00011011
28	1C	00011100
29	1D	00010101
30	1E	00011110
31	1F	00011111
32	20	00100000
33	21	00100001
34	22	00100010
35	23	00100011
36	24	00100100
37	25	00100101
38	26	00100110
39	27	00100111
40	28	00101000
41	29	00101001
42	2A	00101010
43	2B	00101011
44	2C	00101100
45	2D	00100101
46	2E	00101110
47	2F	00101111
48	30	00110000
49	31	00110001
50	32	00110010
51	33	00110011
52	34	00110100
53	35	00110101
54	36	00110110
55	37	00110111
56	38	00111000
57	39	00111001
58	3A	00111010
59	3B	00111011
60	3C	00111100
61	3D	00110101
62	3E	00111110
63	3F	00111111
64	40	01000000
65	41	01000001
66	42	01000010
67	43	01000011
68	44	01000100
69	45	01000101
70	46	01000110
71	47	01000111
72	48	01001000
73	49	01001001
74	4A	01001010
75	4B	01001011
76	4C	01001100
77	4D	01000101
78	4E	01001110
79	4F	01001111
80	50	01010000
81	51	01010001
82	52	01010010
83	53	01010011
84	54	01010100
85	55	01010101
86	56	01010110
87	57	01010111
88	58	01011000
89	59	01011001
90	5A	01011010
91	5B	01011011
92	5C	01011100
93	5D	01010101
94	5E	01011110
95	5F	01011111
96	60	01100000
97	61	01100001
98	62	01100010
99	63	01100011
100	64	01100100
101	65	01100101
102	66	01100110
103	67	01100111
104	68	01101000
105	69	01101001
106	6A	01101010
107	6B	01101011
108	6C	01101100
109	6D	01100101
110	6E	01101110
111	6F	01101111
112	70	01110000
113	71	01110001
114	72	01110010
115	73	01110011
116	74	01110100
117	75	01110101
118	76	01110110
119	77	01110111
120	78	01111000
121	79	01111001
122	7A	01111010
123	7B	01111011
124	7C	01111100
125	7D	01110101
126	7E	01111110
127	7F	01111111
128	80	10000000
129	81	10000001
130	82	10000010
131	83	10000011
132	84	10000100
133	85	10000101
134	86	10000110
135	87	10000111
136	88	10001000
137	89	10001001
138	8A	10001010
139	8B	10001011
140	8C	10001100
141	8D	10000101
142	8E	10001110
143	8F	10001111
144	90	10010000
145	91	10010001
146	92	10010010
147	93	10010011
148	94	10010100
149	95	10010101
150	96	10010110
151	97	10010111
152	98	10011000
153	99	10011001
154	9A	10011010
155	9B	10011011
156	9C	10011100
157	9D	10010101
158	9E	10011110
159	9F	10011111
160	A0	10100000
161	A1	10100001
162	A2	10100010
163	A3	10100011
164	A4	10100100
165	A5	10100101
166	A6	10100110
167	A7	10100111
168	A8	10101000
169	A9	10101001
170	AA	10101010
171	AB	10101011
172	AC	10101100
173	AD	10100101
174	AE	10101110
175	AF	10101111
176	B0	10110000
177	B1	10110001
178	B2	10110010
179	B3	10110011
180	B4	10110100
181	B5	10110101
182	B6	10110110
183	B7	10110111
184	B8	10111000
185	B9	10111001
186	BA	10111010
187	BB	10111011
188	BC	10111100
189	BD	10110101
190	BE	10111110
191	BF	10111111
192	C0	11000000
193	C1	11000001
194	C2	11000010
195	C3	11000011
196	C4	11000100
197	C5	11000101
198	C6	11000110
199	C7	11000111
200	C8	11001000
201	C9	11001001
202	CA	11001010
203	CB	11001011
204	CC	11001100
205	CD	11000101
206	CE	11001110
207	CF	11001111
208	D0	11010000
209	D1	11010001
210	D2	11010010
211	D3	11010011
212	D4	11010100
213	D5	11010101
214	D6	11010110
215	D7	11010111
216	D8	11011000
217	D9	11011001
218	DA	11011010
219	DB	11011011
220	DC	11011100
221	DD	11010101
222	DE	11011110
223	DF	11011111
224	E0	11100000
225	E1	11100001
226	E2	11100010
227	E3	11100011
228	E4	11100100
229	E5	11100101
230	E6	11100110
231	E7	11100111
232	E8	11101000
233	E9	11101001
234	EA	11101010
235	EB	11101011
236	EC	11101100
237	ED	11100101
238	EE	11101110
239	EF	11101111
240	F0	11110000
241	F1	11110001
242	F2	11110010
243	F3	11110011
244	F4	11110100
245	F5	11110101
246	F6	11110110
247	F7	11110111
248	F8	11111000
249	F9	11111001
250	FA	11111010
251	FB	11111011
252	FC	11111100
253	FD	11110101
254	FE	11111110
255	FF	11111111

Return to Index | Return to The World of DCC

Copyright 1997-2007 by Tried & True Trains, Inc.
This page is written and maintained by: Stan Ames
For more information contact Debbie Ames,  President tttrains