This document received approval from the NMRA Boardof Trustees in July 1995, March 1997, July 2003 andJanuary 2006. Changes since the last approved version
are indicated by change bars on the left or right side ofthe document. Minor edit changes made to agree with S9.2.2. Edit changes made July 2009. Changed toStandard July 2012.

NMRA STANDARD

Extended Packet Formats
For Digital Command Control,
All Scales
July 2012

S-9.2.1

5

The NMRA Communications Standard for Digital Communications (S-9.2) provides a minimal, basic packet format
required for interoperability. This STANDARD provides Extended Packet Formats that provide the framework to
enable realistic operations to occur. These formats adhere to the general packet format as defined in S-9.2. Whilethe baseline packet has a length of 3 data bytes separated by a "0" bit, a packet using the extended packet format
definition may have a length of between 3 and 6 data bytes each separated by a "0" bit.

10

Format Definitions
Within this Standard, bits within the address and data bytes will be defined using the following abbreviations.
Individual bytes within a specific packet format are separated by spaces. Bytes which are within square [] brackets
can occur one or more times as necessary. Bits are numbered from right to left with bit 0 (the right most bit) being
the least significant bit (LSB) and bit 7 (the left most bit) being the most significant bit (MSB).

15

20

25

30

A = Address bit
0 = Bit with the value of "0"
1 = Bit with the value of "1"
U = bit with undefined value either a "1" or a "0" is acceptable
B = Bit Position / Address
C = Instruction Type field
F = Flag to determine Instruction Implementation
L = Low Order Binary State Control Address
H = High Order Binary State Control Address
S = Decoder Sub Address
V = CV Address Bit
D = Data
X = Signal Head Aspect Data Bit
E = Error Detection Bit

A: Address Partitions
The first data byte of an Extended Packet Format packet contains the primary address. In order to allow fordifferent types of decoders this primary address is subdivided into fixed partitions as follows.
Address 00000000 (0):Broadcast address

Addresses 00000001-01111111 (1-127)(inclusive):

Multi-Function decoders with 7 bit
addresses

35

40

45

Addresses 10000000-10111111 (128-191)(inclusive): Basic Accessory Decoders with 9 bit
addresses and Extended Accessory Decoders with 11-bit addresses
Addresses 11000000-11100111 (192-231)(inclusive):

Multi Function Decoders with 14 bit
addresses

Addresses 11101000-11111110 (232-254)(inclusive):

Reserved for Future Use

Address 11111111 (255):

Idle Packet

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NMRA Technical Department #635

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50

B: Broadcast Command for Multi Function Digital Decoders
The format for this packet is:{preamble} 0 00000000 0 {instruction-bytes} 0 EEEEEEEE 1

55

Instructions addressed to "broadcast address" 00000000 must be executed by all Multi Function Digital Decoders.
The single instruction has the same definition as defined by the Multi Function Digital Decoder packet and can beone, two, or three bytes in length depending on the instruction. Digital Decoders should ignore any instruction they
do not support. The manufacturer must document which broadcast commands a decoder supports.

C: Instruction Packets for Multi Function Digital Decoders
60

The formats for these packets are:{preamble} 0 [ AAAAAAAA ] 0 {instruction-bytes} 0 EEEEEEEE 1

65

Multi Function Digital Decoders are used for the purpose of controlling one or more motors and/or accessories.
The packet format used to control these devices consists of between 3 and 6 bytes where the first bytes are address
bytes followed by one or two instruction bytes and ended by an error control byte.

70

The first address byte contains 8 bits of address information. If the most significant bits of the address are "11"andthe remaining bits are not “111111”, then a second address byte must immediately follow. This second address byte
will then contain an additional 8 bits of address data. When 2 bytes of address information are present they areseparated by a "0" bit. The most significant bit of two byte addresses is bit 5 of the first address byte. (bits #6 and#7 having the value of "1" in this case.

75

Instruction-bytes are data bytes used to send commands to Multi Function Digital Decoders. Although it is unlikelythat all Digital Decoders will implement all instructions, it is important that if they support packets with more than asingle instruction, they can sufficiently parse the packet to be able to recognize if a byte is a new instruction or thesecond byte of a previous instruction.
Each instruction (indicated by {instruction-bytes}) is defined to be:80{instruction-bytes} =

85

90

95

CCCDDDDD,
CCCDDDDD 0 DDDDDDDD, or
CCCDDDDD 0 DDDDDDDD 0 DDDDDDDD

Each instruction consists of a 3-bit instruction type field followed by a 5-bit data field. Some instructions have one
or two or three additional bytes of data. The 3-bit instruction type field is defined as follows:
000 Decoder and Consist Control Instruction
001 Advanced Operation Instructions
010 Speed and Direction Instruction for reverse operation
011 Speed and Direction Instruction for forward operation
100 Function Group One Instruction
101 Function Group Two Instruction
110 Future Expansion
111 Configuration Variable Access Instruction
The last byte of the packet is the Error Detection Byte, which is calculated the same as is done in the baseline packet
using all address, and all instruction bytes (see S-9.2).

100

Decoder and Consist Control Instruction (000)With the exception of the decoder acknowledgment function (00001111), only a single decoder and consist control
instruction may be contained in a packet.
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Decoder Control (0000)
The decoder control instructions are intended to set up or modify decoder configurations.
105

This instruction has the format of{instruction byte} = 0000CCCF, or {instruction byte} = 0000CCCF DDDDDDDD
110

This instruction (0000CCCF) allows specific decoder features to be set or cleared as defined by the value of D ("1"indicates set). When the decoder has decoder acknowledgment enabled, receipt of a decoder control instruction
shall be acknowledged with an operations mode acknowledgment.
CCC = 000

115

120

CCC = 001
125

130

CCC = 010
CCC = 011
CCC = 100
CCC = 101
CCC = 110
CCC = 111

D = "0": Digital Decoder Reset - A Digital Decoder Reset shall erase all volatile memory(including and speed and direction data), and return to its initial power up state as defined in S9.2.4 section A. Command Stations shall not send packets to addresses 112-127 for 10 packet
times following a Digital Decoder Reset. This is to ensure that the decoder does not start
executing service mode instruction packets as operations mode packets (Service Mode
instruction packets have a short address in the range of 112 to 127 decimal.)
D = "1": Hard Reset - Configuration Variables 29, 31 and 32 are reset to its factory default
conditions, CV#19 is set to "00000000" and a Digital Decoder reset (as in the above instruction)
shall be performed.
Factory Test Instruction - This instruction is used by manufacturers to test decoders at thefactory. It must not be sent by any command station during normal operation. This
instruction may be a multi-byte instruction.
Reserved for future use
Set Decoder Flags (see below)
Reserved for future use
Set Advanced Addressing (CV#29 bit 5)
Reserved for future use
D= "1": Decoder Acknowledgment Request-

Set Decoder Flags
135

This instruction is under re-evaluation by the NMRA DCC Working Group.
Manufacturers should contact the NMRA DCC Coordinator before implementing this
instruction.
Set Decoder Flags is an expanded decoder control function that allows for the command station to turn on or
off flags within a specific decoder or within a group of decoders.
140

Format:{instruction bytes} = 0000011D CCCC0SSS

145

SSS is the decoder’s sub-address. This allows for up to 7 decoders to share the same decoder primary address,
yet have certain functions (such as Configuration Variable Access Instructions) be performed on an individual
basis. If SSS = 000 then the operation affects all decoders within the group. The decoder sub-address is definedin CV15.

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150

CCCC is defined in the following table:

155

CCCC
0000

Meaning
Disable 111 Instructions1

Action if D = 1
Instruction is ignored for all sub addresses.

0100

Disable Decoder
Acknowledgement
Request Instruction

0101

Activate Bi-Directional
Communications

Acknowledgement and Address are nottransmitted in response to a Decoder
Acknowledgement Instruction for all sub
addresses.
B-Directional Communications are enabled
per CVs (See note below).

1000

Set Bi-Directional
Communications

1001

Set 111 Instruction

1111

Accept 111 Instructions

Bi-Directional communications are enabled
for specified sub address; all other sub
addresses are disabled. (Not valid at Consist
Address).
Enables 111 Instructions for specified subaddress, all other sub addresses are disabled.
(Not valid at Consist Address).
All multi-CV programming instructions are
now valid.

Scope
Until next Digital Decoder
Reset Packet is received.
Until power is removed longenough to power down the
decoder.
Permanent (sets CV 29 (or541), bit 3) or internal Flag
if sent to Consist Address.
Permanent (sets CV 3216,
bit 0)

Permanent (sets CV 3216,
bit 1)
One-Time

Note: This command is valid at both the decoder’s base address and (if active) the consist address. If sent tothe base address, the command affects both the base address and the active consist address (if any). If
sent to the consist address, and D=0 this command has no effect on the base address. If sent to theconsist address, and D=1 this command has no effect.

Consist Control (0001)
160

This instruction controls consist setup and activation or deactivation.
When Consist Control is in effect, the decoder will ignore any speed or direction instructions addressed to its
normal locomotive address (unless this address is the same as its consist address). Speed and direction instructions
now apply to the consist address only.

165

Functions controlled by instruction 100 and 101 will continue to respond to the decoders baseline address.
Functions controlled by instructions 100 and 101 also respond to the consist address if the appropriate bits in CVs#21,22 have been activated.
170

175

By default all forms of Bi-directional communication are not activated in response to commands sent to the consist
address until specifically activated by a Decoder Control instruction. Operations mode acknowledgement and Data
Transmission applies to the appropriate commands at the respective decoder addresses.
The format of this instruction is:{instruction bytes} = 0001CCCC 0 0AAAAAAA
A value of “1” in bit 7 of the second byte is reserved for future use. Within this instruction CCCC contains a consist
setup instruction, and the AAAAAAA in the second byte is a seven bit consist address. If the address is "0000000"
then the consist is deactivated. If the address is non-zero, then the consist is activated.

180

If the consist is deactivated (by setting the consist to ‘0000000’), the Bi-Directional communications settings are set
as specified in CVs 26-28.
When operations-mode acknowledgement is enabled, all consist commands must be acknowledged via operationsmode acknowledgement.
1 This instruction also applies to accessory decoders.

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185

The format for CCCC shall be:
CCCC=0010

Set the consist address as specified in the second byte, and activate the consist. The consist
address is stored in bits 0-6 of CV #19 and bit 7 of CV #19 is set to a value of 0. The directionof this unit in the consist is the normal direction. If the consist address is 0000000 the consist
is deactivated.

CCCC=0011

Set the consist address as specified in the second byte and activate the consist. The consist
address is stored in bits 0-6 of CV #19 and bit 7 of CV#19 is set to a value of 1. The directionof this unit in the consist is opposite its normal direction. If the consist address is 0000000 theconsist is deactivated.

190

195

All other values of CCCC are reserved for future use.

Advanced Operations Instruction (001)200

These instructions control advanced decoder functions. Only a single advanced operations instruction may becontained in a packet.
The format of this instruction is 001CCCCC 0 DDDDDDDD
The 5-bit sub-instruction CCCCC allows for 32 separate Advanced Operations Sub-Instructions.

205

210

215

CCCCC = 11111: 128 Speed Step Control - Instruction "11111" is used to send one of 126 Digital Decoder speed
steps. The subsequent single byte shall define speed and direction with bit 7 being direction ("1" is forward and "0"is reverse) and the remaining bits used to indicate speed. The most significant speed bit is bit 6. A data-byte valueof U0000000 is used for stop, and a data-byte value of U0000001 is used for emergency stop. This allows up to 126
speed steps. When operations mode acknowledgment is enabled, receipt of a 128 Speed Step Control packet mustbe acknowledged with an operations mode acknowledgement.
CCCCC = 11110: Restricted Speed Step Instruction - Instruction “11110” is used to restrict the maximum speed ofa decoder. Bit 7 of the data byte (‘DDDDDDDD’ above) is used to enable (‘0’) or disable (‘1’) restricted speed
operation. Bits 0-5 of the Data byte are the Speed Steps defined in S-9.22. When operations mode acknowledgment
is enabled, receipt of a Restricted Speed Instruction must be acknowledged with an operations mode
acknowledgement.
CCCCC = 11101: Analog Function Group220

The format of this instruction is 001CCCCC 0 VVVVVVVV 0 DDDDDDDD where
VVVVVVVV - Analog Function Output (0-255)
DDDDDDDD - Analog Function Data (0-255)
225

Analog Output Use:
00000001 Volume Control
230

All other values of VVVVVVVV are reserved. This function must not be used to control the speed of amobile decoder.
When operations mode acknowledgment is enabled, receipt of a Analog Function Group Instruction mustbe acknowledged with an operations mode acknowledgement.

235

The remaining 29 instructions are reserved for future use.

2 In 128 speed step mode, the maximum restricted speed is scaled from 28 speed mode.

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Speed and Direction Instructions (010 and 011)
240

245

250

255

These two instructions have these formats:for Reverse Operation 010DDDDD
for Forward Operation 011DDDDD
A speed and direction instruction is used send information to motors connected to Multi Function Digital Decoders.
Instruction "010" indicates a Speed and Direction Instruction for reverse operation and instruction "011" indicates a
Speed and Direction Instruction for forward operation. In these instructions the data is used to control speed withbits 0-3 being defined exactly as in S-9.2 Section B. If Bit 1 of CV#29 has a value of one (1), then bit 4 is used as
an intermediate speed step, as defined in S-9.2, Section B. If Bit 1 of CV#29 has a value of zero (0), then bit 4 shallbe used to control FL3. In this mode, Speed U0000 is stop, speed U0001 is emergency stop, speed U0010 is thefirst speed step and speed U1111 is full speed. This provides 14 discrete speed steps in each direction.
If a decoder receives a new speed step that is within one step of current speed step, the Digital Decoder may select a
step half way between these two speed steps. This provides the potential to control 56 speed steps should thecommand station alternate speed packets.
Decoders may ignore the direction information transmitted in a broadcast packet for Speed and Direction commands
that do not contain stop or emergency stop information.
When operations mode acknowledgment is enabled, receipt of any speed and direction packet must be
acknowledged with an operations mode acknowledgement.

Function Group One Instruction (100)4
The format of this instruction is 100DDDDD
260

265

Up to 5 auxiliary functions (functions FL and F1-F4) can be controlled by the Function Group One instruction. Bits
0-3 shall define the value of functions F1-F4 with function F1 being controlled by bit 0 and function F4 being
controlled by bit 3. A value of "1" shall indicate that the function is "on" while a value of "0" shall indicate that thefunction is "off". If Bit 1 of CV#29 has a value of one (1), then bit 4 controls function FL, otherwise bit 4 has no
meaning.
When operations mode acknowledgment is enabled, receipt of a function group 1 packet must be acknowledged
according with an operations mode acknowledgement.

270

Function Group Two Instruction (101)5This instruction has the format 101SDDDD

275

280

Up to 8 additional auxiliary functions (F5-F12) can be controlled by a Function Group Two instruction. Bit 4
defines the use of Bits 0-3. When bit 4 (S) is ‘1’, Bits 0-3 (DDDD) shall define the value of functions F5-F8 withfunction F5 being controlled by bit 0 and function F8 being controlled by bit 3. When bit 4 (S) is ‘0’, Bits 0-3(DDDD) shall define the value of functions F9-F12 with function F9 being controlled by bit 0 and function F12
being controlled by bit 3. A value of "1" shall indicate that the function is "on" while a value of "0" shall indicatethat the function is "off".
When operations mode acknowledgment is enabled, receipt of function group 2 packet shall be acknowledged
according with an operations mode acknowledgement.

Feature Expansion Instruction (110)The instructions in this group provide for support of additional features within decoder. (See TN-3-05)
285

3 FL is used for the control of the headlights.
4 Any function in this packet group may be directionally qualified.
5 Any function in this packet group may be directionally qualified.

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The format of two byte instructions in this group is: 110CCCCC 0 DDDDDDDD
The format of three byte instructions in this group is: 110CCCCC 0 DDDDDDDD 0 DDDDDDDD
The 5-bit sub-instruction CCCCC allows for 32 separate Feature Expansion Sub-instructions.
290

295

300

305

310

CCCCC = 00000: Binary State Control Instruction long form – Sub instruction "00000" is a three byte instruction
and provides for control of one of 32767 binary states within the decoder. The two bytes following this instruction
byte have the format DLLLLLLL 0 HHHHHHHH". Bits 0-6 of the first data byte (LLLLLLL) shall define thelow order bits of the binary state address; bits 0-7 of the second data byte (HHHHHHHH) shall define the highorder bits of binary state address. The addresses range from 1 to 32767. Bit 7 of the second byte (D) defines thebinary state. A value of "1" shall indicate that the binary state is "on" while a value of "0" shall indicate that thebinary state is "off". The value of 0 for the address is reserved as broadcast to clear or set all 32767 binary states.
An instruction "11000000 0 00000000 0 00000000" sets all 32767 binary states to off.
Binary states accessed with all high address bits set to zero would be the same as accessed by the short form of thebinary state control. Command stations shall use the short form in this case, i.e. Binary State Controls 1 to 127
should always be addressed using the short form. Decoders supporting the long form shall support the short form as
well.
CCCCC = 11101: Binary State Control Instruction short form – Sub-instruction “11101” is a two byte instruction
and provides for control of one of 127 binary states within the decoder. The single byte following this instruction
byte has the format DLLLLLLL. Bits 0-6 of the second byte (LLLLLLL) shall define the number of the binary
state starting with 1 and ending with 127. Bit 7 (D) defines the binary state. A value of "1" shall indicate the binary
state is "on" while a value of "0" shall indicate the binary state is "off". The value of 0 for LLLLLLL is reserved as
broadcast to clear or set all 127 binary states accessible by the short form of the binary state control. An instruction"11011101 0 00000000" sets all 127 binary states accessed by this instruction to off.
Binary State Controls are quite similar to Functions, as they may control any output, sound or any other feature ofdigital nature within a decoder in direct response to a packet received. But Binary State Controls do have a different
access method and function space. Therefore they have a different name.

315

Binary state control packets – both short and long form – will not be refreshed. Therefore non-volatile storage ofthe function status is suggested. When operations mode acknowledgment is enabled, receipt of a Binary State
Control packet shall be acknowledged accordingly with an operations mode acknowledgment. Consult theTechnical Note(s) for additional information on this instruction. (See TN-4-05)
320

325

330

335

340

CCCCC = 11110: F13-F20 Function Control – Sub-instruction “11110” is a two byte instruction and provides forcontrol of eight (8) additional auxiliary functions F13-F20. The single byte following this instruction byte indicates
whether a given function is turned on or off, with the least significant bit (Bit 0) controlling F13, and the mostsignificant bit (bit 7) controlling F20. A value of “1” in D for a given function shall indicate the function is “on”
while a value of “0” in D for a given function shall indicate a given function is “off”. It is recommended, but notrequired, that the status of these functions be saved in decoder storage such as NVRAM. It is not required, andshould not be assumed that the state of these functions is constantly refreshed by the command station. Command
Stations that generate these packets, and which are not periodically refreshing these functions, must send at least
two repetitions of these commands when any function state is changed. When operations mode acknowledgment isenabled, receipt of an F13-F20 Function Control packet shall be acknowledged accordingly with an operations
mode acknowledgement. Consult the Technical Note(s), TN-4-05, for additional information on this instruction.
CCCCC = 11111: F21-F28 Function Control – Sub-instruction “11111” is a two byte instruction and provides forcontrol of eight (8) additional auxiliary functions F21-F28. The single byte following this instruction byte indicates
whether a given function is turned on or off, with the least significant bit (Bit 0) controlling F21, and the mostsignificant bit (bit 7) controlling F28. A value of “1” in D for a given function shall indicate the function is “on”
while a value of “0” in D for a given function shall indicate a given function is “off”. It is recommended, but notrequired that the status of these functions be saved in decoder storage such as NVRAM. It is not required, andshould not be assumed that the state of these functions is constantly refreshed by the command station. Command
Stations that generate these packets, and which are not periodically refreshing these functions, must send at least
two repetitions of these commands when any function state is changed. When operations mode acknowledgment isenabled, receipt of an F21-F28 Function Control packet shall be acknowledged accordingly with an operations
mode acknowledgement. Consult the Technical Note(s), TN-4-05, for additional information on this instruction.
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345

The remaining 28 sub-instructions are reserved by the NMRA for future use.6

Configuration Variable Access Instruction (111)350

The Configuration Variable Access instructions are intended to set up or modify Configurations Variables either on
the programming track or on the main line. There are two forms of this instruction. The short form is for modifying
selected frequently modified Configuration Variables. The long form is for verifying or modifying any selected
Configuration Variable. Only a single configuration variable access instruction may be contained in a packet.

Configuration Variable Access Acknowledgment
If a configuration variable access acknowledgment is required, and the decoder has decoder operations-mode
acknowledgment enabled, the decoder shall respond with an operations mode acknowledgment.

Configuration Variable Access Instruction - Short Form
355

This instruction has the format of
1111CCCC 0 DDDDDDDD

360

The 8 bit data DDDDDDDD is placed in the configuration variable identified by CCCC according to the following
table.

365

CCCC = 0000 - Not available for use
CCCC = 0010 - Acceleration Value (CV#23)
CCCC = 0011 - Deceleration Value (CV#24)
CCCC = 1001 – See S-9.2.3, Appendix B
The remaining values of CCCC are reserved and will be selected by the NMRA as need is determined.

370

Only a single packet is necessary to change a configuration variable using this instruction. If the decoder
successfully receives this packet, it shall respond with an operations mode acknowledgment.

Configuration Variable Access Instruction - Long Form
The long form allows the direct manipulation of all CVs7. This instruction is valid both when the Digital Decoder
has its long address active and short address active. Digital Decoders shall not act on this instruction if sent to its
consist address. The format of the instructions using Direct CV addressing is:
375

1110CCVV 0 VVVVVVVV 0 DDDDDDDD

380

385

390

The actual Configuration Variable desired is selected via the 10-bit address with the 2-bit address (VV) in the firstdata byte being the most significant bits of the address. The Configuration variable being addressed is the provided
10-bit address plus 1. For example, to address CV#1 the 10 bit address is “00 00000000”.
The defined values for Instruction type (CC) are:
CC=00 Reserved for future use
CC=01 Verify byte
CC=11 Write byte
CC=10 Bit manipulation
Type = "01" VERIFY BYTE
The contents of the Configuration Variable as indicated by the 10-bit address are compared with the data byte
(DDDDDDDD). If the decoder successfully receives this packet and the values are identical, the Digital Decoder
shall respond with the contents of the CV as the Decoder Response Transmission, if enabled.
6 The NMRA shall not issue a NMRA Conformance Warrant for any product that uses an instruction or sub-instruction that has been reserved by the NMRA.
7 Because of the length of this instruction, care must be taken to ensure that the maximum time between packets is

not exceeded.
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395

400

Type = "11" WRITE BYTE
The contents of the Configuration Variable as indicated by the 10-bit address are replaced by the data byte
(DDDDDDDD). Two identical packets are needed before the decoder shall modify a configuration variable8.
These two packets need not be back to back on the track. However any other packet to the same decoder willinvalidate the write operation. (This includes broadcast packets.) If the decoder successfully receives this second
identical packet, it shall respond with a configuration variable access acknowledgment.
Type = "10" BIT MANIPULATIONThe bit manipulation instructions use a special format for the data byte (DDDDDDDD):
111CDBBB

405

Where BBB represents the bit position within the CV, D contains the value of the bit to be verified or written, and C
describes whether the operation is a verify bit or a write bit operation.
C = "1" WRITE BIT
C = "0" VERIFY BIT

410

415

The VERIFY BIT and WRITE BIT instructions operate in a manner similar to the VERIFY BYTE and WRITEBYTE instructions (but operates on a single bit). Using the same criteria as the VERIFY BYTE instruction, an
operations mode acknowledgment will be generated in response to a VERIFY BIT instruction if appropriate. Using
the same criteria as the WRITE BYTE instruction, a configuration variable access acknowledgment will be
generated in response to the second identical WRITE BIT instruction if appropriate.

D: Accessory Digital Decoder Packet Formats
Accessory Digital Decoders are intended for control of a number of simple functions such as switch machine
control or turning on and off lights. It is permissible to develop Digital Decoders that respond to multiple addresses
so that more devices can be controlled by a single Digital Decoder.
420

Basic Accessory Decoder Packet Format
The format for packets intended for Accessory Digital Decoders is:{preamble} 0 10AAAAAA 0 1AAACDDD 0 EEEEEEEE 1

425

430

435

Accessory Digital Decoders can be designed to control momentary or constant-on devices, the duration of time each
output is active being controlled by configuration variables CVs #515 through 518. Bit 3 of the second byte "C" isused to activate or deactivate the addressed device. (Note if the duration the device is intended to be on is less thanor equal the set duration, no deactivation is necessary.) Since most devices are paired, the convention is that bit "0"of the second byte is used to distinguish between which of a pair of outputs the accessory decoder is activating ordeactivating. Bits 1 and 2 of byte two are used to indicate which of 4 pairs of outputs the packet is controlling. The
most significant bits of the 9-bit address are bits 4-6 of the second data byte. By convention these bits (bits 4-6 ofthe second data byte) are in ones complement.
If operations-mode acknowledgement is enabled, receipt of a basic accessory decoder packet must be acknowledged
with an operations-mode acknowledgement.

Extended Accessory Decoder Control Packet Format
The Extended Accessory Decoder Control Packet is included for the purpose of transmitting aspect control to signal
decoders or data bytes to more complex accessory decoders. Each signal head can display one aspect at a time.
{preamble} 0 10AAAAAA 0 0AAA0AA1 0 000XXXXX 0 EEEEEEEE 1
440

8 Note that CV 17 and CV 18 are a “paired CV”. A “paired CV” refers to a pair of CVs which taken together holdone piece of data. A WRITE BYTE instruction to CV17 will take effect only when CV18 is written. Other paired
CVs will work in a similar manner. See S-9.2.2 for more information on paired CVs.
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XXXXX is for a single head. A value of 00000 for XXXXX indicates the absolute stop aspect. All other aspects
represented by the values for XXXXX are determined by the signaling system used and the prototype being
modeled.
445

If operations-mode acknowledgement is enabled, receipt of an extended accessory decoder packet must be
acknowledged with an operations-mode acknowledgement.

Broadcast Command for Accessory Decoders
Broadcast Command for Basic Accessory Decoders
The format for the broadcast instruction is:
450{preamble} 0 10111111 0 1000CDDD 0 EEEEEEEE 1
This packet shall be executed by all accessory decoders. CDDD is defined as specified in the paragraphon Basic Accessory Decoder Packet Format.
455

Broadcast Command for Extended Accessory Decoders
The format for the broadcast instruction is:{preamble} 0 10111111 0 00000111 0 000XXXXX 0 EEEEEEEE 1
460

All extended accessory decoders must execute this packet. XXXXX is defined as specified in theparagraph on Extended Accessory Decoder Packet Format.

465

470

Accessory Decoder Configuration Variable Access Instruction
Accessory decoders can have their Configuration variables changed in the same method as locomotive decoders
using the Configuration Variable Access Instruction - Long Form instruction defined above. For the purpose ofthis instruction, the accessory decoders’ address is expanded to two bytes in the following method. If operationsmode acknowledgement is enabled, the receipt of an Accessory Decoder Configuration Variable Access instruction
must be acknowledged in the same manner as the Configuration Variable Access Instruction – Long Form.
Basic Accessory Decoder Packet address for operations mode programming

475

10AAAAAA 0 1AAACDDD
Where DDD is used to indicate the output whose CVs are being modified and C=1.
If CDDD= 0000 then the CVs refer to the entire decoder. The resulting packet would be480{preamble} 10AAAAAA 0 1AAACDDD 0 (1110CCVV 0 VVVVVVVV 0 DDDDDDDD) 0 EEEEEEEE 1
Accessory Decoder Address (Configuration Variable Access Instruction)
Error Byte

485

Extended Decoder Control Packet address for operations mode programming
10AAAAAA 0 0AAA0AA1

490

Please note that the use of 0 in bit 3 of byte 2 is to ensure that this packet cannot be confused with the legacy
accessory-programming packets. The resulting packet would be:{preamble} 10AAAAAA 0 0AAA0AA1 0 (1110CCVV 0 VVVVVVVV 0 DDDDDDDD) 0 EEEEEEEE 1
Signal Decoder Address (Configuration Variable Access Instruction)
Error Byte

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495

E: Operations Mode Acknowledgment
The operations-mode acknowledgment mechanism as defined in S-9.3.1 and S-9.3.2 are the only valid
acknowledgement in operations mode. Whenever an acknowledgment is requested, the decoder shall respond using
this mechanism described in S-9.3.1 and S-9.3.2.

500

Copyright -2006 - 2012, by National Model Railroad Association, Inc.

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Appendix A.
This Appendix contains additional useful information and/or legacy instructions. A DCC product need not
implement any items described in this appendix.

Accessory Decoder Configuration Variable Access Instruction9505

The following command is included for backward compatibility for some older accessory decoders. Its use is
discouraged in new decoder designs.
The format for Accessory Decoder Configuration Variable Access Instructions is:510{preamble} 0 10AAAAAA 0 0AAA11VV 0 VVVVVVVV 0 DDDDDDDD 0 EEEEEEEE 1
Where:515

520

A = Decoder address bits
V = Desired CV address - (CV 513 = 10 00000000)
D = Data for CV
The bit patterns described by VV VVVVVVVV in the second and third bytes and DDDDDDDD in the fourth byte
are also identical to the corresponding bits in the Configuration Variable Access Instruction - Long Form (see S9.2.1).
The purpose of this instruction is to provide a means of programming all parameters of an accessory decoder after it
is installed on the layout. It is recommended that Command Stations exercise caution if changes to the address (CV513 and CV 521) are allowed.

9 For backward compatibility, decoders should check the length of instruction packets when bit 7 of byte 2 is zero.

Copyright -2006 - 2012, by National Model Railroad Association, Inc.

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