This standard covers the format of the information sent via Digital Command Stations to Digital Decoders. A Digital Command Station transmits this information to Digital Decoders by sending a series of bits using the NMRA digital signal described in S-9.1. This sequence of bits, termed a packet, is used to encode one of a set of instructions that the Digital Decoder operates upon. Packets must be precisely defined to ensure that the intended instructions can be properly encoded and decoded.
General Packet Format
The following sequence of bits defines a valid NMRA packet. Any sequence of bits not meeting the full specifications of this general packet format is not, for the purpose of this standard, a "packet". Digital decoders should not act on any instructions that are not contained within a valid packet while in the NMRA digital mode of operation1. Note, portions within square [ ] brackets must occur one or more times.
Preamble: The preamble to a packet consists of a sequence of "1" bits. A digital decoder must not accept as a valid, any preamble that has less than 10 complete one bits, or require for proper reception of a packet with more than 12 complete one bits. A command station must send a minimum of 14 full preamble bits.
Packet Start Bit: The packet start bit is the first bit with a value of "0" that follows a valid preamble. The Packet Start Bit terminates the preamble and indicates that the next bits are an address data byte.
Address Data Byte: The first data byte of the packet normally contains eight bits of address information 2. The first transmitted address bit shall be defined to be the most significant bit of the address data byte. Address Data Bytes with values 00000000, 11111110, and 11111111 are reserved for special operations and must not be transmitted except as provided in this Standard or associated Recommended Practices.
Data Byte Start Bit: This bit precedes a data byte and has the value of "0".
Data Byte: Each data byte contains eight bits of information used for address, instruction, data, or error detection purposes. The first transmitted data bit of each data byte shall be defined to be the most significant bit of the data byte. ]
Packet End Bit: This bit marks the termination of the packet and has a value of "1"3.