Tunnel 8 bit bytes over LocoNet: Generic 8

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About

A lot of people are complaining about the fact, that LocoNet message payloads may not have the 8th bit set. This message is a generic container to carry any sequence of bytes over LocoNet, as long as it fits into the maximum size LocoNet packet.

Status: Proposal

Message format

The message uses a not defined opcode. This is not communicated to Digitrax, so if you use it, you will not be guarantied interoperabillity with any commercial devices. The opcode is likely to change in the future.

Byte Index 0 1 2 3 4 5 6 7 8 9 10 11 12 LEN-1
Mnemonic 0xFF LEN PXCT₀ D₀0 D₀1 D₀2 D₀3 D₀4 D₀5 D₀6 PXCT₁ D₁0 D₁1 ... CHK

Byte Index	0	1	2	3	4	5	6	7	8	9	10	11	12		LEN-1
Mnemonic	0xFF	LEN	PXCT₀	D₀0	D₀1	D₀2	D₀3	D₀4	D₀5	D₀6	PXCT₁	D₁0	D₁1	...	CHK

Usage of data fields

Mnemonic Usage
LEN This is the length byte of the LocoNet packet. The value may be in the range [5..127]. A packet of length 5 carries just a single byte of payload; a packet of length 127 is the biggest that LocoNet can handle and it carries 108 bytes of payload.
PXCT_N

Most significant bits of D_N[0..6].

Bit index Value
0 MSB of D_N0
1 MSB of D_N1
2 MSB of D_N2
3 MSB of D_N3
4 MSB of D_N4
5 MSB of D_N5
6 MSB of D_N6
7 0 (required by LocoNet)

The LocoNet message carries a sequence of 8 byte groups: each group consists of one byte carrying the MSBs of 7 payload bytes followed by 7 payload bytes with stripped MSB. The last group does not have to be complete, it can contain 1..7 bytes of payload.
The naming of the mnemonic PXCT is derived from similar fields in the Digitrax specification.

D_NB 7 lower significant bits of the N*7+Bth byte of the payload

Examples

Smallest possible message with 1 byte of payload: 0x99

Byte Index 0 1 2 3 4
Field Opcode Length PXCT₀ D₀0 XOR-Check
Value 0xFF 0x05 0x01 0x19 0x1D
Payload 0x99

Byte Index	0	1	2	3	4
Field	Opcode	Length	PXCT₀	D₀0	XOR-Check
Value	0xFF	0x05	0x01	0x19	0x1D
Payload				0x99

Biggest possible message with 108 bytes of payload with incremental data [1..108]:

Byte Index 0 1 2 3 4 5 6 ... 121 122 123 124 125 126 127
Field Opcode Length PXCT₀ D₀0 D₀1 D₀2 D₀3 ... D₁₄6 D₁₄7 PXCT₁₅ D₁₅0 D₁₅1 D₁₅2 XOR-Check
Value 0xFF 0x7F 0x00 0x01 0x02 0x03 0x04 ... 0x68 0x69 0x00 0x6A 0x6B 0x6C CHK
Payload Index 1 2 3 4 ... 104 105 106 107 108

Byte Index	0	1	2	3	4	5	6	...	121	122	123	124	125	126	127
Field	Opcode	Length	PXCT₀	D₀0	D₀1	D₀2	D₀3	...	D₁₄6	D₁₄7	PXCT₁₅	D₁₅0	D₁₅1	D₁₅2	XOR-Check
Value	0xFF	0x7F	0x00	0x01	0x02	0x03	0x04	...	0x68	0x69	0x00	0x6A	0x6B	0x6C	CHK
Payload Index				1	2	3	4	...	104	105		106	107	108

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Author of this page: Stefan Bormann.