1. A network for controling model railroads
LocoNet
A network for controling
model railroads
by Stefan Bormann

2. Contents LocoNet? System overview. Physical layer Packet layer
Contents
LocoNet? System overview.
Physical layer
Packet layer
Application layer – an overview
Loco operation
Example: FRED
References.

3. 1a) System overview
DCC
C:>
LocoNet .
Command
Station

4. 1b) What is LocoNet? invented by Digitrax proprietory Peer to peer
CSMA/CD
~3 Layers.

5. 2a) Physical layer - electrical
master
any device
+12V
15mA 3V . RX TX
Net
Ground

6. 2b) Physikal layer - timing
16600 Baud, 60µs bit time
8N1
600µs .
60µs 1
Start D0 D1 D2 D3 D4 D5 D6 D7
Stop 1

7. 3a) Packet layer bytes are transmitted back-to-back
Opcode
Data
Check
...
bytes are transmitted back-to-back
packets consist of 2, 4, 6 or N bytes
first byte is „opcode“
last byte is checksum (xor of all other bytes).

8. 3b) Opcode D7 D6 D5 D4 D3 D2 D1 D0 5 bits to encode 32
possible opcodes.
always 1 (all other bytes have D7=0)
6 bytes 1
4 bytes
N bytes
2 bytes
packet length
(including opcode and checksum)

9. 3c) Packet timing, net accessCD 20
bit times MD 6 PD
0..20 .
CD: carrier detect (defines end of packet)
MD: master delay (only master may transmit)
PD: priority delay (high prio. = small PD)

10. 3d) Packet timing, collisions
Collisions are avoided, by shortening the time between still observing the net is idle until the first bit is transmitted onto the net.
Collision is detected, when a 0 is seen on the net, while a 1 is being transmitted
On detection of a collision, a break (15 bit times 0) is transmitted.

11. 4. Application layer -- an overview
turnouts
signals
message exchange
decoder programming
fast clock
loco operation
station local
not used
control locally
well, interesting...
YES
Only limited mission for layout-wide LocoNet.

12. 5a) Loco operation Table with loco data in command station
Slot # = Index in table = Handle
Slot # is a 7 bit number
128 Slots:
are loco slots
0 dispatch slot
>120 are special slots (fast clock, programming)
loco slots have fields:

13. 5b) Fields of a slot loco address (14 bits)
state of slot (free/common/idle/IN_USE)
protocol selection (# of speed steps, ...)
state of loco
speed (7 bit)
functions (9 bits, F0, F1, ..., F8)
consist tree
ID (14 bits, can be a device ID of controller)

14. 5c) Operations on slots (requests)
requests to master:
OPC_LOCO_ADR: get slot by address
OPC_RQ_SL_DATA: get slot by slot #
OPC_WR_SL_DATA: write slot by slot #
OPC_MOVE_SLOTS: move slot from # to #
dispatch put (destination=0)
dispatch get (source=0)
NULL move (source=destination!=0)
OPC_SLOT_STAT1: modify slot status

15. 5d) Operations on slots (responses, operation)
responses from master:
OPC_LONG_ACK: simple acknowledge
OPC_SL_RD_DATA: slot data
realtime operation
OPC_LOCO_SPD: 7 bits of speed
OPC_LOCO_DIRF: direction and F0, ..., F4
OPC_LOCO_SND: F5, ..., F8

16. 6a) FRED FRemos Einfacher Drehregler Simple to use
Can only control one engine
speed,
direction,
F0, F1, F2.

17. 6b) FRED: Example for protocol (acquire loco)
master
FRED
master
FRED
Dispatch Get
Dispatch Get
Slot Read
NACK.
Write Slot
ACK

18. 6c) FRED: Example of protocol (reconnect)
master
FRED
master
FRED
if slot data changed
Get Slot
By Address
Null Move
Slot Read
Slot Read
Write Slot
ACK.

19. 7. References This clinic: Digitrax homepage:
stefan.bormann/dcc/dcc_e.html
Digitrax homepage:
LocoNet-Hackers mailing list: