CS 452/652 Winter 2026 - Lecture 3

CAN bus and Märklin protocol

Jan 13, 2026 prev next

MCP - MCP2515 Data Sheet
MRK - Märklin CAN Protocol

Controller Area Network (CAN)

• network protocol and standard
  • physical specifications (Layer 1)
    • low bitrate → robust against electrical noise
    • 10s kbit/s to 10s Mbit/s range, compare 100s Gbit/s in modern Ethernet
    • bit-synchronous encoding and transmission
  • medium access control and addressing (Layer 2)
    • multi-leader serial bus: each participant autonomously decides to send
    • collision detection and arbitration needed (cf. original Ethernet)
    • message identifier instead of source/destination addressing
    • priority encoding ensures realtime capabilities
    • 0-bit is dominant → lower identity number wins arbitration
• message types
  • data frame: regular data frame
  • remote frame: request data (with priority of requested data)
  • error frame: transmitted by any node detecting an error
  • overload frame: pacing, tell sender to slow down
• message format
  • identifier (4 bytes), data length (1 byte), payload (up to 8 bytes)
  • base/standard vs extended header format
    • 11 bits vs. 29 bits identifier
    • Märklin uses extended format
  • wire format (vs. device format) - see MCP Page 10
    • bit count - ID: 11/29, Meta: 6/8, DLC: 4, CRC: 16, EOF: 7, IFS: 3
    • but stuffing done by hardware: every 6th bit must flip
      • due to non-return-to-zero (NRZ) encoding
    • 64 bits header/etc + 64 bits data + 3 bits IFS + stuffing = 160 bits
    • 250 kbit/s → approx. 1562 frames/sec; 640 usec/frame
• network byte order: always big endian
  • use __htonl, etc. (man 3 htonl)

MCP 2515 controller

• configuration and control setup: see iotest
  • no RX filtering
  • no internal priorities
• 3 TX message buffers: TXB0/1/2
  • control register: TXBnCTRL
    • bit 3: TXREQ
    • check for 0 before read, set to 1 after write
    • priority & other conditions
  • message registers (BCM Pages 20-22)
    • SIDH: top 8 bits standard identifier
    • SIDL: low 3 bits standard identifier, EXIDE, bits 17/16 of extended identifier
    • EID8, EID0: extended identifier
    • DLC: 4 bits data length field
    • Dm: 8 registers, one data byte each
  • SPI interface (MCP Section 12.5): sequential writes possible
  • using TXB0 should be sufficient
• 2 RX message buffers: RXB0/1 (plus internal assembly buffer)
  • control register: RXBnCTRL
    • configure rollover, not used for filtering
  • (interrupt) flag register: CANINTF
    • bits 0,1: RX data available in respective buffer
    • check for 1 before read, clear after read
    • other conditions
  • message registers (BCM Pages 30-32):
    • SIDH: top 8 bits standard identifier
    • SIDL: low 3 bits standard identifier, EXIDE, bits 17/16 of extended identifier
    • EID8, EID0: extended identifier
    • DLC: 4 bits data length field
    • Dm: 8 registers, one data byte each
  • SPI interface (MCP Section 12.3): sequential reads possible
  • use RXB0, and RXB1 for overflow
• controller status instruction (MCP Section 12.8 and Figure 12-8)
  • obtain RX0IF, RX1IF, TX0REQ, TX1REQ, TX2REQ

Märklin CAN protocol

• message format (MRK Page 5)
  • extended identifier (29 bits)
    • priority (4 bits), can be 0
    • command (8 bits)
    • response indicator (1 bit)
    • hash value (16 bits): collision resolution, can use 0xC300
    • CAN bus priority not really used/needed...
  • dlc (4 bits): as needed
  • data (0-8 bytes): depending on command
• CS3 responses
  • same command, dlc, data
  • response bit set
  • different hash
  • simple pacing: wait for response before sending next command
• speed (command 0x04, MRK Page 30)
  • dlc = 6
  • data 0-3: train number
  • data 4-5: speed level 0..1000
    • CS3 ↔ locomotive: 14 steps? (see Page 10)
    • level = 1 + (step - 1) * 77
• direction (command 0x05, MRK Page 31)
  • dlc = 5
  • data 0-3: train number
  • data 4: 1 forward, 2 backward, 3 reverse
• trains slow down gradually
  • do not directly reverse a rolling train
• light (command 0x06, MRK Page 32)
  • dlc = 6
  • data 0-3: train number
  • data 4: function 0 (light)
  • data 5: 0 off, 1 on
• switch (command 0x0B, MRK Page 37)
  • dlc = 6
  • data 0-3: 0x3000 + switch number
  • data 4: 0 curved, 1 straight
  • data 5: 1 (engage)
  • response with data 5 set to 1: response/confirm
  • response with data 5 set to 0: solenoid off
  • middle "double" switches
    • avoid C/C state (and S/S is not very helpful either)
    • if curved left (CS), and want curved right (SC)
      • first set C switch to S (SS)
      • then set other switch to C (SC)
• sensors (command 0x11, MRK Page 40)
  • 5 banks of 16 up to 16 sensors
    • banks labeled as A, B, C, D, E on track
    • individual sensors labeled A13, A14
    • sensors are directional: A15 →, A16 ←
  • asynchronous incoming message
  • response bit set
  • dlc = 8
  • data 0,1: ignore (unused identification)
  • data 2,3: (bank - 'A') * 16 + (number - 1) + 1
  • data 4,5: old, new state (0 free, 1 occupied)
  • data 6,7: ignore (time stamp)
• control and configuration
  • command 0x00
  • data 0-3: 0 (broadcast)
  • subcmd in data 4
    • 0x01: go (same as button)
    • 0x02: halt - set speed 0 to all
    • 0x00: stop (same as button)
• other incoming messages
  • command 0x18 and response bit: ping
  • command 0x00, subcmd 0x00, no response: stop via button
  • command 0x00, subcmd 0x01, no response: go via button

Notes

• turn on the headlights! (verify basic communication and train direction)
• initialize track & turnouts to well-defined state
  • captive loop, lights on, speed zero, forward direction
• can use wireshark to observe traffic on CAN bus