HBus
Home automation interface based on Arduino, UARTs and CAN transceivers.
Introduction
HBus is a wired home automation interface. Nodes made as simple and as cheap as possible. Minimum requirement - Arduino Pro Mini modules (Atmega 328P) with 78L05 regulator and a CAN transceiver. Alternatevily, dedicated HBbus modules (AVR64DD32) can be used, HBnode Pro Mini or HBnode Nano. HBnode modules have more memory, they offer 12-bit ADC and they can update their firmware via HBus.
Once configured, HBus system is self-contained, eg it can operate without any host or server, like KNX, C-Bus or Velbus.
Basics
- Baud rate 19.2 kbps.
- Communication by ordinary UARTs with CAN transceivers, dominant/recessive states. CAN protocol not used, CAN controllers not required.
- CSMA/CD (collision detection) allowed for cost sensitive implementations. CSMA/CA (collision avoidance) recommended, but is not mandatory. Common Arduino modules require an additional low-cost microcontroller for CSMA/CA. HBnode modules implement CSMA/CA without additional microcontrollers.
- Flow control by byte-stuffing.
- Two modes of operation:
- Configuration: point-to-point messages, master/slave communication model - “HBus mode”.
- Operation: broadcast messages, producer/consumer communication model - “MQTT-SN mode”.
- All messages covered by CRC-16-CCITT.
- Encryption is optional.
Wiring
Wiring made by Cat5/Cat6 cables. Twisted pairs used as follows:
- (orange + orange/white) = HBus signal:
- orange - HBH (HBus High)
- orange/white - HBL (HBus Low)
- (blue + blue/white) = common
- (brown + brown/white) = +Vsupply (12V…24V)
- (green + green/white) = not used.
Free topology is allowed. Total length of cables should not exceed 500 m. A single terminating resistor 100 Ohm should be placed somwhere in a middle of the network.
Cat5/Cat6 cables have limited current carrying capability. To prevent fire in case of short circuit, power supply +12V connected to the bus should have current limit or a slow-blow fuse. Cat5/Cat6 single core cable has wires 0.5 mm diameter or 24 AWG (0.511 mm diameter). Single AWG24 wire can carry 2…3.5 A, depending on allowed temerature rise. For a short time AWG24 can carry 5A or more, but cable insulation material will be degraded rapidly.
Thus, when 2 wires in parallel are used for power distribution, Cat5/Cat6 can carry up to 4..7 A of current. However, it would be a bit risky, because it is hard to quarantee that both cores are always connected. Safe and reliable applications should not distribute more than 3 A via Cat5/Cat6 cable, even when 2 or 3 cores connected in parallel.
More expensive EIB/KNX cable can be used too. With 0.8 mm wires it can safely distribute up to 10A of current.
CAN driver restrictions
Many CAN drivers has embedded mechanism to prevent long dominant state. If dominant state lasts longer than a pre-defined time-out, driver automatically goes into recessive state. Bit interval is 52.08 us at baud rate of 19.2 kbps. Start bit and a data byte of 0x00 produce dominant state for 9 bit-intervals, or for 0.47 ms. The following CAN transceivers are suitable for HBus:
- MCP2542 - 0.8 ms min
- ATA6560, ATA6561 - 0.8 ms min
- TJA1057 - 0.8 ms min
- NCV7342 - 1.3 ms min
- NCV7351 - 1.5 ms min
- TLE7250V - 4.5 ms min
- MCP2551 - 1.25 ms min, slope control
- ADM3054 - no limit, isolated
- IFX1050GVIO - no limit
- SN65HVD230 - no limit, slope control
- L9615 - no limit, slope control
- PCA82C250 - no limit, slope control
Transceivers with slope control produce less noise on the bus, ie have better EMC.