Hardware design recommendations
Points concerning hardware design are listed here. Though they are not mandatory to follow, they are recommended for consideration to ensure interoperability of UAVCAN-compatible devices from different vendors.
Physical connector definition
This standard defines the following connector types, though the specification can be extended later with other connectors. It is highly recommended to use only connectors specified below to ensure compatibility.
UAVCAN Micro connector
This connector incorporates CAN bus signals as well as stabilized, low voltage, low current power supply.
- Connector type: JST GH, 4 circuits
- Cable: twisted pair #30 to #26 AWG, insulation outer diameter 0.8 to 1.0 mm.
- CAN PHY: ISO 11898-2, also known as high-speed CAN
- Power supply: +5 VDC ±10%, up to 1 A per node
|1||pair 1 tip||+5 VDC|
|2||pair 2 ring||CAN H|
|3||pair 2 tip||CAN L|
|4||pair 1 ring||GND|
It is highly recommended to provide two identical connectors per each CAN interface per node, so that the node can be connected to the bus without need to use T-connectors. T-connectors should be avoided when possible because generally they add an extra point of failure and increase the stub length and weight.
Each power input must be protected with an overcurrent protection circuit (e.g. fuse), so that an occasional short circuit on the device will not bring down the power on the entire bus. If a device incorporates redundant bus interfaces, it must prevent direct current flow between power inputs from different interface connectors, so that if one bus suffers a power failure (e.g. short circuit) it won’t be propagated to other buses.
Nodes with different number of redundant interfaces
Mission critical nodes and non-mission critical nodes often may need to co-exist on the same UAVCAN network. Non-mission critical nodes are likely to be equipped with non-redundant CAN bus interface, which often creates a situation when multiple nodes with different number of redundant interfaces need to be connected to the same UAVCAN network.
If multiple nodes with different number of interfaces need to co-exist on the same UAVCAN network, the following rules should be followed:
- Each available CAN bus (UAVCAN supports up to 3) is assigned a level of importance (primary or backup).
- All nodes should be connected to the primary CAN bus.
- Only nodes with redundant interfaces should be also connected to the backup bus/buses.
The figure below shows a doubly redundant CAN bus, but the same considerations apply to a triply redundant bus:
CAN bus parameters
UAVCAN is bit rate agnostic, so technically any bit rate can be used as long as it is supported by the physical layer. However, only recommended bit rates from the table below should be used to ensure compatibility.
All recommended bit rates and other relevant CAN bus parameters are listed in the table below.
|Bit rate||Valid range for location of sample point||Recommended location of sample point||Max bus length||Max stub length|
|1 Mbit/s (preferred)||75% to 90%||87.5%||25 m||0.3 m|
|500 kbit/s||85% to 90%||87.5%||100 m||0.3 m|
|250 kbit/s||85% to 90%||87.5%||250 m||0.3 m|
|125 kbit/s||85% to 90%||87.5%||500 m||0.3 m|
The estimated bus length limits are based on the assumption that propagation delay does not exceed 5 ns/m, not including additional delay times of CAN transceivers and other components.
Automatic bit rate detection
It is encouraged to implement CAN auto bauding when applicable. Please refer to the CiA 801 application note for recommended practices.
It should be noted that UAVCAN enables the use of the simple bit time measuring approach, as it is guaranteed that any functioning UAVCAN network will always contain node status message, which can be expected to be published at a rate no lower than 1 Hz, and that contains a suitable alternating bit pattern in the CAN ID field. Please refer to the chapter dedicated to standard data types for details.