CAN Line Design Recommendations
- Definitions. CAN bus is a two-wire cable with a general reverse wire, to both ends of which the terminal resistances representing characteristic resistance of line are connected. The cable conductors can be parallel, twisted and/or screened depending on the required line length, and data rate. Application of shielded twisted pair is preferable.
- The maximum length of CAN segment is 1 km. Bridges and repeaters (not passive transceivers!) for increase of distance between nodes (more than 1 km) or for provision of galvanic isolation is allowed.
Maximum line length hardly depends on the number of sensors in the network, this dependence impacts only at small lengths when line capacitance is comparable to input capacitances of CAN receivers. On a long-distance line (>100 m) maximum length almost does not depend on the number of blocks and even increases during increase of the number of sensors.
Maximum line length strongly depends on capacitance parameters of cable and is around 50 m at the linear load of 120 pF/m and 100 Kbps rate.
Maximum line length also depends on linear cable resistance; therefore it is more preferable to choose cables with large cross-section.
- Stable operation of the CAN network requires proper coordination (against wave resistance), correct screening, and correct line topology. Physical topology of the CAN network is bus-type (see figure below):
Bus topology must be as compact as possible for bus structure to minimize reflections.
- The cable branches of bus knots must be as short as possible (if line length allows), especially at high rates. It is recommended to connect network subscribers in accordance with the figure:
- The GCAN grounding inputs (signal ground, but not the shield) of all subscribers must be connected, except for cases of galvanic isolation where the grounding inputs can be disconnected. The cable shield must be grounded on one side.
- Recommended cable parameters
Bus length Cable type Cable Terminal
Cross-Section m mΩ/m mm2 Ω±1% Kbps 0 to 40 AWG23
70 0.25 to 0.34 124 1000 at 40 m 40 to 300 AWG22
<60 0.34…0.6 150 to 300 >500 at 100 m 300 to 600 AWG20 <40 0.5 to 0.6 150 to 300 >100 at 500 m 600 to 1,000 AWG18 <26 0.75 to 0.8 150 to 300 >50 at 1 km
(1) Recommended cable specifications: 120 Ohm impedance and 5 ns/m spreading delay.
- For branch cables, a recommended section is 0.25 to 0.34mm² .
- Maximum lengths of various cables:
|Wire section, mm2||Maximum length, m (1)||Maximum length, m (2)|
(1) 20% resistance reserve;
(2) 10% resistance reserve.
9. In accordance with ISO 11898-2, the cables used in CAN lines must have 120 Ohm impedance and 5 ns/m signal spreading delay. Linear resistance of wires in a cable must not exceed 70 mOhm/m. General allowable cable resistance (i.e. cable length per linear resistance) must not exceed 150 Ohm.