SN65HVD255, SN65HVD256, SN65HVD257

SLLSEA2D – DECEMBER 2011 – REVISED MAY 2015

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10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

10.1 Application Information

10.1.1 Bus Loading, Length, and Number of Nodes

The ISO 11898 standard states that a CAN bus should have a maximum of 30 nodes, be less than 40 meters

from end to end, and should have no stubs greater than 0.3 meters. However, with careful design, users can

have longer cables, longer stub lengths, and many more nodes to a bus. A large number of nodes requires a

transceiver with high input impedance, such as the SN65HVD25x family devices.

Many CAN organizations and standards have scaled the use of CAN for applications outside the original

ISO11898 standard. They have made system level trade-offs for data rate, cable length, and parasitic loading of

the bus. Examples of some of these specifications are: ARINC825, CANopen, DeviceNet, and NMEA200.

A CAN network design is a series of trade-offs, but these devices operate over wide common-mode range. In

ISO11898-2, the driver differential output is specified with a 60-

Ω load (the two 120-Ω termination resistors in

parallel) and the differential output must be greater than 1.5 V. The SN65HVD25x devices are specified to meet

the 1.5-V requirement with a 45-

Ω load incorporating the worst case including parallel transceivers. The

differential input resistance of the SN65HVD25x devices is a minimum of 30 K

Ω. If 167 SN65HVD25x family

transceivers are in parallel on a bus, this is equivalent to a 180-

Ω differential load worst case. That transceiver

load of 180

Ω in parallel with the 60 Ω gives a total 45 Ω. Therefore, the SN65HVD25x family theoretically

supports over 167 transceivers on a single bus segment with margin to the 1.2-V minimum differential input at

each node. However, CAN network design margin must be given for signal loss across the system and cabling,

parasitic loadings, network imbalances, ground offsets, and signal integrity; thus a practical maximum number of

nodes is typically much lower. Bus length may also be extended beyond the original ISO11898 standard of 40 m

by careful system design and data-rate tradeoffs. For example, CAN open network design guidelines allow the

network to be up to 1 km with changes in the termination resistance, cabling, less than 64 nodes, and a

significantly lowered data rate.

This flexibility in CAN network design is one of the key strengths of the various extensions and additional

standards that have been built on the original ISO11898 CAN standard. In using this flexibility comes the

responsibility of good network design and balancing these tradeoffs.

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