Datasheet

SN65HVD255
SN65HVD256, SN65HVD257
SLLSEA2C –DECEMBER 2011–REVISED SEPTEMBER 2013 www.ti.com
BUS LOADING, LENGTH AND NUMBER OF NODES
The ISO11898 Standard specifies a maximum bus length of 40m and maximum stub length of 0.3m with a
maximum of 30 nodes. 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.
Many CAN organizations and standards have scaled the use of CAN for applications outside the original
ISO11898. 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 tradeoffs, but these devices operate over wide common-mode range. In
ISO11898-2 the driver differential output is specified with a 60Ω load (the two 120Ω termination resistors in
parallel) and the differential output must be greater than 1.5V. The SN65HVD25x family is specified to meet the
1.5V requirement with a 45Ω load incorporating the worst case including parallel transceivers. The differential
input resistance of the SN65HVD25x is a minimum of 30KΩ. If 167 SN65HVD25x family transceivers are in
parallel on a bus, this is equivalent to a 180Ω differential load worst case. That transceiver load of 180Ω in
parallel with the 60Ω gives a total 45Ω. Therefore, the SN65HVD25x family theoretically supports over 167
transceivers on a single bus segment with margin to the 1.2V minimum differential input at each node. However
for CAN network design margin must be given for signal loss across the system & 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 40m by careful system
design and datarate tradeoffs. For example CANopen network design guidelines allow the network to be up to
1km with changes in the termination resistance, cabling, less than 64 nodes and 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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