9.2.2 Detailed Design Procedure

9.2.2.1 Bus Loading, Length and Number of Nodes

The ISO 11898-2 Standard specifies a maximum bus length of 40 m and maximum stub length of 0.3 m.

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 transceivers with high input impedance such as the ISO1044B

transceiver.

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

11898-2 Standard. These organizations and standards 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 NMEA2000.

The ISO1044B device is specified to meet the 1.5-V requirement with a 50-Ω load, incorporating the worst case

including parallel transceivers. The differential input resistance of the ISO1044B device is a minimum of 30 kΩ. If

100 ISO1044B transceivers are in parallel on a bus, this requirement is equivalent to a 300-Ω differential load

worst case. That transceiver load of 300 Ω in parallel with the 60 Ω gives an equivalent loading of 50 Ω.

Therefore, the ISO1044B device theoretically supports up to 100 transceivers on a single bus segment.

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

loadings, network imbalances, ground offsets and signal integrity, therefore a practical maximum number of

nodes is typically much lower. Bus length may also be extended beyond the original ISO 11898 standard of 40 m

by careful system design and data-rate tradeoffs. For example, CANopen 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 ISO 11898-2 CAN standard. Using this flexibility requires the

responsibility of good network design and balancing these tradeoffs.

9.2.2.2 CAN Termination

The ISO11898 standard specifies the interconnect to be a single twisted pair cable (shielded or unshielded) with

120-Ω characteristic impedance (ZO). Resistors equal to the characteristic impedance of the line should be used

to terminate both ends of the cable to prevent signal reflections. Unterminated drop-lines (stubs) connecting

nodes to the bus should be kept as short as possible to minimize signal reflections. The termination may be in a

node, but if nodes are removed from the bus, the termination must be carefully placed so that it is not removed

from the bus.

Node 1

CAN

Transceiver

MCU or DSP

CAN

Controller

Node 2

CAN

Transceiver

MCU or DSP

CAN

Controller

Node 3

CAN

Transceiver

MCU or DSP

CAN

Controller

Node n

(with termination)

CAN

Transceiver

MCU or DSP

CAN

Controller

RTERM

RTERM

Figure 9-2. Typical CAN Bus

Termination may be a single 120-Ω resistor at the end of the bus, either on the cable or in a terminating node. If

filtering and stabilization of the common-mode voltage of the bus is desired, then split termination can be used.

ISO1044

SLLSFB0A – MARCH 2020 – REVISED JULY 2020

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