DESIGNER’S HANDBOOK 4189350049C EN
Page 35 of 206
CAN bus
bit rate
Theoretical
max CAN cable length
(optimal installation)
Recommended max
CAN cable length
(practical installation)
Max. drop cable
length
Max.
accumulated
drop cable length
250 kbps
250 m
150 m
6 m
30 m
125 kbps
500 m
300 m
10 m
60 m
50 kbps
1000 m
500 m
15 m
100 m
20 kbps
2500 m
1000 m *)
20 m
250 m
*) For cable length >1000 m, a CAN buffer or repeaters should be used.
4.4.4.1 Specifications of the data wire pair (twisted pair):
Gage:
Not less than AWG24/0.205 mm
2
(approx. 90 m
Ω/m), thicker cable is recommended as long as the entity
parameters are considered.
Characteristic impedance:
120
Ω +/-10 % up to at least 500 kHz
Cable loss:
The AC signal attenuation must be less than 24 dB/100 m up to 16 MHz
Propagation delay:
Maximum 5 ns/m
4.4.4.2 Recommendation for the power wire pair:
Gage: Not less than AWG20/0.5 mm
2
(approx. 33 m
Ω/m). Where long supply cables are used, thicker wire is
recommended and worst case calculations of supply voltage drop in the cable should be made.
Guidelines for selecting CAN bus cable can also be found in ISO11898-2.
4.4.5
Shielding and grounding of the CAN bus cables
4.4.5.1 Cable shield
Where CAN cables are connected, the cable shielded must be interconnected. The cable shield must not be
connected to the CAN GND terminal on the XDi. CAN GND is a “common” terminal that
must only be used if there
is an extra “common mode wire” included in the CAN cable (
that is the twisted pair for data + one common wire).
This extra wire reduces common mode voltage between CAN devices on the bus, but it is only rarely used in
marine applications.
4.4.5.2 Grounding of the CAN bus cable
It is recommended only to connect the shield, of the total CAN bus network, to th
e ship’s ground
in one single
location.
It is important that the ground connection used is free from noise and transients from other devises using the
same ground connection. If a good and noise-free ground connection is not available, it is normally better not to
connect the CAN bus cable shield to ground at all.
If redundant CAN bus is used, the two CAN bus cables should be routed separately and in a
safe distance from each other to reduce the risk of a single event damaging both CAN bus
cables.
Using multiple ground connections on the CAN bus cable may create electrical noise loops
disturbing the CAN bus communication.
