Additional Installation Tips
7.7 Cable and Bus Termination RS-485
•
If maximum transfer distance and maximum transfer speed is needed,
shielded and twisted pair cable should be used. The mutual capacitance
may not exceed 52.5 pF/m, and the cable area should be at least 0.25 mm
2
(AWG24).
•
0 V, the reference voltage for communication should be included in
the cabling. With two-way communication use two pairs; one pair for
communication and one pair for 0 V.
•
The shield must be grounded at one end. The other end is usually grounded,
but with longer distances or when there is a difference in the ground potential,
the shield should be connected to the ground via 0.1 μF/250 V film capacitor
to prevent ground current in the braided shield. A number of manufacturers
recommend that the shield be grounded at each node. Various manufacturers
have different systems for bus termination.
Depending on the recipients’ design, the bus wires may be on the same level or
require pull-up or pull-down to ensure that no faulty signals are detected when the
bus is in resting mode (all transmitters are disconnected).
7.8 CAN Interface
7.8.1
Cable Recommendations
•
Use shielded, twisted pair cable to improve noise immunity and to reduce
radiated emissions.
Characteristic impedance: 120 Ohm (typical)
Propagation delay: 5 ns/m
Mutual capacitance: 40 pF/m (typical, wire to wire)
•
Recommended cable cross section:
Bus length 0-40 m: 0.25-0.34 mm² (AWG23, AWG22), 70 mOhm/m
Bus length 40-300 m: 0.34-0.6 mm² (AWG22, AWG 20), < 60 mOhm/m
Bus length 300-600 m: 0.5-0.6 mm² (AWG20), < 40 mOhm/m
Bus length 600-1000 m: 0.75-0.8 mm² (AWG18), < 26 mOhm/m
•
GND should be included in the cabling to provide a reliable reference
Ground for each CAN node (potential equalization). Use two wire pairs; one
pair for communication and one pair for GND. Do not use the cable shield to
connect GND.
7.8.2
Termination
•
To achieve the best signal integrity, noise immunity and reliable bus
performance, the CAN network must be terminated at its two most distant
CAN nodes with 118 Ohm... 130 Ohm resistors. The recommended resistor
value is 120 Ohm/1%/0.25W on each end.
•
Place the termination resistor near the CAN bus connector (as close as
possible).
•
This product has no integrated termination resistors. They must be added
externally.
Beijer Electronics, MAEN228
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