Additional Installation Tips
7.7 CableandBusTerminationRS-485
• If maximum transfer distan ce 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 sh ould 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 mus t 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,
theshieldshouldbeconnectedtothegroundvia0.1μF/250Vfilmcapacitor
to prevent ground current in the braided shield. A number of manufacturers
recommendthattheshieldbegroundedateachnode.Variousmanufacturers
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 CANInterface
7.8.1 CableRecommendations
• 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
pairforcommunicationandonepairforGND.Donotusethecableshieldto
connect GND.
7.8.2 Termination
• To achieve the best signal integrity, noise immunity and reliable bus
performance, th e 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.
BeijerElectronics, MAEN249B
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