c) Vim - the common mode voltage between any pair of RS485 chips may not exceed
10 V.
d) A physical earth connection between all RS485 circuits will reduce noise.
2.3.2
Bus segment termination network
The termination network below required at each end of each Bus Ph-segment.
en03000112.vsd
ExV+
Signal B
Signal A
DGND
Ru = 390 ohm
1/4 W, 2%
Rt = 220 ohm
1/4 W, 2%
Rd = 390 ohm
1/4 W, 2%
ExV is supplied by the Node at end of the Bus Segment
IEC03000112 V1 EN-US
Figure 30: RS-485 bus segment termination
ExV is supplied by the Node at end of the Bus Segment
The specifications of the components are:
a) Ru
+ 5 V to Signal B
= 390
W
, 0.25 W ±2.5%
b) Rt
Signal B to Signal A
= 220
W
, 0.25 W ±2.5%
c) Rd
Signal A to GND
= 390
W
, 0.25 W ±2.5%
2.3.3
Bus power distribution
The end node in each Ph-segment applies 5 V bus excitation power to the Termination
network via the Excitation pair (ExV+ and GND) used in the Type 3 Physical layer
specification.
6.3.3
Data on RS485 serial communication module cable
SEMOD176342-280 v2
Type:
Twisted-pair S-STP (Screened – Screened Twisted Pair)
Shield:
Individual foil for each pair with overall copper braid
Length:
Maximum 1200 m (3000 ft) from one system earth to the next system earth (includes
length from platform point to system earth on both sides)
Temp:
According to application
Impedance:
120
W
Capacitance:
Less than or equal to 42 pF/m
Example:
Belden 9841,Alpha wire 6412, 6413
Section 6
1MRK 514 027-UEN D
Connecting
46
650 series
Installation manual
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