Siemens SIMATIC NET Installation Manual page 28

and RS422 can accommodate multi-drop networks, for master-slave serial network
communications. For both RS485/RS422 connections, the following general
guidelines should be followed:
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Note
The RUGGEDCOM RMC30 features built-in pull-up and pull-down resistors. As such,
external bias resistors are only recommended when connecting the RUGGEDCOM
RMC30 to third-party serial devices that do not have built-in pull-up and pull-down
resistors.
Note
Transient protection is provided on all terminals. Lightning strikes and ground
surge currents can cause large momentary voltage differences between ends of
communication links. For maximum reliability of the entire link, all equipment should
have similar transient protection installed.
The following shows the recommended RS485 wiring.
RUGGEDCOM RMC30
Installation Manual, 04/2021, C79000-G8976-1008-10
To minimize the effects of ambient electrical noise, use shielded cabling.
The correct polarity must be observed throughout a single sequence or ring.
The number of devices wired should not exceed 32, and total distance should be
less than 1219 m (4000 ft) at 100 kbps.
The Common terminals should be connected to the common wire inside the
shield.
The shield should be connected to earth ground at a single point to avoid loop
currents.
The twisted pair should be terminated at each end of the chain (typically with a
120 Ohm resistor and a 10nF capacitor in series across the twisted pair).
Communication Ports
4.2.2 RS485/422 Data Ports
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