2.4.2 RS422 and RS485 Data Ports
The RMC20 is equipped with a single RS485 / RS422 data port. In half duplex mode (See section
2.5 for RMC20 DIP switch settings) the RS485 connections (Rx +, Rx -, COM) should be
connected. In full-duplex mode the RS422 connections (Rx+, Rx -, Tx+, Tx-, COM) should be
connected. Both RS485 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:
1.
To minimize the effects of ambient electrical noise, shielded cabling is recommended
2.
The correct polarity must be observed throughout the daisy chain
3.
The number of devices wired should not exceed 32, and total distance should be less than
4000 feet (at 100Kbps)
4.
The COM terminal should be connected to the common wire inside the shield.
5.
The shield/COM should be grounded at ONE single point to avoid loop currents
6.
The twisted pair should be terminated at each end of the chain. (Typically with a 120Ohm
resistor and a 10nF capacitor in series across the twisted pair)*
* Both data terminal pairs (Rx +/- and Tx +/-) are terminated by default from the factory. To
remove termination: Open the cover and remove jumper JP1 and/or JP2 (JP1 for Rx+, Rx-
terminals, JP2 for Tx+, Tx- terminals) depending on which port termination is NOT required.
Termination provided is a 120 Ohm resistor in series with a 10nF capacitor as per the ModBus 1.0
specification. In general termination should be left in place unless it is detrimental to
communications.
Note: Transient protection is provided on all RMC20 terminals. Lightning strikes and ground surge
currents can cause large momentary voltage differences between ends of communication links. To
ensure maximum reliability of the entire link, all equipment should have similar transient protection
installed.
Figure 2.4.2.1 details an example RS485 wiring diagram.
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2006 RuggedCom Inc. All rights reserved
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