Rs485 Interface; Electrical Interface - GE 350 Communications Manual

GE
Digital Energy
SR350 Feeder Protection System
Chapter 2: RS485 interface

Electrical Interface

NOTE:
350 FEEDER PROTECTION SYSTEM – COMMUNICATIONS GUIDE

RS485 interface

The hardware or electrical interface in the 350 is two-wire RS485. In a two-wire link, data is
transmitted and received over the same two wires. Although RS485 two wire
communication is bi-directional, the data is never transmitted and received at the same
time. This means that the data flow is half duplex.
RS485 lines should be connected in a daisy chain configuration with terminating networks
installed at each end of the link (i.e. at the master end and at the slave farthest from the
master). The terminating network should consist of a 120 W resistor in series with a 1 nF
ceramic capacitor when used with Belden 9841 RS485 wire. Shielded wire should always
be used to minimize noise. The shield should be connected to all of the 350 s as well as the
master, then grounded at one location only. This keeps the ground potential at the same
level for all of the devices on the serial link.
Polarity is important in RS485 communications. The '+' (positive) terminals of every device
must be connected together.
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