Section 6: Data Comms Connections; Rs485 Connection - Siemens 7SR18 Solkor Manual

7SR18 Installation Guide

Section 6: Data Comms Connections

6.1

RS485 Connection

The RS485 communication port is located on the rear of the relay and can be connected using a suitable RS485
120Ω screened twisted pair cable.
The RS485 electrical connection can be used in a single or multi-drop configuration. The RS485 master must
support and use the Auto Device Enable (ADE) feature.
The last device in the connection must be terminated correctly in accordance with the master driving the
connection. A terminating resistor is fitted in each relay, when required this is connected in circuit using an
external wire loop between terminals 18 and 20 of the power supply module.
The polarity of the signal terminals is marked as A and B in line with the RS485 standard. The polarity is that
when the bus is in the quiescent state and no communication is taking place, the B terminal is more positive than
A. This can be used to identify the polarity of any equipment to be connected, typically measured at each terminal
in turn to ground. Connection of the device to a termination network at the end of the bus will also be to suit the
quiescent state as shown below.
The polarity marking is often found to be reversed or marked as +/- on other equipment so care is required. If the
devices are connected in reverse, communication to all devices will be disturbed but no damage will occur. If
problems are experienced during commissioning, the connections should be tried in reverse.
Up to 64 relays can be connected to the RS485 bus.
The RS485 data communications link with a particular relay will be broken if the relay element is withdrawn from
the case, all other relays will still communicate.
Figure 6.1-1 RS485 Data Comms Connections Between Relays
Unrestricted Page 20 of 24 ©2018 Siemens Protection Devices Limited