Chapter 4 - 7SR45 Data Communications Definitions
© 2018 Siemens Protection Devices Limited
Chapter 4 Page 5 of 20
USB Data Cable
USB Type A
USB Type B
USB Type B
socket on Relay
USB Type A
Socket on PC
Figure 2-1
Communication to Front USB Port
2.3
RS485 Interface
The 2-wire RS485 communication port is located on the rear of the relay and can be connected using
a suitable RS485 120 Ohm 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 device driving the connection. This can be
done via the internal 120 ohm terminating resistor, which can be connected between (A) and (B) by
fitting an external wire loop between terminals 1 and 2 on the X4 connector.
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 A terminal is
more positive than B. 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 in the diagram 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.
The maximum number of relays that can be connected to the bus is 32.
The following settings, on the COMMUNICATIONS MENU, must be configured when using the RS485
interface.
Setting Name Range/Options
Default
Setting Notes
RS485
Protocol
OFF
IEC60870-5-103
MODBUS-RTU
MODBUS-RTU
The protocol used to
communicate on the
standard RS485
connection.
RS485
Station
Address
0 to 254 for IEC60870-5-103
1 to 247 for Modbus RTU
1
An address within the
range of the relevant
protocol must be
given to identify the
relay. Each relay in a
network must have a
unique address.