Technical data

1MRS751108-MUM
REX 521
41
Protection Relay
Technical Reference Manual, General
The physical bus topology is star-based. A star-coupler unit, for example RER 125,
is needed for connecting the REX 521 unit to a control system.The line idle state of
the fibre-optic interface is configurable (“light on” and “light off”) with a parameter
Tx mode.
For more information about the IEC 60870-5-103 bus, see “Appendix A: IEC
60870-5-103 bus” on page 65.
5.1.12.8. Modbus
The REX 521 has a slave communication interface for Modbus system. Both ASCII
and RTU mode are supported.
The communication technique used in Modbus protocol is a master-slave technique.
The master can be connected to the slaves either directly or via modems by using a
compatible serial interface. However, only the direct connection can be used with
REX 521.
At physical level, the Modbus systems may use different physical interfaces, for
example, RS-485 or RS-232 (see Fig. 5.1.12.8.-1). Both the RS-485 two-wire
interface and the optical RS-232 interface are supported by REX 521. The
recommended RS-485 bus topology with Modbus is a chain of devices with line
terminations at the both ends of the bus.
A051913
Fig. 5.1.12.8.-1 Example of a Modbus system
REX 521
RER 133
RS232 RS485
RS485 INTERFACE
12345678
4WIRE TERMINATED
S1
OFFON ONOFF OFF NC NC NC
S2 ON ON ON ON ON ON NC NC
4 WIRE WITHOUT TERMINATION
S1
OFFON ONOFF OFF NC NC NC
S2 OFFOFFOFFOFFOFFOFFNCNC
2WIRE TERMINATED
S1
ONOFF ON OFFOFFNC NC NC
S2 OFFOFFOFFONONONNCNC
2 WIRE WITHOUT TERMINATION
S1
ONOFF ON OFFOFFNC NC NC
S2 OFFOFFOFFOFFOFFOFFNCNC
S1
S2
ON
ON
OFF
OFF
2WIRE
8A +
7B 
6A +
5B 
4 AGND
3NC
2 Shield GND C
1 Shield GND
4WIRE
RX
TX

RER133_front_a
RER 133
RER 133
RER 133
REX 521
READY START TRIP
FEEDER PROTECTION
REX 521
READY START TRIP
FEEDER PROTECTION
REX 521
READY START TRIP
FEEDER PROTECTION
REF 541
Uaux = 80...265 Vdc/ac
fn = 50 Hz
In = 1/5 A (I)
1MRS xxxxxx
98150
9509
Ion = 1/5 A (Io)
Un = 100/110 V (U)
Uon = 100/110 V (Uo)
21 kV
CB OK
AROFF
REF 541
Uaux = 80...265 Vdc/ac
fn = 50 Hz
In = 1/5 A (I)
1MRS xxxxxx
98150
9509
Ion = 1/5 A (Io)
Un = 100/110 V (U)
Uon = 100/110 V (Uo)
21 kV
CB OK
AROFF
REF 541
Uaux = 80...265 Vdc/ac
fn = 50 Hz
In = 1/5 A (I)
1MRS xxxxxx
98150
9509
Ion = 1/5 A (Io)
Un = 100/110 V (U)
Uon = 100/110 V (Uo)
21 kV
CB OK
AROFF
RER 133
RS232 RS485
RS485 INTERFACE
12345678
4 WIRE TERMINATED
S1
OFFONONOFFOFFNCNCNC
S2 ON ON ON ON ON ON NC NC
4 WIRE WITHOUT TERMINATION
S1
OFFONONOFFOFFNCNCNC
S2 OFFOFFOFFOFF OFFOFF NC NC
2 WIRE TERMINATED
S1
ONOFF ON OFFOFFNC NC NC
S2 OFFOFFOFF ON ON ON NC NC
2 WIRE WITHOUT TERMINATION
S1
ONOFF ON OFFOFFNC NC NC
S2 OFFOFFOFFOFF OFFOFF NC NC
S1
S2
ON
ON
OFF
OFF
2WIRE
8A +
7B 
6A +
5B 
4 AGND
3NC
2 Shield GND C
1 Shield GND
4WIRE
RX
TX

RER133_front_a
RER 133
RS232 RS485
RS485 INTERFACE
12345678
4 WIRE TERMINATED
S1
OFFON ONOFF OFF NC NC NC
S2 ON ON ON ON ON ON NC NC
4 WIRE WITHOUT TERMINATION
S1
OFFON ONOFF OFF NC NC NC
S2 OFFOFFOFFOFF OFFOFF NC NC
2 WIRE TERMINATED
S1
ONOFF ON OFFOFFNC NC NC
S2 OFFOFFOFF ON ON ON NC NC
2 WIRE WITHOUT TERMINATION
S1
ONOFF ON OFFOFFNC NC NC
S2 OFFOFFOFFOFF OFFOFF NC NC
S1
S2
ON
ON
OFF
OFF
2WIRE
8A +
7B 
6A +
5B 
4AGND
3NC
2 Shield GND C
1 Shield GND
4WIRE
RX
TX

RER133_front_a
RS-485 twisted pair
Modbus Master
REF 54_ terminals