ABB Industrial IT enabled REX 521 Technical Reference Manual, General page 32

5(; 

32
3URWHFWLRQ 5HOD\
Technical Reference Manual, General
At physical level, the Modbus systems may use different physical interfaces, for
example, RS-485 or RS-232 (see Figure 4.1.12.8.-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.
Modbus master
RS-485 twisted-pair
REX 52x REX 52x
REX 521
)LJ  ([DPSOH RI D 0RGEXV V\VWHP
The adjustable parameters via HMI for Modbus communication in REX 521 are
CRC Order, Modbus Mode, Unit Address, Baud rate, No of stop
bits, No of data bits, End of frame TO, and Parity.
When using the RTU mode, every time the bit rate parameter (the Baud
!
rate) is reconfigured (value is changed) the software of REX 521 also
recalculates the default value for timeout parameter (End of frame
TO). Therefore, to manually enforce the timeout value, the timeout
parameter must always be set after configuring the Baud rate
parameter. In this case it is recommended to set the parameter value for
longer timeout than the calculated default value with the Baud rate
parameter in question.
For more information of Modbus, refer to
IRU 5(;  7HFKQLFDO 'HVFULSWLRQ
7LPH V\QFKURQL]DWLRQ
The time synchronization (Configuration\Time) of the protection relay can
be given from different sources:
1. The time can always be given manually from the front panel.
RS-485/
RS-232
REX 52x
REX 52x
REM 54_ relays
0RGEXV 5HPRWH &RPPXQLFDWLRQ 3URWRFRO
(see "References" on page 51).
1MRS 751108-MUM
RS-485/ RS-485/
RS-232
RS-232
REX 52x
REX 52x
Modbus_syst_a