Date Code 20150130 Instruction Manual SEL-787 Relay
Appendix E
Modbus RTU Communications
Instruction Manual
Overview
This appendix describes the Modbus
®
RTU communications features
supported by the SEL-787 Transformer Protection Relay. Complete
specifications for the Modbus protocol are available from the Modbus user’s
group website at www.modbus.org.
Enable the Modbus TCP protocol with the optional Ethernet port settings. The
SEL-787 supports as many as two Modbus TCP sessions. The TCP port
number for each session is selected with the Ethernet port settings. The default
TCP port number is the Modbus TCP registered port 502. Modbus TCP uses
the device IP address as the Modbus identifier and accesses the data in the
relay by using the same function codes and data maps as Modbus RTU.
Enable Modbus RTU protocol with the serial port settings. When Modbus
RTU protocol is enabled, the relay switches the port to Modbus RTU protocol
and deactivates the ASCII protocol.
Modbus RTU is a binary protocol that permits communication between a
single master device and multiple slave devices. The communication is half
duplex—only one device transmits at a time. The master transmits a binary
command that includes the address of the desired slave device. All of the slave
devices receive the message, but only the slave device with the matching
address responds.
The SEL-787 Modbus communication allows a Modbus master device to do
the following:
➤ Acquire metering, monitoring, and event data from the relay.
➤ Control SEL-787 output contacts.
➤ Read the SEL-787 self-test status and learn the present
condition of all the relay protection elements.
➤ Read most of the relay settings and modify the relay settings.
The BLOCK MODBUS SET setting is used to block relay settings changes
via Modbus or DeviceNet protocols. The factory-default setting,
BLKMBSET := NONE, allows all setting changes via Modbus or DeviceNet
communications. The BLKMBSET := R_S setting prevents Modbus or
DeviceNet communications from resetting to the factory-default settings. The
BLKMBSET := ALL setting blocks all changes to the settings via the Modbus
or the DeviceNet protocol.
You are strongly advised to change the BLKMBSET (BLOCK MODBUS
SET) := ALL if you do not want the PLC (Programmable Logic Controller) or
DCS (Distributed Control System) to send the settings to the SEL-787 relay.
There is a strong possibility that under special conditions like a reboot, the
NOTE: Be aware of the following
setting in the relay:
Under Global settings category
Access Control, there is a setting
called BLOCK MODBUS SET.
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