D.10
SEL-749M Relay Instruction Manual Date Code 20080918
Modbus RTU Communications Protocol
Communications Protocol
Modifying Relay
Settings Using
Modbus
The SEL-749M does not provide password protection. It is assumed that
because the interface is a binary protocol with CRC-16 protection, the
interface is being handled by an intelligent master system. Therefore, the
master would provide password protection.
Any of the settings listed in the Modbus Register Map (Table D.23) can be
changed. The high and low limits provided in the table might cover a wider
range than what is acceptable by the particular model or configuration. The
settings are not saved as and when they are received. The relay acknowledges
the write operation, but it does not change the relay settings. The relay holds
these settings until there are no further edits for a time specified by
SETTINGS TIMEOUT register (4010h). After this timeout, the relay attempts
to save the settings. If there are no errors, the settings are saved. If, however, a
setting interdependency rule is violated, the settings are not saved. The relay
will set the Config Fault bit in the TRIP STATUS HI register to indicate that
the save settings operation has failed. The relay will also set ERROR
REGISTER (4016h) flags to indicate the type of error.
Parameters such as date and time can be changed with the appropriate
registers by using Modbus Function Code 06h or 10h.
Controlling Output
Contacts Using
Modbus
The SEL-749M includes registers for controlling some of the outputs. See
LOGIC COMMAND (2000h), RESET COMMAND (2001h), and registers in
the Reset Settings region for the control features supported by the relay. Use
Modbus function codes 06h or 10h to write appropriate flags. Remember that
when writing to the Logic command register with output contacts, it is not a
bit operation. All the bits in that register need to be written together to reflect
the state you want for each of the outputs.
Reading History Data
Using Modbus
Through use of the Modbus Register Map (Table D.23), you can download a
complete history of the last five events via Modbus. The history contains the
date and time stamp, type of event that triggered the report, currents, and
voltages at the time of the event. Please refer to the Historical Data section in
the map.
To use Modbus to download history data, write the event number (1–5) to the
EVENT LOG SEL register at address 0176h. Then read the history of the
specific event number you requested from the registers shown in the Historical
Data section of the Modbus Register Map (Table D.23).
Courtesy of NationalSwitchgear.com
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