E.17
Date Code 20081022 Instruction Manual SEL-787 Relay
Modbus RTU Communications
Communications Protocol
7Eh NOP Command
This function code has no operation. This allows a Modbus master to perform
a control operation without any other Modbus command. This is only used
inside of the 7Dh when no regular Modbus query is required.
Reading Parameter
Information and Value
Using Modbus
Through use of Modbus commands, you can read the present value of a
parameter as well as parameter name, units, low limit, high limit, scale, and
even the enumeration string (if the parameter is an enumeration type). This
means that you can use a general user interface to retrieve and display specific
parameter details from the relay. Use the 60h, 61h, and 62h commands to
retrieve parameter information, and use the 03 command to retrieve values.
Modifying Relay
Settings Using
Modbus
The SEL-787 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 E.32) 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 and ERROR
ADDRESS Register to indicate the register that caused the issue.
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-787 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.
Table E.30 7Eh NOP Command
Bytes Field
An example of a 7D message response using 7E will have the following format:
1 byte Slave Address
1 byte Function Code (7Dh)
2 bytes Status Information
1 byte Function Code (7Eh)
2 bytes CRC-16
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