Sel 751A - Reading Parameter Information and Value Using Modbus; Modifying Relay Settings Using Modbus; Table E.29 7 Dh Encapsulated Packet Query Errors; Table E.30 7 Eh NOP Command

E.19
Date Code 20100129 Instruction Manual SEL-751A Relay
Modbus RTU Communications
Communications Protocol
Table E.29 shows the format of the relay responses to errors in the query.
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-751A 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 (Tab le 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
Table E.29 7Dh Encapsulated Packet Query Errors
Bytes Field
Queries from the master must have the following format:
1 byte Slave Address
1 byte Function Code (7Dh)
2 bytes Status Information (Register 2100h or 2101h based on Bit 3 in
Control Command Word)
1 byte Modbus Function with Error Flag
1 bytes Function Error Code
a
a
If the embedded function code is invalid, then an illegal function code is returned here and
the illegal function counter is incremented. This error code is returned by the embedded
function for all valid embedded functions.
2 bytes CRC-16
Ta b le E . 3 0 7 E h N O P Com m a n d
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