Sel 751A, Instruction Manual

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E.15
Date Code 20100129 Instruction Manual SEL-751A Relay
Modbus RTU Communications
Communications Protocol
The relay responses to errors in the query are shown below.
60h Read Parameter
Information
Command
The SEL-751A uses this function to allow a Modbus master to read parameter
information from the relay. One parameter (setting) is read in each query.
The Parameter Descriptor field is defined in Table E.21 .
Table E.19 10h Preset Multiple Registers Query Error Messages
Error Error Code Returned
Communication
Counter Increments
Illegal register to set Illegal Data Address (02h) Invalid Address
Illegal Write
Illegal number of registers to set Illegal Data Value (03h) Illegal Register
Illegal Write
Incorrect number of bytes in
query data region Illegal Data Value (03h) Bad Packet Format
Illegal Write
Invalid register data value Illegal Data Value (03h) Illegal Write
Table E.20 60h Read Parameter Information Command
Bytes Field
Queries from the master must have the following format:
1 byte Slave Address
1 byte Function Code (60h)
2 bytes Parameter Number
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (60h)
2 bytes Parameter Number
1 byte Parameter Descriptor
1 byte Parameter Conversion
2 bytes Parameter Minimum Settable Value
2 bytes Parameter Maximum Settable Value
2 bytes Parameter Default Value
2 bytes CRC-16
Table E.21 60h Read Parameter Descriptor Field Definition (Sheet 1 of 2)
Bit Name Description
0 RO: Read-only 1 when the setting is read-only
1 H: Hidden 1 when the setting is hidden
2 DBL: 32-bit 1 when the following setting is a fractional value of this setting
3 RA: RAM-only 1 when the setting is not saved in nonvolatile memory