E.16
SEL-787 Relay
Instruction Manual
Date Code 20081022
Modbus RTU Communications
Communications Protocol
The relay responses to errors in the query are shown below.
7Dh Encapsulated
Packet With Control
Command
The SEL-787 uses this function to allow a Modbus master to perform control
operations and another Modbus function with one query. The Device Net card
will transmit this command periodically to achieve high-speed I/O processing
and establish a heartbeat between the DeviceNet card and the main board.
shows the format of the relay responses to errors in the query.
Table E.27
61h Read Parameter Enumeration Text Query Error Messages
Error
Error Code Returned
Communication
Counter Increments
Illegal parameter to read
Illegal Address (02h)
Invalid Address
Illegal enumeration in index
Illegal Data Value (03h)
Illegal Register
Table E.28
7Dh Encapsulated Packet With Control Command
Bytes
Field
Queries from the master must have the following format:
1 byte
Slave Address
1 byte
Function Code (7Dh)
2 bytes
Control Command (same as write to 2000h)
1 byte
Embedded Modbus Function
n
bytes
Optional Data to Support Modbus Function (0–250)
2 bytes
CRC-16
A successful response from the slave will 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
Embedded Modbus Function
n
bytes
Optional data to support the Modbus function (0–250)
2 bytes
CRC-16
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
Summary of Contents for SEL-787
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