Sel SEL-787 - Table E.18 08 H Loopback Diagnostic Command; Table E.19 Responses to 08 H Loopback Diagnostic Query Errors; Table E.20 10 H Preset Multiple Registers Command

E.13
Date Code 20150130 Instruction Manual SEL-787 Relay
Modbus RTU Communications
Communications Protocol
08h Loopback
Diagnostic Command
The SEL-787 uses this function to allow a Modbus master to perform a
diagnostic test on the Modbus communications channel and relay. When the
subfunction field is 0000h, the relay returns a replica of the received message.
The relay responses to errors in the query are shown in Table E.19.
10h Preset Multiple
Registers Command
This function code works much like code 06h, except that it allows you to
write multiple registers at once, as many as 100 per operation. If you are
accustomed to 4X references with the function code, for six-digit addressing,
simply add 400001 to the standard database addresses.
Table E.18 08h Loopback Diagnostic Command
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (08h)
2 bytes Subfunction (0000h)
2 bytes Data Field
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (08h)
2 bytes Subfunction (0000h)
2 bytes Data Field (identical to data in Master request)
2 bytes CRC-16
Table E.19 Responses to 08h Loopback Diagnostic Query Errors
Error Error Code Returned
Communication Counter
Increments
Illegal subfunction code Illegal Data Value (03h) Invalid Function Code/Op
Code
Format error Illegal Data Value (03h) Bad Packet Format
Table E.20 10h Preset Multiple Registers Command (Sheet 1 of 2)
Bytes Field
Queries from the master must have the following format:
1 byte Slave Address
1 byte Function Code (10h)
2 bytes Starting Address
2 bytes Number of Registers to Write
1 byte Number of Bytes of Data (n)
n bytes Data
2 bytes CRC-16