F.8
SEL-734 Meter Instruction Manual Date Code 20090730
Modbus RTU Communications Protocol
Modbus RTU Communications Protocol
08h Loopback
Diagnostic Command
The SEL-734 uses this function to allow a Modbus master to perform a
diagnostic test on the Modbus communications channel and meter. When the
subfunction field is 0000h, the meter returns a replica of the received message.
The meter responses to errors in the query are shown in Table F.1 9.
10h Preset Multiple
Registers Command
This function code works much like code 06h, except that it allows you to
write multiple registers at once, up to 100 per operation. Refer to the Modbus
Register Map, Control I/O Commands, in Table F.2 3 for a list of registers that
can be written using this function code. If you are accustomed to 4X
references with the function code, for six-digit addressing, simply add 400001
to the standard database addresses.
Table F.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 F.19 Responses to 08h Loopback Diagnostic Query Errors
Error Error Code Returned
Communication Counter
Increments
Illegal subfunction code Illegal Data Value (03h) Illegal Function Code/Op
Code
Format error Illegal Data Value (03h) Bad Packet Format
Table F.20 10h Preset Multiple Registers Command (Sheet 1 of 2)
Bytes Field
Requests 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 Bytes of Data (n)
n bytes Data
2 bytes CRC-16
Courtesy of NationalSwitchgear.com
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