F.2
SEL-734 Meter Instruction Manual Date Code 20090730
Modbus RTU Communications Protocol
Modbus RTU Communications Protocol
Modbus Responses
The slave device sends a response message after it performs the action
requested in the query. If the slave cannot execute the command for any
reason, it sends an error response. Otherwise, the slave device response is
formatted similarly to the query and includes the slave address, function code,
data (if applicable), and a cyclical redundancy check value.
Supported Modbus
Function Codes
The SEL-734 supports the Modbus function codes shown in Table F.2.
Modbus Exception
Responses
The SEL-734 Meter sends an exception code under the conditions described
in Table F.3.
In the event that any of the errors listed in Table F.3 occur, the meter
assembles a response message that includes the exception code in the data
field. The meter sets the most significant bit in the function code field to
indicate to the master that the data field contains an error code, instead of the
requested data.
Cyclical
Redundancy Check
The SEL-734 calculates a 2-byte CRC value by using the device address,
function code, and data fields. It appends this value to the end of every
Modbus response. When the master device receives the response, it
recalculates the CRC. If the calculated CRC matches the CRC sent by the
SEL-734, the master device uses the data received. If there is not a match, the
check fails and the message is ignored. The devices use a similar process
when the master sends queries.
01h Read Coil
Status Command
Use function code 01h to read the On/Off status of the selected bits (coils).
You can read the status of as many as 2000 bits per query.
Table F.2 SEL-734 Meter Modbus Function Codes
Codes Description
01h
Read Coil Status
02h
Read Input Status
03h
Read Holding Registers
04h
Read Input Registers
05h
Force Single Coil
06h
Preset Single Register
08h
Loopback Diagnostic Command
10h
Preset Multiple Registers
Table F.3 SEL-734 Meter Modbus Exception Codes
Exception Code Error Type Description
01
Illegal Function Code The received function code is either undefined or unsupported.
02
Illegal Data Address The received command contains an unsupported address in the data field.
03
Illegal Data Value The received command contains a value that is out of range.
04
Device Error
The SEL-734 Meter is in the wrong state for the requested function.
06
Busy
The SEL-734 Meter is unable to process the command at this time because of a
busy resource.
08
Memory Error Checksum error on stored data.
Courtesy of NationalSwitchgear.com
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