D.7
Date Code 20020903 SEL-587Z Instruction Manual
Modbus RTU Communications Protocol
Modbus Commands
04h Read
Input Registers
Command
Use function code 04h to read from the Modbus Register map shown in
Table D.18 on page D.16. You may read a maximum of 125 registers at once
with this function code.
The relay responses to errors in the query are shown below:
O5h Force Single
Coil Command
Use function code 05h to set or clear a coil.
The command response is identical to the command request.
The coil numbers supported by the SEL-587Z are listed in Table D.9 on
page D.8. The physical coils (Coils 1–5) are self-resetting. If the relay is
disabled or the breaker jumper is not installed, it will respond with error
code 4 (Device Error). Remote Bit Coils 14–21 return zero when read.
Remote Bit Coils 6–13 can be set or cleared and return to the corresponding
state when read. Pulsing a remote bit already in the set state clears that remote
bit after a one-second delay.
Table D.7 04h Read Holding Register Command
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (04h)
2 bytes Starting Register Address
2 bytes Number of Registers to Read
2 bytes CRC-16
A successful response from the slave will have the following format:
1 byte Slave Address
1 byte Function Code (04h)
1 byte Bytes of data (n)
n bytes Data
2 bytes CRC-16
Error Error Code Returned
Communication
Counter Increments
Illegal register to read Illegal Data Address (02h) Invalid Address
Illegal number of registers to read Illegal Data Value (03h) Illegal Register
Format error Illegal Data Value (03h) Bad Packet Format
Table D.8 05h Force Single Coil Command
Bytes Field
Requests from the master must have the following format:
1 byte Slave Address
1 byte Function Code (05h)
2 bytes Coil Reference
1 byte Operation Code
(FF for bit set, 00 for bit clear)
1 byte Placeholder (00)
2 bytes CRC-16
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