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Protection
Modbus
2 - 172
SKF Multilog O
n-Line System IMx-M
User Manual
Modbus
Modbus (L1, L2) for PRM
The Modbus protocol allows for only one master, but many slaves (See RS485 Modbus).
Modbus slaves respond to requests from the Modbus master.
Currently, IMx-M protection part supports Modbus slave functionality only.
A Modbus slave can export (read) or import (write single) values from Modbus registers,
following requests of the Modbus master.
•
Modbus frame format is RTU.
•
The request is limited to maximum 57 registers per request.
•
Refer to Exported (Read) Modbus Registers for the detailed available functions that
can be read to the Modbus master. Exported Modbus registers provide all of
analog, digital and combined channel measurement values and status, system
status, Modbus statistics and information on protection module, firmware, IMx-M
Manager and PRM configuration.
•
Refer to
Imported (Write Single) Modbus Registers
for the detailed available
functions that can be written from the Modbus master. Note that the imported
Modbus registers are limited to command registers.
To configure RS485 Modbus L1 and L2 settings via IMx- M Manager, refer to Modbus
in Assigning Protection Settings (PRM) chapter.
Exported (Read) Modbus Registers
An IMx-M protection part Modbus slave provides channel status and has all of its
measured analog and digital channels available for export at the request of the Modbus
master.
Important facts
• Only the defined registers can be accessed by a Export/Read function.
•
It is not possible to read multiple register blocks where there are gaps of undefined
registers in the address range between the blocks. e.g. A multiple block of register
numbers from 513 to 576 cannot be read since there are gaps (undefined
registers) between 529 and 560.
•
Exported (Read) registers are Read-only.
Notes on Implementation
•
Modbus registers translate according to the table "Exported (Read) Modbus
registers", below.
•
The read function code is 0x04.