9 MODBUS
This section describes how the MODBUS standard is applied to the Px40 platform. It is not a description of
the standard itself. The level at which this section is written assumes that the reader is already familiar with
the MODBUS standard.
The MODBUS protocol is a master/slave protocol, defined and administered by the MODBUS Organization
For further information on MODBUS and the protocol specifications, please seethe Modbus web site
(www.modbus.org).
9.1 PHYSICAL CONNECTION AND LINK LAYER
Two connection options are available for MODBUS
● Rear Port 1 (RP1) - for permanent SCADA connection via RS485
● Optional fibre port (RP1 in slot A) - for permanent SCADA connection via optical fibre
The MODBUS interface uses ‘RTU’ mode communication rather than ‘ASCII’ mode as this provides more
efficient use of the communication bandwidth. This mode of communication is defined by the MODBUS
standard.
The IED address and baud rate can be selected using the front panel menu or by the settings application
software.
When using a serial interface, the data format is: 1 start bit, 8 data bits, 1 parity bit with 1 stop bit, or 2 stop
bits (a total of 11 bits per character).
9.2 MODBUS FUNCTIONS
The following MODBUS function codes are supported:
● 01: Read Coil Status
● 02: Read Input Status
● 03: Read Holding Registers
● 04: Read Input Registers
● 06: Preset Single Register
● 08: Diagnostics
● 11: Fetch Communication Event Counter
● 12: Fetch Communication Event Log
● 16: Preset Multiple Registers 127 max
These are interpreted by the MiCOM IED in the following way:
● 01: Read status of output contacts (0xxxx addresses)
● 02: Read status of opto inputs (1xxxx addresses)
● 03: Read setting values (4xxxx addresses)
● 04: Read measured values (3xxxx addresses
● 06: Write single setting value (4xxxx addresses)
● 16: Write multiple setting values (4xxxx addresses)
9.3 RESPONSE CODES
MCode MODBUS Description MiCOM Interpretation
01 Illegal Function Code The function code transmitted is not supported by the slave.
Chapter 7 - SCADA Communications MiCOM P747
210 P747-TM-EN-1
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