SIMPRO-100
Modbus
™
RTU Communications Protocol
C
PRIM-2400C 159
C Modbus
™
RTU Communications Protocol
C.1 Introduction
This appendix describes Modbus RTU
communications features supported by the
SIMPRO-100 Relay at the rear-panel EIA-485
port. Complete specifications for the Modbus
protocol are available from Modicon on their web
site: www.modicon.com.
The SIMPRO-100 Relay supports Modbus RTU
protocol when you enable Modbus protocol using
the rear-panel serial port settings. When Modbus
protocol is enabled, the relay activates the
rear-panel EIA-485 serial port and deactivates
the EIA-232 serial port.
Modbus RTU is a binary protocol that permits
communication between a single master device
and multiple slave devices. The communication is
half-duplex: only one device transmits at a time.
The master transmits a binary command that
includes the address of the desired slave device.
All of the slave devices receive the message, but
only the slave device having the matching
address responds.
The SIMPRO-100 Relay Modbus communication
allows a Modbus master device to:
• Acquire metering, monitoring, and event data
from the relay
• Control SIMPRO-100 Relay output contacts
and selected internal logic variables
• Read the SIMPRO-100 Relay self-test status
and learn the present condition of all the relay
protection elements
C.2 Modbus RTU
Communications
Protocol
C.2.1 Modbus Queries
Modbus RTU master devices initiate all
exchanges by sending a query. The query
consists of the fields shown in Table C.1.
Table C.1 Modbus Query Fields
The SIMPRO-100 Relay SLAVEID setting defines
the device address when the relay rear-panel port
is set for Modbus communication. Set this value
to a unique number for each device on the
Modbus network. For Modbus communication to
operate properly, no two slave devices may have
the same address.
Function codes supported by the SIMPRO-100
Relay are described in Table C.2 on page 160.
The cyclical redundancy check is an error
detection method that validates the data received
by the slave device and helps ensure that the
packet received is identical to the packet sent by
the master device. The CRC-16 Cyclical
Redundancy Check algorithm is used.
C.2.2 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 including the slave address, function
code, data if applicable, and a cyclical
redundancy check value.
Field Number of Bytes
Slave Device Address 1 byte
Function Code 1 byte
Data Region 0 – 251 bytes
Cyclical Redundancy Check 2 bytes
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