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Perle Systems Technical Note
Modbus Gateway Support
IOLAN DS, TS, SDS, STS and SCS Device Servers
Modbus Overview
Modbus is a legacy serial protocol used in industrial automation
by equipment such as SCADA, HMI, OPC Servers, PLC’s, RTU’s and HMI’s.
ModBus comes in 2 formats ;
- Modbus ASCII ( each data byte converted to 2 ASCII characters )
- Modbus RTU ( 8 bit data )
The Modbus protocol contains device (host) addressing and enables communication
between multiple entities on same serial line ( IE: RS485 ). Modbus masters
poll modbus slaves in order to send and retrieve information.
The Industrial Automation industry created the Modbus/TCP protocol which
enables direct attachment of equipment over Ethernet. Equipment with support
for Modbus/TCP could then be used on standard Ethernet networks providing
interconnection over greater distances and with more flexibility.
Valuable legacy equipment that is limited to serial modbus connections may
require attachment to ModbusTCP or Modbus/UDP compliant device (hosts) over Ethernet.
Perle IOLAN Device Servers provide full support for attaching Modbus serial
to Modbus/TCP equipment via the Modbus Gateway features.
IOLAN Modbus Gateway Feature Set
- Defined at the port level, Perle IOLAN’s can operate as either a Modbus Master
Gateway or a Modbus Slave Gateway.
- The Modbus Gateway function can operate with equipment across TCP or
UDP.
- Both Modbus RTU and ASCII forms of the Modbus protocol are supported
- As a master gateway, up to 248 UID’s are addressable per master gateway
configured serial ports
- E.g.: 2 serial ports configured as master gateways can have up to 2
x 248 = 496 addressable slaves on the IP network
- As a slave gateway, up to 248 UID’s per unit addressable from an IP network
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