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Chapter 13 Configuring the Digi Device for IA (Industrial Automation) 63
Configuring Modbus
Modbus is a protocol that defines how devices in an industrial automation
(IA) environment communicate. It specifies that a controlling unit, called a
master, manages one or more units, called slaves. The protocol specifies
that only the master may initiate communication. Slaves may only respond.
The protocol defines the structure of Modbus messages, determines how
the master requests information from the slave or specifies an action for
the slave to take, defines how the slave is to respond, specifies addressing
conventions, and deals with many of the other details required for
communication to occur. Modbus defines two encoding schemes, Modbus
ASCII and Modbus RTU. Each Modbus device uses one or the other.
Designed to function over a serial communication cable, Modbus has been
extended in recent years to function over an Ethernet network using
Modbus/TCP, which defines a method of encapsulating Modbus ASCII or
Modbus RTU messages in IP packets for transport over the network.
The extermely flexible Digi implementation includes support for Modbus
ASCII, Modbus RTU, Modbus/TCP, and two other methods of transport
over a network, TCP socket and UDP socket communication. The
implementation enables multiple network-based masters to concurrently
initiate communication with serial-based slaves using any of the supported
network protocols.
Modbus Configuration Procedure
Use the setia command from the See the Digi One/PortServer TS
Command Reference for information.
• Configure the serial port for the serial communication parameters
(baud rate, data bits, parity and stop bits) required by the connected
IA device.
• Choose Modbus ASCII or Modbus RTU as the serial port protocol,
depending on the requirements of the IA device connected to the
port.
• If you configure the port for a slave, you do not have to configure a
network-based master. Communication with the master just works.
(If the master is connected to a serial port, it must be configured,
however.)
• If you configure a port for a master and the slaves are located on the
network, TCP sockets, UDP sockets, and Modbus/TCP are all sup-
ported. Use the protocol required by the master.
Configuring the User-Defined Protocol
Digi One\PortServer TS 2/4 MEI devices support what Digi calls the “user-
defined” protocol, which is any IA serial-port protocol with the following
attributes:
Chapter 13
Configuring the Digi Device for IA
(Industrial Automation)