Nortech EasyGrid V1, User Manual

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EasyGrid User Guide
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10.2 MODBUS
Modbus Protocol is a messaging structure used to establish master-slave/client-server communication
between intelligent devices. It is a de facto standard, open and widely used in the industrial
manufacturing environment. It has been implemented on different devices to transfer discrete/analog
I/O and register data between control devices.
Modbus is used in multiple master-slave applications to monitor and program devices; to communicate
between intelligent devices and sensors and instruments; to monitor field devices using PCs and HMIs.
Modbus is also an ideal protocol for RTU applications where wireless communication is required. For this
reason, it is used in innumerable gas and oil and substation applications. Modbus is not only an industrial
protocol, building, infrastructure, transportation and energy applications also make use of its benefits.
The protocol defines a message structure that controllers recognize and use, regardless of the type of
networks over which they communicate. It describes the process a controller uses to request access to
another device, how it will respond to requests from the other devices, and how errors will be detected
and reported. It establishes a common format for the layout and contents of message fields.
The Modbus protocol provides the internal standard that the Nortech EasyGrid unit uses for parsing
messages. During communications on a Modbus network, the protocol determines how each controller
will know its device address, recognize a message addressed to it, determine the kind of action to be
taken, and extract any data or other information contained in the message. If a reply is required, the
controller will construct the reply message and send it using Modbus protocol.
The Nortech EasyGrid unit can be setup to communicate on standard Modbus networks using either of
two transmission modes: ASCII or RTU. Users select the desired mode, along with the serial port
communication parameters (baud rate, parity mode, etc.), during configuration of each controller. The
mode and serial parameters must be the same for all devices on the network.
When using ASCII mode, each byte in a message is sent as two ASCII characters. The main advantage of
this mode is that it allows time intervals of up to one second to occur between characters without
causing an error.
When controllers are setup to communicate on a Modbus network using RTU (Remote Terminal Unit)
mode, a byte in a message contains two 4
–
bit hexadecimal characters. The main advantage of this mode
is that its greater character density allows better data throughput than ASCII for the same baud rate.
Each message must be transmitted in a continuous stream.