Glossary of MODBUS TCP/IP terms
MODBUS TCP/IP, like many other networking systems, has a set of unique terminology. Table below
contains a few of the technical terms used in this guide to describe the MODBUS TCP/IP interface. They
are listed in alphabetical order.
ADU
Application Data Unit, it is the data frame of the MODBUS
protocol. It takes the form of a 7 byte header (MBAP Header:
transaction iden protocol iden length field +
unit identifier), and the Protocol Data Unit (PDU: function
code + data). The MODBUS TCP/IP ADU is inserted into the
data field of a standard TCP frame and sent via TCP on
registered port 502, which is specifically reserved for MODBUS
applications. Thus, this packet is encapsulated by the data
frames imposed by the TCP/IP stack of protocols (TCP/IP/MAC)
before being transmitted onto the network. Refer to page 35.
Application Process
The Application Process is the task on the Application Layer.
Application protocol
MODBUS is an application protocol or messaging structure
that defines rules for organizing and interpreting data
independent of the data transmission medium. TCP/IP only
guarantees that application messages are transferred between
the devices over the Ethernet Local-Area Network (LAN), it
does not guaranty that the devices actually understand or
interoperate with one another. For MODBUS TCP/IP, this
capability is provided by the application layer protocol
MODBUS.
Broadcast address
An IP address with a host portion that is all ones.
Bus
A bus is a communication medium connecting several nodes.
Data can be transferred via serial or parallel circuits, that is,
via electrical conductors or fiber optic.
Client
A Client is any network device that sends data requests to
servers.
MODBUS TCP/IP follows the Client/Server model. MODBUS
Masters are referred to as Clients, while MODBUS Slaves are
Servers.
Data encoding
MODBUS uses a ‘big-Endian’ representation for addresses and
data items. This means that when a numerical quantity larger
than a single byte is transmitted, the most significant byte is
sent first. Refer to page 35.
Determinism
It is the ability of the communication protocol to guaranty
that a message is sent or received in a finite and predictable
amount of time.
Deterministic
Communication
It describes a communication process whose timing behaviour
can be predicted exactly. I.e. the time when a message reaches
Summary of Contents for EMC58 MT
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