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DESCRIPTION
COMMUNICATION MODE
PROTOCOL BUILDING
BLOCKS
D485 MODBUS TO DEVICENET CONVERTER – USER GUIDE
Protocol configuration
In order to be able to communicate on the Modbus sub-network, the D485 must be
supplied with a description of the required sub-net protocol. To accomplish this, the
EnerVista P485/D485 Setup software features a flexible protocol-programming system,
allowing the D485 to interpret and exchange data with almost any serial device on the
Modbus sub-network.
The D485 supports the Modbus Master communication mode. In this mode, the D485 is
setup to use the Modbus RTU protocol and implements a Modbus master for data
exchange between the fieldbus and one or more devices on the sub-network. Refer to
Chapter 5 for additional details.
A description of the building blocks used to describe the sub-net protocol is shown below.
Node
Transaction
Transaction
•
Node: In the D485, a node holds all transactions and parameters for a particular
device on the sub network.
•
Transaction: Transactions contains messages to be transmitted on the sub-network.
A transaction consists of one or more message frames (see figure above), and has a
few general parameters to specify how and when the transaction should be used on
the sub-network.
•
Commands: A command is a pre-defined transaction that has been stored in a list in
the EnerVista P485/D485 Setup software. This improves readability as well as
simplifying common operations by allowing transactions to be stored and reused.
•
Message frame: The message frame contains a description of what is actually
transmitted on the sub-network and consists of frame objects (see figure above).
•
Frame object: Frame objects are used to compose a message frame. Frame objects
include fixed values, dynamic values retrieved from a specified memory location in
the D485, or strings.
Message frame
Byte
Word
Data
Message frame
Byte
Word
Data
Message frame
Byte
Word
Data
Figure 3-5: Modbus protocol blocks
DATA EXCHANGE
Frame objects
CRC
Byte
Frame objects
CRC
Byte
Frame objects
CRC
Byte
3–5
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