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DESCRIPTION
COMMUNICATION MODE
PROTOCOL BUILDING
BLOCKS
P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE
•
Status register (0x0000 to 0x0001): If enabled, this register occupies the first two
bytes in the input data area. For more information, see Control and status registers on
page 8–1.
•
Input data area (0x002 to 0x00F3): This area holds data that should be sent to the
fieldbus (see the status and control registers).
•
Control register (0x0200 to 0x0201): If enabled, these register occupies the first two
bytes in the output data area. For more information, see Control and status registers
on page 8–1.
•
Output data area (0x200 to 0x2F3): This area holds data received from the fieldbus.
Data cannot be written to this area.
•
General data Area (0x0400 to 0x7FF): This data cannot be accessed from the fieldbus,
and should be used for transfers between nodes on the sub-network, or as a general
"scratch pad" for data.
Protocol configuration
In order to be able to communicate on the Modbus sub-network, the P485 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 P485 to interpret and exchange data with almost any serial device on the
Modbus sub-network.
The P485 supports the Modbus Master communication mode. In this mode, the P485 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 P485, 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
Byte
Word
Data
Message frame
Byte
Word
Data
Message frame
Byte
Word
Data
Figure 3-3: Modbus protocol blocks
DATA EXCHANGE
Frame objects
CRC
Byte
Frame objects
CRC
Byte
Frame objects
CRC
Byte
3–3
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