Ge P485 User Manual download pdf (Page 21)

DATA EXCHANGE

P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE 3–3

• 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

DESCRIPTION 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.

COMMUNICATION MODE 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.

PROTOCOL BUILDING

BLOCKS

A description of the building blocks used to describe the sub-net protocol is shown below.

Figure 3-3: Modbus protocol blocks

• 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.

Node

Transaction

Message frame

Byte

Word Data CRC Byte

Frame objects

Message frame

Byte Word Data CRC Byte

Frame objects

Message frame

Byte

Word Data CRC Byte

Frame objects

1 2 ... 16 17 18 19 20 21 22 23 24 25 26 ... 75 76

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