GE P485 instruction manual Protocol configuration, Blocks

DATA EXCHANGE

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

BLOCKS

Node Message frame

Transaction

Byte Word Data CRC Byte Frame objects

Message frame

Transaction

Byte	Word	Data	CRC	Byte	Frame objects
	Message frame
					Frame objects
Byte	Word	Data	CRC	Byte

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.

P485 MODBUS TO PROFIBUS CONVERTER – USER GUIDE 3–3