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Modbus Communications

1.3 Implementation Basics

Modbus protocol provides control and data acquisition, through query and response, between master

and slave devices. This protocol comprises the rules for communication, controlling the message

format between devices, how master and slave devices initiate communications, as well as unit

identification, message-handling and error-checking.

The Liebert IntelliSlot 485 card acts as a slave device on a network. This network can be a multidrop

configuration over EIA-485, where multiple slaves reside on a common wire or loop.

1.4 Transmission Format

The Liebert IntelliSlot 485 interface card supports Modbus Remote Terminal Unit (RTU)

transmission modes. See Table 2 below.

Table 2 Modbus Remote Transmission Unit settings for Liebert IntelliSlot 485 interface card

Physical Port

Transmission

Mode

Baud Rate

Data Bits

Parity Bits

Stop Bits

Start

Bits

EIA-485/422

2 wire

RTU 9600, 19200 or 38400 8 None 1 1

1.5 Modbus Packet Format

Each Modbus packet consists of these fields:

• Device Address

• Function Code

• Data Field(s)

• Error Check Field

1.5.1 Device Address

The address field immediately follows the beginning of the frame and consists of 8-bits (RTU). This

bit indicates the user-assigned address of the slave device that is to receive the message from the

attached master device.

Each slave must be assigned a unique address. Only the addressed slave will respond to a query that

contains its address.

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Emerson Liebert FPC Manual de usuario Pagina 6