Emerson Autochangeover Controllers AC8 Manuel d'utilisateur télécharger pdf (Page 10)

Revision B.33 2/25/2011

Liebert Monitoring Group – Development

Page 10 of 149

Modbus Packet Format

Each Modbus packet consists of the following fields:

Device Address

Function Code

Data Field(s)

Error Check Field

Device Address:

The address field immediately follows the beginning of the frame and consists of 8-bits (RTU) or 2 characters

(ASCII). These bits indicate the user assigned address of the slave device that is to receive the message sent by the

attached master device.

Each slave must be assigned a unique address and only the addressed slave will respond to a query that contains its

address.

Function Code:

The function code field tells the addressed slaves what function to perform. Function codes are specifically

designed invoke a specific action by the slave device. The function code range is from 1 to 127. However, the

SiteLink module primarily uses Function Code 3 (Read Holding Registers), Function Code 6 (Preset Single

Data Field(s):

The data field varies in length depending on whether the message is a request or a response to a packet.

This field typically contains information required by the slave device to perform the command specified or

to pass back data to the master device.

Error Check Field:

The Error Check Field consists of a 16-bit (2 byte) Cyclical Redundancy Check (CRC16). It allows the

receiving device to detect a packet that has been corrupted with transmission errors.

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Emerson Autochangeover Controllers AC8 Manuel d'utilisateur Page 10