Emerson OpenComms-485 Manuel d'utilisateur télécharger pdf (Page 35)

34 NIC Setup and Testing

Exception Response

Server

Address

Function Code

(0x80)

Exception

code

16 bit CRC

1 byte 1 byte 1 bytes 2 bytes

Exception code is one of the following:

01: Unsupported Function code

02: Out-of-range data addresses

03: Out-of-range data value (write to holding

registers)

04: There is currently no LMD device connected to

the server

05: Server is composing response but cannot

response to the client within the time limit required

by Modbus specification.

Modbus Packet Formats

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. 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 OpenComms NIC must be assigned a unique address

and only the addressed slave will respond to a query that

contains its address.

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Emerson OpenComms-485 Manuel d'utilisateur Page 35