Emerson MONITORING OpenComms Manuel D’Utilisation

The input status block contains read-only status accessible to the 

users. It can be read by using function code 0x02 in the same as read-

ing coil status.

According to the Modbus specification, the OpenComms NIC composes 

the following exception responses to the client if an exception or error 

occurs.

Exception codes are:

• 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.

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.
Function Code
The function code field tells the addressed slaves what function to per-

form. Function codes are specifically designed invoke a specific action 

by the slave device. The function code range is from 1 to 127. However, 

1 byte

1 byte

1 bytes

2 bytes