Emerson MONITORING OpenComms - Manual (Page 29)

Modbus Communications and Connectivity
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5.4.6 Read Input Status (0x02)
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.
5.4.7 Error Handling
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.
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.
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,
Table 4 Exception response
Server
Address
Function Code
(0x80)
Exception
Code
16 Bit
CRC
1 byte 1 byte 1 bytes 2 bytes