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Reference MVI56-MCM ♦ ControlLogix Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 135 of 159
July 24, 2008
Response
The normal response to a function 16 query is to echo the address, function
code, starting address and number of registers to be loaded.
Adr Func Hi Addr Lo Addr Quantity Error Check Field
11 10 00 87 00 02 56
8.9.10 MODBUS Exception Responses
When a client device sends a request to a server device it expects a normal
response. One of four possible events can occur from the master's query:
ƒ If the server device receives the request without a communication error, and
can handle the query normally, it returns a normal response.
ƒ
If the server does not receive the request due to a communication error, no
response is returned. The client program will eventually process a timeout
condition for the request.
ƒ If the server receives the request, but detects a communication error (parity,
LRC, CRC, ...), no response is returned. The client program will eventually
process a timeout condition for the request.
ƒ If the server receives the request without a communication error, but cannot
handle it (for example, if the request is to read a non-existent output or
register), the server will return an exception response informing the client of
the nature of the error.
The exception response message has two fields that differentiate it from a
normal response:
Function Code Field:
In a normal response, the server echoes the function
code of the original request in the function code field of the response. All function
codes have a most-significant bit (MSB) of 0 (their values are all below 80
hexadecimal). In an exception response, the server sets the MSB of the function
code to 1. This makes the function code value in an exception response exactly
80 hexadecimal higher than the value would be for a normal response.
With the function code's MSB set, the client's application program can recognize
the exception response and can examine the data field for the exception code.
Data Field:
In a normal response, the server may return data or statistics in the
data field (any information that was requested in the request). In an exception
response, the server returns an exception code in the data field. This defines the
server condition that caused the exception.
The following table shows an example of a client request and server exception
response.
Request Response
Field Name (Hex) Field Name (Hex)
Function 01 Function 81
Starting Address Hi 04 Exception Code 02
Starting Address Lo A1
Quantity of Outputs Hi 00
Quantity of Outputs Lo 01