ProSoft Technology MVI69-MCM - Manual (Page 144)

Reference MVI69-MCM ♦ CompactLogix or MicroLogix Platform
User Manual Modbus Communication Module
Page 144 of 167 ProSoft Technology, Inc.
March 22, 2011
5.6.11 MODBUS Exception Responses
When a Modbus Master sends a request to a slave device, it expects a normal
response. One of four possible events can occur from the Master's query:

If the slave device receives the request without a communication error and
can handle the query normally, it returns a normal response.

If the slave does not receive the request due to a communication error, no
response is returned. The Master program will eventually process a timeout
condition for the request.

If the slave receives the request but detects a communication error (parity,
LRC, CRC, ...), no response is returned. The Master program will eventually
process a timeout condition for the request.

If the slave receives the request without a communication error but cannot
handle it (for example, if the request is to read a non-existent address or read
too many points), the slave will return an Exception Response informing the
Master of the nature of the error by using a specific Exception Code in the
response.
An Exception Response message has two fields that differentiate it from a
normal response:
Function Code
Field: In a normal response, the slave 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 slave sets the MSB of the Function
Code byte 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 Master application program can
recognize the Exception Response and can examine the data field for the
Exception Code.
Data Field: In a normal response, the slave may return data or statistics in the
data field (any information that was requested by the request). In an Exception
Response, the slave returns an Exception Code in the data field. This tells what
slave condition caused the exception.
The following table shows an example of a Master request and slave 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
In this example, the Master addresses a read request to slave device. The
Function Code (01) is for a Read Coil Status operation. It requests the status of
the output coil at address 1245 (04A1 hex). Note that only that one coil is to be
read, as specified by the numbers in the output fields (0001).