CHAPTER
13
MODBUS COMMUNICATION EXCEPTION
CODES
Introduction
When a master device sends a query to a responder device it expects
a normal response. One of four possible events can occur from the
master’s query:
n
Responder device receives the query without a communication
error and can handle the query normally.
It returns a normal response.
n
Responder does not receive the query due to a communication
error No response is returned. The master program will eventually
process a time-out condition for the query.
n
Responder receives the query but detects a communication error
(parity, LRC or CRC).
No response is returned. The master program will eventually
process a time-out condition for the query.
n
Responder receives the query without a communication error but
cannot handle it (i.e., request is to a non-existent coil or register).
The responder will return with an exception response informing
the master of the nature of the error (Illegal Data Address.)
The exception response message has two fields that differentiate it
from a normal response:
Function Code Field
In a normal response, the responder echoes the function code of the
original query in the function code field of the response. All function
codes have a most-significant bit (MSB) of 0 (their values are below
80 hex). In an exception response, the responder sets the MSB of the
function code to 1. This makes the function code value in an
exception response exactly 80 hex higher than the value would be for
a normal response.
With the function code’s MSB set, the master’s application program
can recognize the exception response and can examine the data field
for the exception code.
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