ES58 • EM58 MODBUS®
7.2 Exception response and codes
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.
•
If the Server device receives the request without a communication error
and can handle the query normally, it returns a normal response.
•
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.
•
If the Server receives the request, but detects a communication error, no
response is returned. The Client program will eventually process a
timeout condition for the request.
•
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 about 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 code, the Server returns an exception code in the data field. This
defines the Server condition that caused the exception.
MAN ESx58x_EMx58x MB E 1.3
Programming parameters
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