ProSoft Technology MVI56E-MCM User Manual

Reference MVI56E-MCM ♦ ControlLogix Platform
User Manual Modbus Communication Module
Page 178 of 209 ProSoft Technology, Inc.
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
6.8.11 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 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 Master program will eventually process a timeout
condition for the request.
 If the server 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 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 Master 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 Master'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.

ProSoft Technology MVI56E-MCM Specifications

General

Brand	ProSoft Technology
Model	MVI56E-MCM
Category	Control Unit
Language	English