Despatch Protocol Plus Modbus Communications - Manual (Page 41)

Chromalox Instruments and Controls
A-51643 Rev. 6 10/06/03 35
5.4 Exception Response Description
Except for broadcast messages, when a master device sends a query to a slave it expects a normal
response. One of four possible events can occur from the master query:
1. If the slave device receives the query without a communication error, and can handle the query
normally, it returns a normal response.
2. If the slave does not receive the query due to a communication error, no response is returned. The
master program will eventually process a timeout condition for the query.
3. If the slave receives the query, but detects a communication error (parity, CRC), no response is
returned. The master program will eventually process a timeout condition for the query.
4. If the slave receives the query without a communication error, but cannot handle it (for example, if
the request is to read a nonexistent output or register), the slave will return an exception response
informing the master of the nature of the error.
The master device‘s application program has the responsibility of handling exception responses. Typical
processes are to post subsequent retries of the message, to try diagnostic messages to the slave, and to
notify operators. The exception codes supported in the Protocol TM Plus controller are summarized in
Table 10 ProtocolTM Plus Exception Code Set.
In an exception response, the slave sets the MSB of the function code to 1. This makes the function code
value in an exception response exactly $80 higher than the value in the query. With 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.
In an exception response, the slave returns an exception code in the data field. The code defines the
condition that caused the exception.
Example of a query with an exception response:
Query
Field Name RTU Example Data Description
Slave Address $06 Slave number 6
Function $01 Read Output Status
Starting Address Hi $00 Start reading at output 119
Starting Address Lo $76
No. of Outputs Hi $00 Read 8 successive outputs