Exception Responses, Request Message to slave

Communication-capable Circuit Breakers

WL Circuit Breaker

Exception Responses

Except for broadcast messages, when a master device sends a query to a slave device, it expects a normal response. One of four possible events can occur from the master's 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 or 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 non-existent coil or register), the slave 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 slave 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 all below 80 hex). In an exception response, the slave sets the MSB of the function code to 1 (adds 80 hex to the function code). 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 slave may return data or statistics in the data field (any information that was requested in the query). In an exception response, the slave returns an exception code in the data field. This defines the slave condition that caused the exception.

Example:

Request Message to slave

The following is an example of a request to read Dataset 1 (Diagnostic Information) from a COM16 slave device at MODBUS address 7. Dataset 1 has 8 registers, but in this example, the Master tries to read just 6 registers.

07H	Slave Address
03H	Function Code
01H	Register Start Address “High” (Dataset 1
	address is 0100 hex)
00H	Register Start Address “Low”
00H	Quantity of Registers “High”
06H	Quantity of Registers “Low” (6 registers is not
	valid)
xxH	CRC Check Code “Low”
xxH	CRC Check Code “High”

Reply Message from slave

The response returns the function code with the high bit set indicating an exception response. The Exception Code returned is 03 (Invalid Data Value). This exception code indicates that an illegal amount of data was specified for the requested Dataset.

07H	Slave Address
83H	Function Code
03H	Exception Code (Illegal Register Amount)
xxH	CRC Check Code “Low”
xxH	CRC Check Code “High”

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WL MODBUS Communication and Electronic Accessories • January 2005

UL 489, UL1066 specifications

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Siemens UL1066, UL 489 specifications Exception Responses, Request Message to slave

Models: UL 489 UL1066