1746SC-INO4I Communication module

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 hexadecimal). 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 hexadecimal higher than the value would be for a normal response. With the function code’s MSB set, the master’s application program can recognise 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. Figure B-11 ‘Master Query and Slave Exception Response’ shows an example of a master query and slave exception response. The field examples are shown in hexadecimal.

In this example

the master addresses a query to slave device 10 (0A hex). The function code (01) is for a Read Coil Status operation. It requests the status of the coil at address 1245 (04A1 hex). Note that only that one coil is to be read, as specified by the number of coils field (0001). If the coil address is non-existent in the slave device, the slave will return the exception response with the exception code shown (02). This specifies an illegal data address for the slave. For example, if the slave is a 984-385 with 512 coils, this code would be returned. A listing of Modicon exception codes is in Table C-1 Standard Exception Codes.