Modbus Protocol Specification
tải về 0.61 Mb. Chế độ xem pdf
|
- Điều hướng trang này:
- 4.2 Exception Codes
| 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 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. Here is an example of a master query and slave exception response. The field examples are shown in hexadecimal. http://www.modicon.com/techpubs/excpt7.html (1 of 3) [1/11/2000 10:43:26 PM] 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: Only one coil is to be read, as specified by the number of coils field (0001). If the coil address is nonexistent 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. 4.2 Exception Codes http://www.modicon.com/techpubs/excpt7.html (2 of 3) [1/11/2000 10:43:26 PM] http://www.modicon.com/techpubs/excpt7.html (3 of 3) [1/11/2000 10:43:26 PM] |