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8. Modbus protocol
8. 13. Read non-existent or protected parameters
If a set of parameters is read using a Modbus function, the value returned for non-existent and protected parameters is equal to 16#8000.
If the same Modbus function is used to read parameters, all of which are non-existent or protected, the drive sends back an exception
response with an error code equal to 2.
Example of non-existent or protected parameters being read:
In this example, the same request to read a non-existent parameter followed by 2 existing parameters is used several times in a row in order
to demonstrate the effects of parameter protection.
The "Read Holding Registers" request (3) is addressed to a drive with a Modbus address of 12 (16#0C). The read operation is performed
for 3 consecutive words, starting with address 8400.
Request:
Response:
Scenario no. 1: Parameters CHCF (8401) and COP (8402) not protected
V Successful reading of these two parameters and value equal
to 16#8000 for the non-existent parameter located at address 8400.
Scenario no. 2: Parameter CHCF (8401) protected and COP (8402) not protected
V Successful reading of COP and value equal to
16#8000 for the non-existent parameter located at address 8400 and for parameter CHCF.
Scenario no. 3: Parameters CHCF (8401) and COP (8402) protected
V Exception response (response code = 16#80 + request code), as
all the parameters read are either non-existent, or protected; error code equal to 2 (the word addresses indicated in the request do not exist
in the slave).
Logic address Parameter Value
8400 = 16#20FA Non-existent -
8401 = 16#20D1 [Profile] (CHCF) 3
8402 = 16#20D2 [Copy channel 1
↔ 2] (COP) 2
Slave no. Request code No. of 1st word No. of words CRC16
Hi Lo Hi Lo Lo Hi
0C 03 20 D0 00 03 0E EF
Slave no. Response code No. of bytes read
Value 8400 Value 8401 Value 8402 CRC16
Hi Lo Hi Lo Hi Lo Lo Hi
0C 03 06
80 00 00 03 00 02 17 E4
Slave no. Response code No. of bytes read
Value 8400 Value 8401 Value 8402 CRC16
Hi Lo Hi Lo Hi Lo Lo Hi
0C 03 06
80 00 80 00 00 02 CE 24
Slave no. Response code Error code CRC16
Lo Hi
0C 80+03 = 83 02 51 32