6.2.- MODBUS PROTOCOL
The MODBUS protocol is a communication standard in the industry that enables the network
connection of multiple devices, where there is a master and multiple slaves. It allows individual
master-slave dialogue and also enables commands in broadcast format.
In the MODBUS protocol, the CVM-A device uses the RTU (Remote Terminal Unit) mode.
In the RTU mode, the message start and end are detected with silences of at least 3.5
characters, and the 16-bit CRC error-detection method is used.
The MODBUS functions implemented in the device are:
Function 0x04. Reading registers.
Function 0x05. Writing a relay.
Function 0x10. Writing multiple registers.
Exception codes
If the bit with greatest weight of the byte corresponding to the function in the reply of the device
is 1, this indicates that the next byte is an exception code.
Address Function
Exception
code
CRC
0A 84 01 XXXX
Address: 0A, Peripheral number: 10 in decimal.
Function: 84, Reading function 04 with bit no. 7 at 1.
Exception code: 01, see Table 35.
CRC: 16-bit CRC.
Table 35: Exception code�
Exception code Description
01 Incorrect function. The function number is not implemented.
02 Incorrect address or number of registers out of limits
03 Function not permitted
04
Peripheral error. An error occurred when accessing a peripheral (EEPROM,
card, etc.)
05 Test dependent function
06
Slave busy. The card is processing the response, the query must be re-
peated.
6�2�1�- EXAMPLE OF MODBUS QUERY
Query: Instantaneous value of the L1 phase voltage
Address Function
Initial reg-
ister
No� of regis-
ters
CRC
0A 04 0000 0002 70B0
Address: 0A, Peripheral number: 10 in decimal.
Function: 04, Read function.
228
CVM-A1000 - CVM-A1500
Instruction Manual
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