6.2.- MODBUS PROTOCOL
The MODBUS protocol is a communication standard in the industry that enables the network
connection of multiple units, 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-B unit 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 unit 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 unit
is 1, this indicates that the next byte is an exception code.
Address
Func-
tion
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 23.
CRC: 16-bit CRC.
Table 23: 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.
177
Instruction Manual
CVM-B100 - CVM-B150
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