Modbus protocol – CIRCUTOR CVM-B Series User Manual

Page 177

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

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

Incorrect function. The function number is not implemented.

Incorrect address or number of registers out of limits

Function not permitted

Peripheral error. An error occurred when accessing a peripheral (EEPROM,

card, etc.)

Test dependent function

Slave busy. The card is processing the response, the query must be re-

peated.

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Instruction Manual

CVM-B100 - CVM-B150