4�9�2� Protocol - Circutor Computer SMART III FAST 12Vdc Instruction Manual

4�9�2� PROTOCOL
The Modbus protocol is an industry communication standard which enables networking of multiple
units, with one master and several slaves. It allows individual master-slave dialogue and also enables
commands in broadcast format.
In the Modbus protocol, the Computer SMART III FAST 12Vdc 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 as follows:
Function 01. Reading the status of the relays.
Functions 03 and 04. Reading logs.
Function 05. Writing a relay.
Function 0F. Writing multiple relays.
Function 10. Writing multiple logs.
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.
Example:
Address: 0A, Peripheral number: 10 in decimal.
Function: 84, Reading function 04 with bit no. 7 at 1.
Exception code: 01, see
CRC: 16-bit CRC.
56
Table 12: Exception codes, Modbus communications�
Exception code
01 Incorrect function. The function number is not implemented.
02 Incorrect address or number of logs out of limits
03 Data error. A CRC error has occurred
04 Peripheral error. An error occurred when accessing a peripheral (EEPROM,
card, etc.)
06 Slave error or Slave busy. Retry sending.
Address
0 A
Table 12�
For reasons of operational security of the unit, communication frames of more
than 80 bytes are not accepted (sent or received).
Description
Exception
Function
code
84 01 XXXX
Computer SMART III FAST 12Vdc
CRC
Instruction Manual