4.9.2. PROTOCOL
The Modbus protocol is an industry communication standard which enables networking of multiple
devices, 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 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 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 device is
1, this indicates that the next byte is an exception code.
Table 12: Exception codes, Modbus communications.
Exception code Description
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.
Example:
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 10.
CRC: 16-bit CRC.
For reasons of operational security of the device, communication frames of more
than 80 bytes are not accepted (sent or received).
60
Computer SMART III
Instruction Manual
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