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 Controller MASTER control VAR FAST 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.
Table 11: 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 (EE-
PROM, card, etc.)
06 Slave error or Slave busy. Retry sending.
Example:
Address Function
Exception
code
CRC
0 A 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 9�
CRC: 16-bit CRC.
For reasons of operational security of the unit, communication frames of
more than 80 bytes are not accepted (sent or received).
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