CGCOMMUNICATIONS GUIDE MODBUS RTU PROTOCOL
369 MOTOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE CG41
5.5 Timing
Data packet synchronization is maintained by timing constraints. The receiving device
must measure the time between the reception of characters. If three and one half
character times elapse without a new character or completion of the packet, then the
communication link must be reset (i.e. all slaves start listening for a new transmission from
the master). Thus at 9600 baud a delay of greater than 3.5 × 1 / 9600 × 10 = 3.65 ms will
cause the communication link to be reset.
5.6 Supported Modbus Functions
The following Modbus functions are supported by the 369:
• 03 - Read Setpoints and Actual Values
• 04 - Read Setpoints and Actual Values
• 05 - Execute Operation
• 06 - Store Single Setpoint
• 07 - Read Device Status
• 08 - Loopback Test
• 16 - Store Multiple Setpoints
For detailed Modbus function code descriptions, refer to the Modicon Modbus Protocol
Reference guide.
5.7 Error Responses
When an 369 detects an error other than a CRC error, a response will be sent to the master.
The MSbit of the FUNCTION CODE byte will be set to 1 (i.e. the function code sent from the
slave will be equal to the function code sent from the master plus 128). The following byte
will be an exception code indicating the type of error that occurred.
Transmissions received from the master with CRC errors will be ignored by the 369 Relay.
The slave response to an error (other than CRC error) will be:
SLAVE ADDRESS - 1 byte
FUNCTION CODE - 1 byte (with MSbit set to 1)
EXCEPTION CODE - 1 byte
CRC - 2 bytes
The 369 implements the following exception response codes.
• 01 - ILLEGAL FUNCTION:
The function code transmitted is not one of the functions supported by the 369 Relay.
• 02 - ILLEGAL DATA ADDRESS:
The address referenced in the data field transmitted by the master is not an allowable
address for the 369 Relay.
• 03 - ILLEGAL DATA VALUE:
The value referenced in the data field transmitted by the master is not within range for
the selected data address.
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