KROHNE OPTISONIC V6 Modbus - Manual (Page 4)

2011-07-19
OPTISONIC_V6_Modbus_6.doc 4/29
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ts 2 PHYSICAL COMMUNICATION LAYER
The Modbus over serial line protocol is a master-slave protocol. The physical layer can be half-duplex or full-
duplex.
In case of the OPTISONIC V6 the physical layer is a half-duplex (two-wire) connection according to RS 485
specifications.
The end of a RS485 line has to be terminated by means of a resistor. This terminating resistor is included in
the line driving circuit in the OPTISONIC V6.
Multiple OPTISONIC V6 meters may be connected to the same RS 485 line. In this case only the terminating
resistor in the OPTISONIC V6 at the end of the line should be connected. The terminating resistors in the
other OPTISONIC V6 meters on the line should be disconnected by means of the switch on the RS 485
driver printed circuit board. Default this switch is set to connect the line terminating resistor.
Because of the half-duplex operation, the RS 485 communication circuit in the OPTISONIC V6 is normally
always in data receiving mode. Only in case it is requested to send it will automatically switch to data
transmit mode for the time needed.