KROHNE CORIMASS MFC 85 Interface EN - Manual (Page 16)

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4. Using the Current Output with the RS 485 Bus
The MFC 081/085 Converter is fitted with one current output in addition to the RS 485
connection. This current loop is connected between terminals 6, lout, and 5, 0 volts (refer to
the normal Installation and Operating manual). However extra care must be taken when
connecting this output.
The MFC 081/085 output circuitry is fully floating. It is galvanically isolated from protective
earth, PE and from the converter’s power supply. (See below).
Terminating and
biasing resistors
If just the RS 485 bus is connected then the converter’s output circuitry will float to the
potential of the bus. However the current output shares a common 0 volt reference with the
RS 485 output and hence also with the bus. The current loop receiver must therefore have
a fully floating input. If not it will try to drag the RS 485 bus to some potential of its own.
This could in turn result in interference with stable operation of either, or both of the current
loop and the RS 485 bus.