KROHNE CORIMASS MFC 85 Interface EN - Manual (Page 15)

15
Configuration with multiple slaves:
Computer System
Fig. 5
Please note: If other instruments are to operate on the same Bus, all the devices must use the
same communication protocol.
To avoid interfering signal reflections from the ends of the Bus line, both ends (at computer
system as well as at the last instrument) should be terminated with the typical line resistance
value of 120 ohm.
3.5 Network Biasing Resistors
In RS 485 multi-drop operation, noise may be detected at the master. In a multi-drop
configuration there can be brief periods when no instrument is enabled or addressed, and the
network is therefore allowed to float. Some devices on the network may then be susceptible to
noise and may be liable to float to a potential that is detected as an input.
If this problem is encountered, two extra resistors can be added externally to one end of the
Bus, so that the network is biased to a value of approximately 1 Volt when all devices are
disabled. For non-Ex systems, resistors could be added to the last MFC 081/085 device as
shown in Fig. 6 or to the Master (PC).
4 TX/RX
TX/RX
TX/RX
4.1 TX+/RX+
120R 150R
MFC 081/085
converter
4.2 + 5 V
390R
390R
Biasing resistors of
390 ohm
Screened twisted pair cable
5. OV
Fig. 6
However, for Ex systems, adding resistors within the terminal compartment is not allowed as
this would contravene the Ex requirements. In this case the biasing resistors should be added
at the Master’s end of the bus, provided it is located in a safe area.
Terminating resistor
required
Terminating
resistor
required
No terminating resistors required
Slave Slave Slave Slave