KROHNE CORIMASS MFC 85 Interface EN - Manual (Page 14)

14
3.3 Multi-drop applications
RS 485 allows multiple instruments to be connected in half duplex on a single twisted wire pair
for “party line” type of communications. A method must be used to stop more than one
instrument being online at any time, ensuring that all other instruments are in a high
impedance state. No damage is done when more than one instrument is online
simultaneously, but data will be lost.
In a multiple RS 485 transmitter installation, the application program controls the bi-directional
data communication and selects the instrument to be addressed.
The program control used should be a “master/slave” method. The “master/slave” method
designates one device on the network as master, and this device supervises all transmissions
by communicating with each of the slaves in turn and offering it a transmission slot.
3.4 Bus termination
For proper operation of the RS 485 Bus in half duplex mode in single or multi-drop
communication, it is recommended that a termination resistor (typically 120 ohm) is applied to
both ends of the data line. The simplest form of termination is line to line resistor across the
differential input.
In a multi-drop system, the terminator resistors are only required at the ends of the bus,
usually the master and the last device on the line. The devices in-between do not require
termination.
The Bus configurations are shown in Fig. 4 and Fig. 5.
Configuration with single slave:
Fig. 4
Computer System RS 485 Connection
terminated with 120
ohm resistor between
two lines - RX/TX
Bus System
Screened twisted pair
Slave
MFC 081/085
Terminating resistor
(120 ohm) between
terminals 4 and 4.1