Model 485HF9
Page 19
3.3.3
Multi-drop Applications
RS-422 can provide limited multi-drop capability, using two twisted wire pairs in a broadcast
mode. One wire pair connects one transmitter to multiple receivers, but if duplex operation is
required, only one receiving station can answer back. See figure 3.3.
FIGURE 3.3 RS-422 CONNECTED IN BROADCAST MODE
RS-485 allows multiple (up to 32) transmitters and receivers to be connected in half duplex on
a single twisted wire pair for 'party line' type of communications. See figure 3.4. A method must
be used to stop more than one transmitter being on the line at any time, ensuring that all other
transmitters are in a high impedance state, although the circuit design is such that no damage
will be done to the transceivers if several transmitters are turned on together. In a multiple RS-
485 transmitter installation, the application program controls the data communication
turnaround and the selection of the bus 'talker'.
Two common ways of providing program control are the 'Master / Slave' and 'Token Ring'
methods. 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. In token ring operation, each device knows its ID neighbour and
only talks directly to this next device. Thus communication only occurs between adjacent pairs
of devices and this makes for a flexible network but can be a more difficult method to
implement.
The Model 485 Adaptor provides a method of communication turnaround by enabling the 'ON'
transmitter using an external control signal or RTS from the RS-232 port. Default condition of
the Model 485 Adaptor enables the transmitter, so that for full duplex operation of RS-422/485
no control signal is required.
Model 485
Adaptor
Model 485
Adaptor
120
Ω
Model
485
Adaptor
Model
485
Adaptor
Model
485
Adaptor
120
Ω
=
Twisted
Wire Pair
Data
Transmitter
Master
Answer-back Tx
EN
1
GND 2
TX 3
TX
4
RX
5
RX 6
GND 7
VDC 8
EN
1
GND 2
TX 3
TX
4
RX
5
RX 6
GND 7
VDC 8
EN
1
GND 2
TX 3
TX
4
RX
5
RX 6
GND 7
VDC 8
EN
1
GND 2
TX 3
TX
4
RX
5
RX 6
GND 7
VDC 8
Summary of Contents for 485HF9
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