Amplicon 485HF9, Инструкция по эксплуатации

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Model 485HF9
Page 14
See the applications section of this manual for a discussion of bus termination.
2.6.1 Network Biasing Resistors
In some instances, particularly in RS-485 multi-drop operation, noise may be detected at the
receiver. In the multi-drop configuration, there can be brief periods when no transmitter is
enabled, and the network is therefore allowed to float. The Model 485HF9 Adaptor is protected
against this condition, but other devices on the network may be susceptible to noise and are
liable to float to a potential that is detected as an input. The programmer can sometimes
overcome this situation by ensuring that the communications protocol flushes the input buffer
until the beginning of the message flag is found.
If this problem is encountered and a software solution is not viable, two extra resistors can be
added externally to the 485HF9 transceiver at one end of the bus, so that the network is biased
to about 1 volt when all transmitters are disabled. The arrangement is shown in figure 2.8.
FIGURE 2.8 NETWORK BIASING RESISTORS
Model
485HF9
Adaptor
R2
120
Ω
180
Ω
+6 to +9 V Supply
180
Ω
Other
RS-485
Device
Other
RS-485
Device
Other
RS-485
Device
SCHEMATIC
180
Ω
180
Ω
TX - RX BUS
+6 to +9 V Supply
WIRING
Model
485HF9
Adaptor
EN 1
GND 2
TX 3
TX 4
RX 5
RX 6
GND 7
VDC 8