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they influence the transfer immunity against interference.
The third wire is the other important component of bus. The wire interconnects
the users signal grounds thereby eliminate the zero potentials differences. This
wire has to be connected to DTEs shields.
There are also the active terminators beside the passive ones. Only one active
terminator can be installed to one metallic section of the link and its role is
important at the moments when no one user is transmitting and all of them are in
high impedance state (so-called the third state of the bus). Active terminator is an
resistor divider which makes voltage higher or equal than 200 mV between A and
B wires. Some types of RS-485 receivers don't guarantee a correct interpretation
of the third state – it must be transferred as IDLE. The active terminator can solve
this problem. It can be realized with one internal resistor 680 Ω (jumper 7-3) and
one external resistor 680 Ω between 5 and 8 contacts.
3.3
Converter setting
To work properly, the converter needs time out setting τ. For this case there are
contacts TAU1, TAU2 and TAU3 (numbers 1, 2 and 6) on the RS-485 connector.
How to use them is shown at the table below:
Data rate
4800
not used not used not used
9600
19200
not used
38400
not used
57600
not used
115200
not used
When the RTS control mode is set it is suitable to minimize the τ influence setting
the smallest τ (connection 1-6 for the maximum data rate).
If DTE sets RTS to „ON" polarity permanently, the converter would have his
transmitter switched on all the time and the RS-485 line couldn't be shared. In
this case the RTS influence can be switched off by the connection between
RTSpol and GND (contacts 4-5). This way a RTS polarity has been changed and
RTS can be active in the state „OFF".
3.4
Power Supply Connection
The converter can be supplied ether through the DC connector 2.35mm, or
through the contacts 9-5 of the RS-485 connector (DB9M). The power take-off
2
1
6
not used
56k
22k
10k
5
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