Advantech ADAM-5550KW Series, Руководство пользователя

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Appendix A RS-485 Network
A-6 ADAM-5510KW Series User’s Manual
A.2 Line Termination
Each discontinuity in impedance causes reflections and distortion.
When a impedance discontinuity occurs in the transmission line the
immediate effect is signal reflection. This will lead to signal
distortion. Specially at line ends this mismatch causes problems.
To eliminate this discontinuity, terminate the line with a resistor.
Figure A-4: Signal distortion
The value of the resistor should be a close as possible to the
charac- teristic impedance of the line. Although receiver devices
add some resistance to the whole of the transmission line, normally
it is sufficient to the resistor impedance should equal the
characteristic impedance of the line.
Example: Each input of the receivers has a nominal input
impedance of 18 k feeding into a diode transistor - resistor
biasing network that is equivalent to an 18 k input resistor tied
to a common mode voltage of 2.4 V. It is this configuration, which
provides the large common range of the receiver required for
RS-485 systems! (See Figure D-5 below).