Advantech ADAM-5550KW Series, Руководство пользователя

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Appendix A RS-485 Network
ADAM-5510KW Series User’s Manual A- 7
Figure A-5: Termination resistor locations
Because each input is biased to 2.4 V, the nominal
common mode voltage of balanced RS-485 systems, the
18 k
on the input can be taken as being in series across
the input of each individual receiver. If thirty of these
receivers are put closely together at the end of the
transmission line, they will tend to react as thirty 36k
resistors in parallel with the termination resistor. The overall
effective resistance will need to be close to the
characteristics of the line. The effective parallel receiver
resistance R
P
will therefore be equal to:
R
P
= 36 x 10
3
/30 = 1200
Ω
While the termination receptor R
T
will equal:
R
T
= R
O
/ [1 - R
O
/R
P
]
Thus for a line with a characteristic impedance of 100
resistor R
T
= 100/[1 - 100/1200] = 110
Ω
Since this value lies within 10% of the line characteristic
impedance.