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Why we add resistors in the f transmission lines?

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sonia sharma

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Hi,

Can anyone tell me why we add resistors in the way f transmission lines?

Thanks
Sonia
 

ckshivaram

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Re: network device

Termination Resistor:
Terminating resistors are used to prevent line reflections. In a multidrop network (a network with multiple devices) one resistor at each end of the network is all that is needed. The RS-485 specification calls for a 100-120 Ohm twisted pair cable. The terminating resistors should be chosen to match the impedance of the transmission line. Our in-line terminators are 120 ohm value (part# 182844-01), and can be purchased on-line.

None of our serial boards come with terminating resistors built-in. The terminating resistor must be placed on either side of the RS-485 network, and since the master 485 port could be in the middle of the bus, it does not make sense to put terminating resistors on these boards.

Biasing Resistor:
A transmission line enters an indeterminate state, if no nodes are transmitting on it . This indeterminate state can cause the receivers to receive invalid data bits from noise picked up on the cable.

Bias resistors are used to pull the lines to a known state. There should be at least two on each transmission pair, one pulling the A line up, and another pulling the B line down. The value of these resistors is calculated to guarantee a 200 mV difference between A and B when the bus is properly terminated. You can distribute this biasing throughout the network by using larger resistor values, as long as the combined parallel resistance is not too low. We use 620 ohm bias resistors to bias a properly terminated 120 ohm transmission line.
 

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