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Does RS-485 need terminating resistor?
As data rates and/or cable lengths increase, which is most cases, termination becomes mandatory. In case of an RS485 twisted pair cable this termination is typically between 120 and 130 Ω. Here is a simple schematic of how the end of the lines should be terminated: “RT” is the 120 Ω termination resistor.
What is the objective of using a termination resistor at the ends of a network?
Reflections are minimized when the end of the conductor is terminated with a resistance equal to the conductor’s characteristic impedance. Hence the terminating resistor is to minimize signal reflection.
Where to place termination resistor on RS-485 cable?
Place a termination resistor matching the cable impedance at a convenient location, as close to the end of the RS-485 line as possible. It’s possible that a node attached to the RS-485 line may have terminating resistors built into it as a configurable setting, so check that when adding termination resistors to any line.
When do I need to use a termination resistor?
As data rates and/or cable lengths increase, which is most cases, termination becomes mandatory. To eliminate the presence of reflections from the end of the cable, it must be terminated at both ends with a resistor across the line (between + and -), in accordance with its characteristic impedance.
What should the impedance of a RS-485 be?
Resistive terminators typically have values from 120 to 130 Ω. Although twisted-pair cable impedances can be as low as 100 Ω, a 100 Ω terminating resistor is too low for RS-485 drivers, because two in parallel yield 50 Ω, and RS-485 drivers are not rated for loads below 54 Ω.
Do you need a RS422 or RS485 resistor?
The receiver in the RS422 or RS485 IC doesn’t be the needed load, the voltage sensing circuit takes very little current from the line, right size resistor is needed. One cannot insert many resistors because in parallel they are together much less than the matched load.