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How to terminate RS485?
The most popular termination option is to connect a single resistor across the conductor pair at each end. The resistor value matches the cable’s differential-mode characteristic impedance. If you terminate the bus in this way, no reflections occur, and the signal fidelity is excellent.
When to use termination resistor RS485?
When to Terminate RS-485 Lines Typically reflections occur on long lines, resulting in a receiver misreading logic levels. Proper termination prevents reflections, improving data integrity. When a voltage is first applied to an RS-485 line, current flows through the line (even if the line wires are open).
Do I need to terminate RS485?
If the data rate is low or cables are short, termination may be unnecessary. 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.
What do terminating resistors do?
Termination resistor prevents signal rebound, like waves hitting a wall. If the value of the resistor does not match the characteristic impedance of the cable, reflection may occur. In most RS485 configurations, two termination resistors are used. One of each is placed at the extreme end of the twisted pair cable.
Why do you need to terminate a cable in RS-485?
Terminating a data cable with a value equal to its characteristic impedance reduces reflections and will make your RS-485 network more reliable. Since any device on the bus can transmit at any given time, it is probable that a node in the middle of the bus will transmit, requiring that termination be applied to both ends of the segment.
When to enable RS485 termination on ltc2870?
When RS485 termination is enabled on the LTC2870 or LTC2871, the 120Ω differential resistors are connected across receiver inputs A and B and also across driver outputs Y and Z. The driver termination is not strictly necessary for the case when this device is actively driving the bus, such as the master in Figure 7.
Can a RS485 resistor be disabled in any mode?
The RS485 120Ω termination resistor can be disabled in any mode by setting TE485 pin low, which is useful if the transceiver is not positioned at the end of the bus. The LTC2871 offers similar controls but since the RS232 and RS485 pins are not shared, all of the termination resistors can be engaged simultaneously if desired.
What’s the data rate for a RS485 transceiver?
Both support data rates as high as 20Mbps for the single RS485 transceiver and 500kbps for two RS232 transceivers. These devices are designed to support a wide variety of applications with features that make them flexible and robust: