When to use termination resistors in RS-422/RS-485 and their implementation on DSBOARD-NX2 Carrier Board

When to use termination resistors in RS-422/RS-485 and their implementation on DSBOARD-NX2 Carrier Board

Jetson Nano | Jetson Xavier NX

24 February 2021
WHAT YOU WILL LEARN?

1- What is a termination resistor

2- Why is it needed in RS422/RS485

3- When and where to use a termination resistor

ENVIRONMENT

Hardware: DSBOARD-NX2



In this blog, we will explain briefly when termination resistors are needed in RS-422/RS-485 along with their applications and implementations on DSBOARD-NX2.


For RS-422/RS-485, applications requiring long distance cables and high data rates, so placing a termination resistor between differential lines is crucial to maintain a healthy communication.


In half-duplex configuration (RS-485 2-wire), termination resistors must only be placed on the devices at both ends, not on the ones in between.




                                                         Figure 3 from Analog Devices Application Note AN-960: Half-Duplex Configuration


In full-duplex configuration (RS-422 or RS-485 4-wire), there must be termination resistors on receiver sides of both master device and the furthest slave device.




                                                    Figure 4 from Analog Devices Application Note AN-960: Full-Duplex Configuration


If you’re planning to use DSBOARD-NX2 at one end as mentioned above, there is an option to place the termination resistors on-board. You can place a 0402 sized 120Ω (or other desired value) resistor in place of R1608 for half-duplex applications or R1609 for full-duplex applications.









Also, by reducing bus speed, you may improve long distance performance substantially or avoid the usage of termination resistors. This option is also available on DSBOARD-NX2. In cases where you don’t need the default 20Mbps data rate, you may limit it to 250kbps by removing R1607 and placing a 0402 sized 0Ω in place of R1611.








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