• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 32 subscribers
  • Views 713 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

what's the function of 10K ohm pull up resistor in "R" PIN of SN65HVD888

Wayne Xu
Prodigy 450 points
Other Parts Discussed in Thread: SN65HVD888, SN65HVD72, SN65HVD82

Customer meet a technical issue about SN65HVD888. 10K ohm pull up resistor in "R" PIN (pin 1) is not added as our reference design. Customer found that 5 units in thousands units can’t communicate with another 485 port module. Packet is lost continually. Then a 10K ohm pull up resistor is added in "R" PIN of SN65HVD888. This issue can be solved. Questions are as below:

1. what’s the function of the 10K ohm pull up resistor in "R" PIN?

2. Why this 10K ohm pull up resistor in "R" PIN is needed?

3. this 10K ohm pull up resistor in "R" PIN is also added in other RS485 device’s reference circuit. Like SN65HVD82, SN65HVD72. Is it a necessary one for our sockets? 

  • 0
    TI__Mastermind 28265 points

    Hello Wayne,

    You are correct when you say the receiver output R (pin 1) is high-impedance when /RE = high. In this case the internal biasing of the receiver output stage is also high-impedance and is susceptible to even small switching noise. This is why you see a long low-pulse when the driver is switching low. In order to maintain the receiver output high, I used a 10k pull-up resistor which provides a stronger bias of the R output line so that this line remains logic high when R is high-impedance.

    So, yes the 10k pull-up resistor is needed also for preventing this line from floating as it connects to a high-impedance CMOS input of the MCU.

     Best regards,

    Thomas