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?