• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 34 subscribers
  • Views 683 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

SN65HVD1474: The resistance limit for pulse-proof resistors into the A and B bus lines

user4782171
Intellectual 1560 points
Part Number: SN65HVD1474

Hi Team,

    Here has one application that need your help for it. My customer wants to put the 30ohm resistance on TX/RX pin(R2~R5), and 22pF cap on C180~C183 as following in order to have good system EMI performance. Here need your help to have the suggestion and comment for including the side effect if any. 

 

Best regards,

Albert Lee.

  • 0
    TI__Intellectual 2090 points
    Albert,

    Thank you for your question. Adding series resistance onto the RS-485 bus lines while increasing the differential capacitance will increase the propagation delay in the network, and it will decrease the DC value of the differential output voltage seen on the bus lines. This may decrease your maximum functional operating speed as well as the maximum cable length over which you may functionally transmit and receive data.
    In order to know how much these changes will affect your specific system, could you provide any information regarding your operating speed and the distance over which you are communicating?

    Best Regards,
    Max Megee
  • 0 in reply to Max Megee
    TI__Intellectual 1560 points
    Hi Max Megee,

    The operation speed is 10MHz and distance is 3 meters. Need your help to have the suggestion and comment for this one.

    Best regards,
    Albert Lee.
  • 0 in reply to user4782171
    TI__Intellectual 2090 points
    Albert,

    For reliable communication, the rise/fall times of the transmitted bus signal should not exceed 1/3 of the total bit time at a given data rate. At 10MHz, this means that the rise/fall times from the transmitter should not exceed ~16ns. The SN65HVD1474 specifies its driver rise/fall times with a bus load of 54ohms and 50pF. The maximum rise/fall time specified under this bus condition is 14ns.
    It doesn't look like you will be frequency limited at 10MHz with your alternate load, but you will want to keep an eye on the capacitance that your cables add to the system. As the cabling capacitance approaches the 200pF range, you may encounter more jitter on your system that will start to affect functionality.
    Do you have any knowledge of the cabling specs in the system?

    Best Regards,
    Max