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THVD1500: Terminal resistance

zhang ping
Genius 3865 points

Part Number: THVD1500
Other Parts Discussed in Thread: SN65LBC184

1. SN65LBC184 is used in RS485 of my system. In order to test thvd1500, I directly used thvd1500 to replace SN65LBC184 in this system, but a 50 ohm terminal resistance must be added to make RS485 work normally. What's the reason? Or which parameter difference is the cause?

2. What is the significance of short circuit current in the design of RS485 system?

  • 0
    TI__Guru 53535 points

    Hi Zhang,

    1. RS-485 drivers are designed to drive a 54-ohm load (two 120-ohm termination plus transceivers' loads) because termination is commonly included at two ends of the system. Both SN65LBC184 and THVD1500 were designed to meet this standard. 
    Could you specify how the two devices behave differently when presented with different loads in your testing? Is THVD1500 unable to communicate when no load is present?

    2. The short-circuit output current specification describes how the device internally limits the driver current in the event of a bus short. This could be a case where a bus line is shorted to ground while the driver is attempting to drive the line high. The device will limit the current to this short to avoid damage and excess power draw during the short event. 

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    TI__Guru 53535 points

    Hi Zhang, 

    Were you able to identify how the different loads impact THVD1500 performance? Did the information above answer your questions or is there anything else you need to know?

    Regards,
    Eric