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RS485 - ISO3086T - high Z across bus during unpowered state?

Jim Wiemeyer
Prodigy 120 points
Other Parts Discussed in Thread: ISO3086T

The RS485 specification suggests that unpowered transceivers on the bus conduct no more than 100µA (i.e. when a node remains unpowered). Does the ISO3086T present a high impedance to the bus lines when powered down (Vcc1 and Vcc2 = 0VDC) ? Secondly, the IC ESD protection circuitry does not power the IC from an active bus with the node device unpowered?

  • 0
    TI__Guru 55575 points

    Hi Jim,

    When ISO3086T is not powered, there is no powered circuitry to hold output at of any states (H, L or Hi-Z) hence output state will be undetermined. When DE = 0V (or VCC1 = 0V), output current would be a maximum of ±1µA with VCC2 = 0V or 5V.

    The ESD circuit has Zener protection of 16V, since bus voltage will mostly be less than 16V ISO3086T would not be powered.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 120 points

    Koteshwar,

    Your SLLA070D Overview of RS-422 and RS-485 standards states in sections 2.1.4 and 2.2.1 that the devices must maintain Unit Load with the driver powered down and with the receiver powered down. Do your ICs fail to comply with this specification? Analog devices datasheets state explicitly that their ICs meet this specification (ADM485).

  • 0 in reply to Jim Wiemeyer
    Prodigy 120 points

    Another example...

  • 0 in reply to Jim Wiemeyer
    TI__Guru 55575 points
    Hi Jim,

    Section 2.1.4 in the document SLLA070D specifies that output leakage current shouldn't exceed 100µA in power down conditions and ISO3086T consumes a maximum of 1µA (Ioz, as stated in the datasheet).

    Section 2.2.1 states that each node should not consume more current than 1 U.L. [(VBUS)/12kΩ] allowing the system to be capable of connecting at least 32 such U.L.s. Unit Load is basically 12kΩ or higher corresponding to a current of max 1mA at 12V bus voltage. ISO3086T consumes a maximum of 130µA (IA or IB, as stated in the datasheet) @ VBUS=12V making the system capable of connecting such 246 U.L.s. Hence, the device very well meets RS-485 standards.

    Best Regards,
    Koteshwar Rao