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ISOW1432: ISO1432 Digital Input usage

ais ais
Prodigy 30 points
Part Number: ISOW1432
Other Parts Discussed in Thread: ISOW1412,

Hi there, 

 I'm aware that this may be an unusual request but here we go... 

We're evaluating the IOW1432 RS485 transceiver. In the current design, we cannot afford to switch the DC/DC off so:

 - It will be permanently ON

- We'll use Fault pin to monitor Dc/DC is working properly.

We still have a spare digital input for whatever we need. We are not using a complex RS485 circuit, so not many things to be monitored ;-)

but leaving  input pin unused means wasting money. 

 

What is the normal usage of this pin?  Any suggestion for the input pin functionality?  Any ideas what this intput could be used for ?

 

Kind regards

  • 0
    TI__Mastermind 49695 points

    Hi Ais Ais,

    Welcome to TI E2E forum!

    Most typical RS-485 applications do not require this additional GPIO and hence, the channel will most likely be unused. But very few applications, especially functional safety and safety critical applications, want to have some indication that things are fine on the RS-485 bus side. To report the status from the bus side to the MCU, they use this GPIO.

    I am listing below some of the common situations which customers might want to report back to MCU.

    1. Monitor VISOOUT voltage with a Schmitt comparator for its regulation within given limits and feedback the output of comparator.
    2. Since OUT pin has a default LOW state, just connecting IN to VISOIN will also give you a status of if the power supply is working. A HIGH output at OUT will indicate working power supply while a LOW state indicates that VISOIN is OFF.
    3. If there are multiple isolated interfaces, the statuses of other devices on the bus side can also be reported back.

    I hope this helps clarify some of the uses of the GPIO. Let me know if you have any questions, thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 3086 points

    Hi Rao, 

    My customer is considering ISOW1412. He has two questions. 

    1. If IN(pin14) and OUT(pin7) is not used, should customer just leave it floating? It is not mentioned in datasheet.

    2. Do we have device that have same spec as ISOW1412 but without IN pin and OUT pin that have lower cost than ISOW1412?

    Thanks.

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 49695 points

    Hi Jo,

    Please see my inputs below.

    Hung Ching Hsu said:
    1. If IN(pin14) and OUT(pin7) is not used, should customer just leave it floating? It is not mentioned in datasheet.

    Floating the pins is not an issue and is perfectly fine. Connecting the input to default state (LOW in this case) of the channel would keep power consumption to minimum. Please see below FAQ on this topic.

    https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/732667/faq-can-unused-channel-pins-on-a-digital-isolator-be-left-floating/

    Hung Ching Hsu said:
    2. Do we have device that have same spec as ISOW1412 but without IN pin and OUT pin that have lower cost than ISOW1412?

    Sorry, we do not have ISOW1412 like device without the additional GPIO channel. I believe the single additional channel isn't really adding any noticeable cost to the device, you can count this channel as almost free. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 3086 points

    Hi Rao，

    Thank you for the comment.

    Let me double-check just in case, the link you shared said “The output pins of all the unused channels are best left unconnected.”

    it means OUT pin is best left floating if not used and it also applicable to pin7 of ISOW1412, right?

    Regards,

    Jo

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 49695 points

    Hi Jo,

    Hung Ching Hsu said:
    it means OUT pin is best left floating if not used and it also applicable to pin7 of ISOW1412, right?

    Yes, your understanding is correct. The concern usually is on what to do with unconnected inputs pins, whether to connect them to HIGH, LOW or OPEN. The data output pins are similar to power sources, it is best for them to be left floating when unused as connecting anything to the output pins is like adding a load. Hence, unused outputs are always best left floating. Thanks.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    TI__Expert 3086 points

    Hi Rao, 

    Thank you for the explanation. 

    Let me confirm two things. 

    1. How can I know the default status of IN pin for ISOW1412? It didn't mentioned in datasheet.

    2. The link you shared said customer better set IN pin to its default status when not used due to noise reason but you said for power consumption reason, which one is correct? 

    Regards,
    Jo 

  • 0 in reply to Hung Ching Hsu
    TI__Mastermind 49695 points

    Hi Jo,

    Thanks for the follow-up, please see my inputs below.

    Hung Ching Hsu said:
    1. How can I know the default status of IN pin for ISOW1412? It didn't mentioned in datasheet.

    Sorry that you couldn't find this information easily. Please note that the default state is define for a channel where the output goes to default state when the input is floating or unpowered. The default state for the channel IN/OUT is given in table 10-5 of ISOW1432 datasheet, I am also copying it below for your reference.

    Hung Ching Hsu said:
    2. The link you shared said customer better set IN pin to its default status when not used due to noise reason but you said for power consumption reason, which one is correct? 

    Both are correct reasons, it helps minimize power and also will avoid any noise to couple into device.

    Let me know if you have any further questions, thanks.


    Regards,
    Koteshwar Rao