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ISO1450: ISO1450 input parameter and failsafe protection application issue

Part Number: ISO1450
Other Parts Discussed in Thread: ISO1410,

Hi Team,

My customer use this part have some question need your help , please see my comment as below;

1. Input side D & R resistor : If they want to connect at least 32 set device , Did he still added the resistor in input pin or just the IC internal is enough ? 

(The link figure1's Rf1 and Rf2 is must to have , if so how much this value? )

2. Failsafe protection : If they have many device connect about this function only the bus side device need to added this Resistor on VCC to A and Gnd to B ? Or each device have to added this resistor ?

3. Failsafe protection : Why the datasheet said the ISO1410 didn't add the resistor can do , but the EVM still had the resistor on there. 

https://www.ti.com/lit/an/slla453/slla453.pdf?ts=1618468234257&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FISO1450%253FkeyMatch%253DISO1450%2526tisearch%253Dsearch-everything%2526usecase%253DGPN

Eddie

  • Hi Eddie,

    Thanks for reaching out. Please see my inputs below, thanks.

    1. The resistors RF1 and RF2 along with CF1 and CF2 are filter components to filter out any noise on the data channels between MCU and RS-485 transceiver. These are only given as an option so that customer can use them if he expects lot of noise on MCU output pins or in the system. This is not a mandatory requirement. Hence, you can ask customer to just leave footprint in their first evaluation PCB as backup. Based on their test results, if everything works fine then they don't need to have these filter components in their final PCBs.
    2. The pull-up resistor on A and pull-down resistor on B are the failsafe biasing resistors which make sure the bus doesn't go to tri-state and always shows a valid state when nobody is driving the bus. These resistors are also optional as most present day RS-485 isolators have internal failsafe bias resistors and so is ISO1450. If at all customer wants to use these external bias resistors, then they should only use one pull-up and one pull-down in the entire RS-485 bus as shown below. Please also refer to the below article for more details.
      1. www.ti.com/lit/slyt324
    3. As mentioned above, ISO1410 or ISO1450 does not require external failsafe biasing resistors. The EVM has it has provision in case if the EVM is connected to an old device which doesn't internal failsafe biasing resistors then the resistors on EVM can bias that device. This is just convenience provision given on EVM for old devices.


    Regards,
    Koteshwar Rao