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AM26LV32: Schematics implementation review

Albin Takami6
Prodigy 160 points
Part Number: AM26LV32

Hello Support Forum,

Based on attached schematics, please review attached implementations.

Tried to follow the reference schematics as much as possible.

Thank you & regards,

-albin

  • 0
    TI__Guru 53555 points

    Hi Albin,

    4A (pin 14) and 4B (pin 15) can disconnected and left open. The device will drive the output (4Y in this case) to a default high state which can be left disconnected.

    If this is the last receiver on this bus, I would recommend including termination resistors on CH1_PHA and CH1_PHB as well to help improve signal integrity for longer cables or high data rates.

    The termination resistance of 220-Ohm on the U19 device is a bit higher than usual. This value normally ranges from 80-Ohm to 120-Ohm depending on the cable. Is this value based off of the cable impedance?

    This device also has internal fail-safe biasing so the external biasing on the second device is providing a larger margin when the bus is idle. Is this providing biasing for other nodes on the same bus?

  • 0 in reply to Eric Schott1
    Prodigy 160 points
    Hi Eric,

    Thanks for your reply.

    Re: "4A (pin 14) and 4B (pin 15) can disconnected and left open. The device will drive the output (4Y in this case) to a default high state which can be left disconnected."

    Would above have to do with internal fail-safe biasing? Kind of unsure if it can safely be disconnected and left open. Please confirm.

    Re: "The termination resistance of 220-Ohm on the U19 device is a bit higher than usual. This value normally ranges from 80-Ohm to 120-Ohm depending on the cable. Is this value based off of the cable impedance?"

    Compared to your suggestion, how would performance and noise immunity be affected if I have 220-Ohm termination resistors on both the transmitter and receiver side based on a standard 4m cable with typical impedance?

    Re: "This device also has internal fail-safe biasing so the external biasing on the second device is providing a larger margin when the bus is idle. Is this providing biasing for other nodes on the same bus?"

    I don’t see these internal fail-safe biasing parts (diagrams in section 8.2?) in datasheet. Please confirm.

    Thanks,

    -albin
  • 0 in reply to Albin Takami6
    TI__Guru 53555 points

    Hi Albin,

    Re: "Would above have to do with internal fail-safe biasing? Kind of unsure if it can safely be disconnected and left open. Please confirm."


    The internal fail-safe bias ensures that if the input to the device is left open, disconnected, or shorted, the receiver output stays in a known state. It is recommended that unused inputs are left open. Externally biasing the input will cause the system draw more current.

    Re: "Compared to your suggestion, how would performance and noise immunity be affected if I have 220-Ohm termination resistors on both the transmitter and receiver side based on a standard 4m cable with typical impedance?"


    Termination resistance is based off the characteristic impedance of the cable being used. This resistor value should be within 20% of this value which ranges between 80-Ohm and 120-Ohm. Most twisted pair cables have a characteristic impedance of 120-Ohm.
    You should only use a termination resistor at the last receiver on the bus. No termination is needed at the driver for RS-422 devices.

    Re: "I don’t see these internal fail-safe biasing parts (diagrams in section 8.2?) in datasheet. Please confirm."


    This diagram only depicts the basic functionality of the receiver. Description of the fail-safe conditions can be found in section 8.4 of the datasheet.

    Let me know if you have any other questions.
    Regards,
    Eric