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SN65HVD12 FailSafe functionality

Atsushi Okui
Genius 4015 points
Other Parts Discussed in Thread: SN65HVD12, SN65HVD72, SN65HVD75

Hello, all

Now we are considering to use SN65HVD12.

I would like to know its FailSafe functionality, with including basic standard for RS485 itself.

Please see the comment below and feedback us with your comment.

1. On Page 11 of this datasheet, I could find out Receiver's parameter table.

On this table, OUTPUT R for "–0.2 V < VID < –0.01 V" is defined as indeterminate.

Does this indeterminate mean that the state will be either High Level or Low Level, and shoud not to be in High Impedance?

2. Do you have any other device which is defined as High Level for "–0.2 V < VID < –0.01 V" condition?

    Or this indeterminate state is following RS485 standard itself?

We thank you in advance for your information.

Best regards,

  • 0
    TI__Mastermind 28265 points

    Okui-san,

    30 years ago the RS-485 standard specified the minimum differential input signal for a receiver with -0.2V to +0.2V. So any bus voltage smaller than this range causes the receiver output to be eith high or low, but which state is unpredictable. The receiver output is not high-impedance because it shows either high or low.

    For a predictable High at the receiver output, old transceivers must have an input voltage of higher than +200 mV, that is the voltage on the A-terminal must be +200mV higher than the voltage at the B-terminal. For a predictable Low, the input must be -200mV, that is the A-line must be 200mV less than the B-line.

    The only way to make the receiver output high-impedance is but applying a high-signal on the receiver enable pin (/RE).

    The HVD12 has a smaller positive voltage threshold of -10mV for a High at the receiver output, but it has still the same -200mV for a low at the receiver output. So for example if you have a slow negative signal of -1V (A-line is 1V more negative than B-line), then the receiver output is Low. If the signal gets smaller to -200mV the receiver output is still Low. Then between -0.2V and -0.01V the receviver output can change to an unpredictable or indeterminate state.

    But when the signal crosses the -10mV level then the receiver output turns high for certain. We have newer 3V devices such as SN65HVD72 and SN65HVD75 for 250kbps and 20Mbps. These transceiver have -20mV positive threshold and -200mV negative threshold. But these device have also a minimum hysteresis of 50mV. So these new transceivers do not require failsafe bias resistors and can tolerate differential noise up to 140mV (which is a very high noise level).

    Please see the data sheet attached.

    Best regards, Thomas

     

     

    sn65hvd72.pdf