SN65HVD1782: Voltage tolerance on bus pins

Christian Ahler

Part Number: SN65HVD1782

Hello,

We would like to be able to disable the driver part of the time and then add an analog signal on bus lines from a DAC IC. This requires tolerance of a differential voltage on the SN65 bus pins. We are however unsure of the max dif level that is tolerated of the IC. Is this be understood as -+30V (differentiel to gnd) or maximum -+7V (differential to gnd) for each pin?

And do you see this as a viable solution?

Thanks for inputs.

Best, Christian

over 8 years ago

0 Lucas Schulte9 over 8 years ago

TI__Expert 6955 points

Christian,

The SN65HVD1782 has a fault protection on the A and B bus pins at +30V/-70V from bus to ground and a max differential voltage of +/- 12V. Disabling the device would cause the A and B buses to enter a high impedance state and should allow you to pass analog signals along those buses granted the voltages do not exceed the limits.

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Thanks. But the datasheet states -7V, and +12V. So are you sure it is not only 7V negative that is tolerated in relation to gnd?

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

Those values are for the operating voltages of the bus terminals when the device active. If you are going to disable the device and pass other signals on the bus lines, then the fault protection voltages would apply to your situation.

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Lucas,

Thanks. This is great. As long as we are not experiencing that some clamping diodes or other non-linear components will affect/load til line when we go below -7V.

Follow up Q to allow for external signals: It seems like there is a permanent differential impedance that is loading the bus lines. But I am mistanken regarding this?

Thanks a lot for the support again. Much appreciated.

Best, Christian

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

What other devices are on the bus lines? Are there termination resistors?

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Hi Lucas,

We are still in the design phase. New sch is to be created in a few weeks. So my question is based on the IC itself. If we should be able to drive the two bus lines externally when the 485 driver is disabled it is important that the 485 driver does not by itself load the two bus lines internally differentially or common mode. And it is not clear to me if it does so? We need it to be true high Z both DM and CM with tolerance up to +/- 10V.

So question is: Is the driver by ifself loading the lines in any way to gnd or towards eachother either when it is enabled and/or when it is disabled?

Best, Christian

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

The device is rated to have a Bus input current (Ibus) of 400µA at 12V and -300µA at -7V all when the driver is disabled. Based off of these numbers for current drawn at the Bus pins when the driver is disabled, you should not experience any significant loading from the driver itself.

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Lucas,
Excellent. Thanks for your precise and elaborate support. It matters a lot that TI has a strong support team. We will therefore most likely use this 485 driver.

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

I'm glad I was able to help! Feel free to reach out to us if you have any more questions.

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Lucas, or somebody else,

SN65HVD1782DR, will this tolerate one node running at 5V being connected to another node running at 3V or will this pose a problem electrically?

Thanks!

Best, Christian

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

So that I understand your question, the proposed signal flow is something like this?

SN65HVD1782DR --> 5V Node --> 3V Node

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Well, I would not write it like that but if that works for you.

One SN65 is 5V supplied. One SN65 is 3V3 supplied . And their RS485 interfaces are connected. Will this work?

Thanks.

Best, Christian

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

The logic thresholds should be compatible with Vcc being 3V3 or 5V for either device.

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Hi Lucas,

Yes I understand this. What I am asking are there any ESD diodes or other reasons that a 3v3 supplied SN65 will have a problem communicating with a 5v supplied sn65? On the A and B pins.

Please consult collegues if you are not sure what I am asking.

Thanks a bunch!

Best, Christian

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

If failsafe biasing (pull-up/pull-down resistances on the A and B lines) is used on both the 5-V and 3.3-V nodes then this could create a path for current to flow from the 5-V rail to the 3.3-V. The amplitude should be small but some power supplies are not designed to handle current sinking well. If this is the case, then remove the biasing resistors from one side or include a blocking diode on the pull-up resistance used on the 3.3-V node.

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Hi Lucas,

Thanks, but my question is much simpler than that. If one 5V supplied RS485 sets its A or B pin high, when connected to a RS485 IC supplied with 3V3. Will this create issues? Or is there compatibility?

Are there no ESD diodes?

Best, Christian

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

The A and B bus lines have an operational tolerance of -7V to 12V regardless of Vcc. I do not foresee any compatibility issues arising with the two devices. The A and B bus lines do not have an internal ESD diode referenced to Vcc so there wouldn't be any issues with the internal ESD diode/cell interrupting operation.

-Lucas

0 Christian Ahler over 8 years ago in reply to Lucas Schulte9

Prodigy 130 points

Excellent. And excellent reply.

Thanks again.

Christian

0 Lucas Schulte9 over 8 years ago in reply to Christian Ahler

TI__Expert 6955 points

Christian,

I'm glad I could help! Let me know if have any further questions.

-Lucas

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SN65HVD1782: Voltage tolerance on bus pins