SN65C1168: RS422 transceiver is power off but the BUS is still powered

Vincent Chen64988

Part Number: SN65C1168
Other Parts Discussed in Thread: CD4097B,

Hello Team,

My customer is asking a question. Below block diagram is customer's application topology, and he wants to know a situation that can damage host side devices or not. The host side can be powered on or off and the slave side is always supplied by power source. When host side's power is off, the RS422 bus still have power that comes from slave side, in this kind of situation the current can flow through ESD diode of host side transceiver output pin then could damage SN65C1168, CD4097B, even MCU since these devices are powered by same power source.

So, is customer worry true? If yes, is there any work around solution can solve this issue?

Thanks very much.

Vincent Chen

over 7 years ago

0 Clemens Ladisch over 7 years ago

Guru 283370 points

The absolute maximum ratings in the SN65C1168 datasheet allow voltages on the RS-422 receiver pins even when VCC = 0. As shown in the input schematics , there are no ESD protection diodes at these pins.

0 Miguel Robertson over 7 years ago in reply to Clemens Ladisch

TI__Guru 62920 points

Vincent,

I agree with Clemens that as long as the voltages at the A and B inputs remain within the specified absolute maximum range, damage to the device should not occur. The SN65C1168 inputs are relatively high impedance when the device is unpowered, sinking at most 100 uA for voltages within the typical RS-422 range.

Please let me know if you have any other questions.

Best regards,
Max

0 Vincent Chen64988 over 7 years ago in reply to Miguel Robertson

TI__Expert 4480 points

Hi Max, Clements,

Got it, thanks very much for your prompt response. I already explain it to customer, I will let your know if customer still have other questions.

Best Regards,
Vincent Chen

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SN65C1168: RS422 transceiver is power off but the BUS is still powered