SN65HVD1473: About difference output failsafe

teritama

Part Number: SN65HVD1473

Tool/software:

Hi team,

In datasheet, it is said "The SN65HVD147x full-duplex family provides internal biasing of the receiver input thresholds in combination with large input-threshold hysteresis." and "Internal biasing of the receiver inputs causes the output to go failsafe-high when the transceiver is disconnected from the bus (open-circuit), the bus lines are shorted (short-circuit), or the bus is not actively driven (idle bus). ".

Does this means it is not required external bias resister for failsafe on differential reciever?

Best regards,

teritama

8 months ago

+1 Clemens Ladisch 8 months ago

Guru 317370 points

Correct. You need external fail-safe biasing resistors only if there are other receivers without fail-safe inputs on the bus, or if you want to increase the fail-safe noise margin.

+1 Ethan Sempsrott 8 months ago

TI__Genius 13952 points

Hi Teritama,

I echo what Clemens said. That range of devices has internal biasing such that external fail-safe biasing is not necessary.

Best,

Ethan

0 teritama 8 months ago in reply to Ethan Sempsrott

Expert 2185 points

Clemens-san, Ethan-san,

Thank you for answer. I understand.

Best regards,

0 teritama 7 months ago in reply to teritama

Expert 2185 points

Clemens-san, Ethan-san,

Sorry for additional question. If we use external fail-safe circuit following below AN, could we use pull-up to 5V on external circuit when device power supply is 3.3V?

Interface Circuits for TIA/EIA-485 (RS-485

I guess it is not muched with internal biasing. So I think it is difficult.

Please your opinion.

Best regards,

teritama

0 Clemens Ladisch 7 months ago in reply to teritama

Guru 317370 points

The SN65HVD1473 has a common-mode range from −7 V to 12 V, so this is allowed.

0 Ethan Sempsrott 7 months ago in reply to teritama

TI__Genius 13952 points

Teritama-san,

A 5V pull-up is allowed with a 3.3V powered device.

No worries about additional questions, all questions are welcome!

Best,

Ethan

0 teritama 7 months ago in reply to Ethan Sempsrott

Expert 2185 points

Clemens-san, Ethan-san,

Thank you for answer again.

>A 5V pull-up is allowed with a 3.3V powered device.

⇒Please teach me for my understanding, is this because that there are bi-directional protection diode in reciever differential input as wrote in figure28?

Best regards,

teritama

+1 Ethan Sempsrott 7 months ago in reply to teritama

TI__Genius 13952 points

Teritama-san,

The reason has to do with the nature of the RS-485 standard. This previous E2E post explains this in more detail (although the original question is asking if 3.3V and 5V RS-485 transceivers are compatible, the background is still the same):

https://e2e.ti.com/support/interface-group/interface/f/interface-forum/93260/rs-485-compatibility-between-3-3v-supply-and-5v-supply-devices

Essentially, all RS-485 drivers output a minimum difference of 1.5V across pins A and B (labeled VOD in the datasheet) regardless of the VCC supply. As long as this difference is inside the common mode range for this device (-7V to 12V), then the receiver will follow the bus correctly.

Best regards,

Ethan

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SN65HVD1473: About difference output failsafe