SN65HVD72: interfacing with SN65176B

Howard Zou

Genius 12365 points

Part Number: SN65HVD72
Other Parts Discussed in Thread: SN65176B,

Hi,

my customer is using SN65HVD72 and SN65176B on one 485 bus.

I've read some article saying that 5V 485 transceiver and 3.3V transceiver can work well on the same bus.

But my question is:

they have add clamp diode on A and B pin of SN65HVD72. Since SN65176B is 5V supplied, and the output on A or B could be as high as 5V, will this lead to current flowing through the clamp diode D18 and D20 on the schematic below? It seems to an issue.

Besides, on one 485 bus, do we need to connect all transceivers GND together? Or we can connect only A and B together.

over 4 years ago

0 Eric Hackett over 4 years ago

TI__Guru 74262 points

Howard,

Our applications is looking at this and we'll have a reply to you by tomorrow. Thank you for your patience.

Regards,

0 Hao L over 4 years ago in reply to Eric Hackett

TI__Mastermind 25705 points

Howard,

Your concern is legit. However RS-485 transceivers (3.3V or 5V) work with -7V to 12V common mode voltage per the standard, which should be within the diode's (like SM712) operation region. With the same reason, if the ground potential difference among nodes doesn't exceed -7V to 12V, you don't have to connect the ground together. Here is an app note for your reference.

Regards,

Hao

0 Howard Zou over 4 years ago in reply to Hao L

TI__Genius 12365 points

Hao,

then do you think we should remove D18 and D20 on the schematic?

0 Hao L over 4 years ago in reply to Howard Zou

TI__Mastermind 25705 points

Howard,

What's the purpose of applying D18 and D20? I think they can be removed if you don't want to clamp the bus voltage.

Regards,

Hao

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SN65HVD72: interfacing with SN65176B