SN65HVD72: MAX485 perturbation

DELMAS Yannick

Part Number: SN65HVD72

Hello,

I use the configuration see in the picture RS485.png.

The SN65HVD72 is the master and is always present, and after we can add 1, 2 or 3 MAX3483.

When I use only 2 MAX3483 and the SN65HVD72, it's OK, all work fine.
But, if I had a third MAX3483, I have a perturbation problem, this MAX3483 emit perturbation on the pin 1 (RO) and so the transmission don't work.

Do you see a mistake in my hardware configuration ?

Best regards

over 7 years ago

0 Hao L over 7 years ago

TI__Mastermind 25705 points

Yannick,

I don't see your configuration picture. Do you mind uploading it one more time? General question: do you have any termination resistors on the bus? What's the data rate? You can only have two terminations on one bus. The other nodes are considered stubs. You can refer to this app note for the detail.

Regards,

Hao

0 Miguel Robertson over 7 years ago in reply to Hao L

TI__Guru 62920 points

Hello,

I just wanted to check back in on this one. Have you had a chance to review Hao's questions? The point about termination is important to consider, since signal integrity issues can be introduced when adding unterminated "stub" nodes to a bus (particularly a higher-speed one).

Additionally, if you could capture the RS-485 signal line (A/B) waveforms while the system is working as well as when this perturbation occurs it may help us better understand the issue's root cause.

Regards,
Max

0 DELMAS Yannick over 7 years ago in reply to Miguel Robertson

Intellectual 320 points

Hello,

Thank you for your help. I find the solution, I modify the R charge between A and B (only 2 R120 Ohms), and now it's work.

Best regards.

0 Miguel Robertson over 7 years ago in reply to DELMAS Yannick

TI__Guru 62920 points

Thanks for following up! I'm glad to hear you've found the solution.

Regards,
Max

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SN65HVD72: MAX485 perturbation