About 485 Communication

Dinesh P Jinjala

Other Parts Discussed in Thread: SN65HVD75

I have two systems. One system is working on 5V and second system is working on the 3.3V. I want to make 485 communication between those two system.

Can you suggest me any IC which can convert the 5V to 3.3V and 3.3V to 5V high speed signal?

Can you give me the brief idea that how can I do that?

Thank you in advance.

Regards,

Dinesh Jinjala

over 8 years ago

0 Clemens Ladisch over 8 years ago

Guru 283680 points

The voltage levels on the bus are not directly related to the supply voltages of the transceiver chips, and are always compatible. You do not need a separate level converter.

0 Dinesh P Jinjala over 8 years ago in reply to Clemens Ladisch

Intellectual 615 points

Thank you.

For +5V system I have MAX485 and on other side I used SN65HVD75DR that is worked on 3.3V. In that configuration I cannot able to receive data from the SN65HVD75DR. MAX485 is fixed so I have only one option that I should change SN65HVD75DR. But I don't know what parameter should I consider while selecting the 485 transceiver. So if you have any information related this then it will be useful for me.

Thank you.

Regards,

Dinesh

0 Clemens Ladisch over 8 years ago in reply to Dinesh P Jinjala

Guru 283680 points

In a correctly designed circuit, this should work.

Show the schematic of your circuit.

0 Dinesh P Jinjala over 8 years ago in reply to Clemens Ladisch

Intellectual 615 points

0 Clemens Ladisch over 8 years ago in reply to Dinesh P Jinjala

Guru 283680 points

The two power supplies are connected through the failsafe biasing resistors, so there will always be a current flowing.

To get a defined voltage level when the bus is idle, you need only one pair. Remove one pair.

And the receivers in both the SN65HVD75 and the MAX485 already have fail-safe circuits; consider dropping both pairs.
(See RS-485 failsafe biasing: Old versus new transceivers (SLYT514).)

0 Michael Peffers over 8 years ago in reply to Dinesh P Jinjala

TI__Mastermind 20820 points

Hi Dinesh,

Thank you Clemens for your insightful information!

I agree with Clemens that the fail safe resistors need to be removed from one of the sides. The combination of the two fail safe networks is giving you a characteristic bus impedance of ~30 ohms and the driver is overloaded. When the driver is overloaded the differential amplitude decreases and communication becomes unreliable. You can either remove R19 and R21 complexity and make R20 120 ohms or you can increase R19/20 to some really large value. Fail safe biasing only needs to be present on the bus (network) once though so I recommend removing one of them.

0 Dinesh P Jinjala over 8 years ago in reply to Michael Peffers

Intellectual 615 points

Thank you very much Clemens Ladisch and Michael Peffers. Both of you provided an important information. I will applied it in my project.

Regards,

Dinesh Jinjala

0 Dinesh P Jinjala over 8 years ago in reply to Michael Peffers

Intellectual 615 points

Thank you very much both Clemens Ladisch and Michael Peffers. Both of you have provided a very important information. I have applied it in my project and its worked.

Thank you so much.

Regards,

Dinesh Jinjala

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