THVD1400: Isolation on bus signal of RS485

Shotaro Sakai

Expert 3390 points

Part Number: THVD1400
Other Parts Discussed in Thread: ISOW1432, ISO1430

Hello team,

Can you give your knowledge for my support?

I am discussing RS485 IC with my customer, and in that, the below question happened. THVD1400 is the first candidate.

In general, logic signal is isolated for RS485 isolation(left figure). On the other hand, do we have way to isolate bus signal of RS485(right figure)? Do you know such usage?

If it is achieved, the customer can make simpler system. Note that each RS485 IC mounted on other PCBs.

Best regards,

Shotaro

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 316920 points

There are no isolators that can be inserted in the middle of an RS-485 bus. But there are isolated RS-485 transceivers like the ISO1430 (or ISOW1432 with integrated power supply).

0 Parker Dodson over 2 years ago

TI__Guru 52262 points

Hi Shotaro,

In general when using a discrete isolator in RS-485 the isolator will be in-between controller and logic pins as I am not aware of any isolator that can handle the potential voltages present. So if you want to have A and B isolated from the logic side of the device you need to use an isolated RS-485 device https://www.ti.com/interface/rs-485-rs-422/products.html#1690=Yes&

If you need a discrete isolator you still need to put that on the logic side.

Please let me know if an isolated RS-485 device meets the needs of the application.

Best,

Parker Dodson

0 Shotaro Sakai over 2 years ago in reply to Parker Dodson

TI__Expert 3390 points

Hi Clements-san and Parker-san,

Thank you for comments from you. I understood isolation RS485 bus is not general.

Do you think if customer take care signal range, we can use "Isolated LVDS buffers" as RS458 bus isolator?

In my understanding, both standards of signal range and speed is different, but LVDS is similar to RS485.

https://www.ti.com/isolation/isolated-interfaces/lvds/products.html

Best regards,

Shotaro

+1 Clemens Ladisch over 2 years ago in reply to Shotaro Sakai

Guru 316920 points

LVDS uses lower currents, so the LVDS output of the isolator will not be able to drive the RS-485 bus properly. (It could work if you changed the entire bus to LVDS.)

+1 Parker Dodson over 2 years ago in reply to Shotaro Sakai

TI__Guru 52262 points

Sakai-san,

LVDS and RS-485 aren't compatible as Clemens mentioned - but you can change the entire bus to LVDS - is that the direct that you wish to look at?

Best,

Parker Dodson

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THVD1400: Isolation on bus signal of RS485