ISO1500: Auto-direction control circuit for ISO1500

Hi Charles,

Thanks for reaching out and trying to provide a solution to customer. I see that this circuit needs TXD to be held HIGH to enable the receiver, it currently doesn't have a termination resistor and I see other concerns as well. Please allow me to review this circuit with the right people and come back to you. Thanks.

Regards,
Koteshwar Rao

Hi Charles:

Thanks for posting on E2E. This is a very interesting way to achieve auto-direction control in RS-485 communication.

Theoretically, this circuit works.

In transmit mode, logic zero is achieved by driving the RS-485 bus low while logic one is achieved by disabling the driver and using the fail-safe resistors to create ~160mV VOD. 

In receive mode is enabled when UART_TXD11 signal is high.

However, there are few things to consider.

• ISO1500 VOD will be 0.5x compared to normal use of RS-485. This will drastically reduce the cable reach and/or data rate.
• Noise immunity during transmit mode logic one will be extremely poor due to 10kOhm impedance looking into the driver.
• The RS-485 bus is not terminated properly. This will induce ringing and reflections further reducing the system margin.

In summary, though this technique seems to work in theory, we'd recommend that the customer performs system level tests to validate the robustness of this solution.

Alternatively, you can propose the circuit architecture used in the reference design below to achieve auto-direction control.

https://www.ti.com/tool/TIDA-01090

Regards,

Dushmantha

Hi Dushmantha,

Thank you for your explanation that this circuit will work theoretically. For the reference you show here, I have discussed it with and they don't want one more IC in their design. For the RS-485 bus is not terminated properly, do you think it is possible to change the resistor value to meet the requirement? Thanks.

BR,

Charles Lin