Why the receiver output voltage low (VOL) is bigger than expected?

Hi ,

How do you know the VOL and VOH levels are causing data errors?

What input VID voltage is on the RS-485 bus pins ?

The test values of 0.4V VOL is based on a VID of 200mV

Best Regards,

Richard

Hi Richard,

Thanks for your reply, here is more information:

We use the USART1 from STM32F373 which is FTf(Five volt tolerant), the input characteristics is as following:

Tjhe picture above tells us that the VIL for USART should be smaller than 1.3V with 3.3V power supply. And when we use the ADM3490, the VOL is about 1.5V, and messages are usually lost. And if we replace the ADM3490 by ADM3488, the VOL becomes 0.85V, and messages are no longer lost. That's why we think it's the cause for message lost.

The VID voltage on the RS485 pin is about +/- 3.3V, as shown in the following photo.

Will the VID cause a receiver output voltage low of 1.5V? Anyway, is there any solution to reduce the voltage?

Thanks.

What data rate are you operating the part at?

Do you also load the STM32F373 UART with TWO ADM3488 RX and see the same issue that you saw with the ADM3490?

Actually I found the reason why a higher VOL was there, for I connected two transeivers to the same USART port.

Like the schematicshown below, I want to add an RS-422 port for monitoring.

I cut off the UART_RX to pin2 of D7, and the problem was gone.

Anyway, can you please help to explain the rationale?

Thanks.

Using one ADM3488 and one ADM3490 to feed the UART will get the VOL to about 0.85V, near the border of the accepted CMOS level. We did no testing with 2 ADM3488.