Hi Dear,
I just set up an RS422 transmitter with ADM3490 between the USART of an ARM Cortex M4 MCU and an RS422 cable to PC. And messages were lost during testing, and finally I found the receiver output voltage low (VOL) was about 1.5V with ADM3490 and 0.85V with ADM3488, both were bigger than expected from the datasheet.
Schematic as following.
Can anyone please help to give an answer about the potential causes of this failure?
Thanks.
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.
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.
