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

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?


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