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

Question

RAnslow 
 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.

roadhitting · Accepted Answer

Part 
 Max data rate 
 VID, Data Rate ? 
 VOL level observed 
 VIL max for STM32F373

ADM3488, 3.3V 
 0.25 Mbps 
 +/-3.3V, 115200 
 0.85 V 
 0.99V to 1.37 V

ADM3490, 3.3V 
 10Mbps 
 +/-3.3V, 115200 
 1.5 V 
 0.99V to 1.37 V

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.)

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