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Upon setting up serial UART between towtwo boards I can only monitor send or receive, not both, Why?

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Upon setting up serial UART between tow boards I can only monitor send or receive, not both, Why?

I have an atmega389B talking to a SAMD21 chip over UART and am trying to monitor the data sent by the SAMD21 and 389B chip. The baud rate used to be 9600 8N1 and I was able to observe both the sent and received data between the two boards using an RS485 to USB sniffer with a 120 Ohm resistor between the RX and TX pins. The baud rate has since gone up to 38400 and Now I can only observe the data sent from one board or the other, not both, which I need to do. I can switch the TX and RX pins and get one set of commands or the other but I cannot receive both. The data is flowing between two RS232 chips and for some reason I was still able to reliably get TX and RX data with an RS485 sniffer. Now with a higher speed I cannot. Does anyone know what might be causing this and a solution to this problem? I tried a UART to usb sniffer but got gibberish and I have tried Teraterm and another cheap RS485 terminal program. I've tested the cables as well and they are working. I'm quite confused and could use some help. Here is a drawing of the setupenter image description here