Unstable read on /dev/rs485 serial port of router

Question

I am having trouble with cat /dev/rs485 on a teltonika router. Sometimes it works, sometimes not. Incomming messages are raw hx data, without \n or \r, emitted by Docklight on PC. I am in half duplex (D+ and R+ connected, D- and R- connected).

This morning, everything starts fine: enter image description here Issuing a few echo “hello” > /dev/rs485, received on PC enter image description here Now I go back to cat /dev/rs485: nothing is read by router, but everything is strangly echoed to PC: enter image description here When I kill the cat, no stange echo anymore on PC: enter image description here Now I disconnect R+ and R- from D+ and D-. No strange echo anymore while running cat /dev/rs485. But no output from cat.

How can it be?

Thanks a lot for your ideas!