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We are investigating a problem where simultaneous traffic in and out on the RUT955 RS485 interface give errors.

Our setup is a measurement and control device connected with 4-wires to the RUT955 RS485 sends ~100 bytes of data periodically every 100ms. Baudrate is 115200. Reception of data at RUT is successful and without errors. But if the RUT is sending data at the same time the recieved data is gettings lots of errors.

Terminal 1:

root@VPN089:~# cat /dev/rs485








/▒j▒▒c▒▒8▒▒                                                  ▒▒▒▒▒▒˜▒"0▒▒$▒▒▒▒▒p▒▒







Terminal 2:
root@VPN089:~# while :; do echo -n 'UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU' > /dev/rs485; done
As you can see recieved data is garbage when data is sent out on the same time.
Using a RS485/422 USB dongle connected to the same RUT does not show any garbage data.
RUT955 is using latest FW. 

1 Answer

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Did you placed Termination resistors at each cable end. Termination resistor, equal in resistance to cable characteristic impedance, must be connected at each end of the cable to reduce reflection. Refer to the link below to find out how to enable termination on the RUT955:


Best regards.


This is not related to termination issues. This is because reciever is disabled while transmitting. Which from my point of view means that RS-485 is not full duplex.

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