Question

I'm trying to read data through the RUT955's  rs485 serial port from another device.  As that didn't work I connected D_P to R_P and D_N to R_N ( as labelled in https://wiki.teltonika.lt/view/RUT955_RS232/RS485#RS485_connector_pin-out).  I opened two ssh terminals to the teltonika.  In one I ran 'cat /dev/ttyATH0' and in the other I ran 'echo "Some text" >/dev/ttyATH0'.  No text appeared in the second terminal but with an oscilloscope I can see a signal is produced.

Note: I used /dev/ttyATH0 as /dev/rs485 is a link to it and I was eliminating that as a failure point.

So can I do a loopback test on this port?

EDIT: If I connect pins 2 and 3 on the RS232 port I can echo and cat messages between terminal windows with the console setting.  Why does this not work on the RS485 with the appropriate connections?  I have tried this on more than one RUT955 now.  Are they all faulty?  how can I troubleshoot this problem?

Answer 1

Loopback test will not work because RS485 unlike RS232 works in half-duplex mode (to support two wire mode) and the receiver is disabled while transmitter is active. To troubleshoot your problem check the wiring (is the ground connected?) and serial device settings- baud rate, parity, etc.

Comments

Thanks for this answer, after several failed experiments and much additional research last week, I had arrived at the same conclusion!  There's an interesting article here about writing a driver for this and bemoaning the fact that receive is often disabled during transmit.

Answer 2

Hi,

I'm not sure, did you use two RUT955 connected to each other via RS485? Or you have connected another RS485 device to router?

Please check attached configuration example, perhaps they could help to you.

https://community.teltonika-networks.com/?qa=blob&qa_blobid=9350254562591128103

https://community.teltonika-networks.com/?qa=blob&qa_blobid=8762858677529852638

Comments

No, I am trying to perform a loopback test as described here for example http://www.ni.com/tutorial/3450/en/  .  There is only one teltonika involved in this test and the connections are wired as described using the teltonika manual's labels.  I think more conventionally the labels would be Tx+ to Rx+ and Tx- to Rx-.  I'm afraid the two configs you linked to are irrelevant in this situation, the port is configured to be in console mode.