Computer Interfacing
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Snoop rs485 traffic (Linux or windows)


       Computer Interfacing Forum Index -> RS422 and RS485
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Mar 09, 2012 1:12 am

I have recently bought one of these rs232/rs485 to usb adapters:

I am using some devices which appear to be communicating at 96008N1

I'm trying to see what the devices are sending/receiving but have had no luck so far connecting the USB adapter to the rs485 circuit. It seems the presence of the USB adapter on the rs485 line breaks the communication of the devices.

I have tried D- to A and D+ to B (and also the other way around), and ground connected to ground, have used minicom and set the parameters to 96008N1.

Anyone have any ideas or trouble-shooting tips?

Many thanks Smile

Mar 09, 2012 4:48 pm

Turns out it was mainly the lack of terminating resistors that was the problem. The manual said to use 330 ohms for less than 300m of cable and 220 ohms for less than 600m of cable etc. I had a couple of 360 ohm resistors (the cables are very short) and these seem to have done the trick. The original setup was working fine without resistors, even with multiple devices but adding my usb-485 adapter seems to have made the use of terminating resistors a necessity.

I also had to play quite a bit with stty settings but can now watch what I think is the traffic just by watching /dev/ttyUSB0. Smile

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