How to debug a device that changes from /dev/ttyUSB0 to /dev/ttyUSB1 at will?

Question

I have a device plugged to a CentOS server that changes randomly from /dev/ttyUSB0 to /dev/ttyUSB1 and vice versa.

The device I'm trying to connect is this one.

How can I debug this?

changes randomly? what do you mean?? i doubt that it changes while the device is plugged in. — umläute, Nov 28 '13 at 14:33
What type of device is this? You can force it to stay to a particular device using udev but something else sounds suspicious, as if the device is faulty and is getting erroneously re-detected. — slm, Nov 28 '13 at 14:48
@slm, it's a usb to RS485 converter. I'll try to force the device address on udev. Will look into that. Thanks! — gtludwig, Nov 28 '13 at 15:53
does the device get reset while plugged in? what does dmesg tell you? — umläute, Nov 28 '13 at 16:08
See this Q&A for udev resources: Where is the official udev source and other dev information? — slm, Nov 28 '13 at 17:29
You don't say what model/brand you're using but this might be useful none the less: http://www.sealevel.com/support/article/AA-00524 — slm, Nov 28 '13 at 17:33
@slm, I added a link to the device's website. Thanks for the sealevel info. — gtludwig, Nov 28 '13 at 19:01

score 1 · Accepted Answer · answered Nov 29 '13 at 22:33

The device probably gets disconnected while you're using it; check your dmesg output after it changed to /dev/ttyUSB1. When it gets disconnected, the old name /dev/ttyUSB0 is not immediately released (I'm not sure why, maybe because some program still has an open handle to the device file...?). Then the device reconnects but now the kernel can't reuse this name so it uses the next free name, so you now it's /dev/ttyUSB1.

Possible reasons for this could be a defective USB device or a defective USB port.

How to debug a device that changes from /dev/ttyUSB0 to /dev/ttyUSB1 at will?

1 Answers1