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I'm having a bit of a problem building a little network-connected UPS monitor. I have two USB to Serial adapters of the pl2303 kind. Latest Raspbian and I'm running them off a tiny powered USB hub just to be safe. I set the port parameters on both /dev/ttyUSB0 and /dev/ttyUSB1 for NUT and each adapter and serial cable works when tested individually. When I connect both and try to start the NUT driver one of them always refuses to work with an error like this:

pl2303 ttyUSB0: pl2303_set_control_lines - failed: -71

Sometimes I get other errors but always -71. I haven't really been able to find any useful information about this. I tried running modprobe -r pl2303 and modprobe pl2303 but it wasn't any real help. I have replaced my old USB hub with a powered ones because I assumed that it might have caused the issues but it didn't. Does anyone have any ideas? (Also, I only seem to find adapters of the pl2303 kind locally, even from different brands, which is annoying because the "just get another type of adapter" is not exactly easily doable at the moment)

Thank you in advance!

P.S.: the issue does not seem to be related to NUT, I can trigger the error by simply trying to set the baud rate on the device that has issues like stty -F /dev/ttyUSB1 2400. It also triggers the error in syslog.

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  1. I found PL2303 USB to serial adpators good for Rpi3B.

pl2302 usb serial 1

  1. Test record.

pl2302 test 1

  1. You might like to read my old post for reference.

How many serial ports are on the Pi 3? - tlfong01 RpiSE, 2019apr19, Viewed 7k times

  1. WARNING to RpiZ/RpiZW users. I did annoyingly found that RpiZ/RpiZW's USB/serial drivers problematic, comparing with those of Rpi3B+. So I would suggest to test everything first on Rpi3/4, before moving on to RpiZ/RpiZW.

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