I am running Ubuntu 14.04.1 x64 with 3.16 kernel. My computer has 2 onboard serial ports which work well and 2 serial ports added by a PCI card which work from time to time.

This is the card

Sometimes after booting it seems like one port of these is not initialised correctly, namely ttyS4. I am not able to access it but at other times it works well. I looked into /var/lib/setserial/autoserial.conf but that looked okay, it adds the PCI serial ports as ttyS4 and ttyS5. IRQ is correct, skip_test is activated.

When the error occurs I am not able to change anything on the port with setserial. It says

/dev/ttyS4: Device or resource busy. 

I have to reboot to solve the error.

It's not only this card or this PC, I have experienced similiar problems (only with addon cards, onboard ports work well) on other systems.

The ports are being used by an emulator.

Any ideas? Or any PCI cards you can recommend that work well with Ubuntu?

I found this on tldp.org but it seems a bit outdated and couldn't help me.

Edit: I have found out that it is an IRQ conflict. I read /proc/interrupts and it says

22: 15 14 IO-APIC-fasteoi uhci_hcd:usb7

which can't be right as I want my serial addon card to use that IRQ. So does anybody know how I may give my serial device the IRQ?


The modemmanager app was causing problems for me. After uninstalling it my problem was solved.

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