hello to everybody I have Ubuntu Lucid 10.04 installed on my Desktop and I have a PCI serial card in my computer and it has only one serial port. Actually I want to use this serial port for the serial communication and for that I executed this command : $ setserial -g /dev/ttyS[0123] and I got this as output: /dev/ttyS0: Device or resource busy /dev/ttyS1, UART: undefined, Port: 0xee00, IRQ: 16 /dev/ttyS2, UART: undefined, Port: 0xed00, IRQ: 17 /dev/ttyS3, UART: unknown, Port: 0xec00, IRQ: 17 in this it is showing that my serisl port named /dev/ttyS0 is busy and can't be used for the serial communication. But it is happening from past couple of days, and before that everything was just fine and on the execution of the previous command it was giving this as output: /dev/ttyS0, UART: undefined, Port: 0xef00, IRQ: 16 /dev/ttyS1, UART: undefined, Port: 0xee00, IRQ: 16 /dev/ttyS2, UART: undefined, Port: 0xed00, IRQ: 17 /dev/ttyS3, UART: unknown, Port: 0xec00, IRQ: 17 So now what is this happening ........................... I also googled a lot and got various hints but nothing worked well , and I tried to know whether the interrupt no. 16 being used by some other device than for that I checked the /proc/interrupts file and got the following result: CPU0 CPU1 0: 69 0 IO-APIC-edge timer 1: 43 5783 IO-APIC-edge i8042 8: 1 0 IO-APIC-edge rtc0 9: 0 0 IO-APIC-fasteoi acpi 12: 455224 0 IO-APIC-edge i8042 14: 0 0 IO-APIC-edge ata_piix 15: 0 0 IO-APIC-edge ata_piix 16: 891 121426 IO-APIC-fasteoi uhci_hcd:usb5, i915, HDA Intel, serial 17: 4 0 IO-APIC-fasteoi 18: 0 0 IO-APIC-fasteoi uhci_hcd:usb4 19: 149907 0 IO-APIC-fasteoi ata_piix, uhci_hcd:usb3 20: 56 49397 IO-APIC-fasteoi ohci1394, eth0 23: 478297 0 IO-APIC-fasteoi ehci_hcd:usb1, uhci_hcd:usb2 NMI: 0 0 Non-maskable interrupts LOC: 1844145 1720957 Local timer interrupts SPU: 0 0 Spurious interrupts PMI: 0 0 Performance monitoring interrupts PND: 0 0 Performance pending work RES: 74493 98624 Rescheduling interrupts CAL: 90 128 Function call interrupts TLB: 7221 7432 TLB shootdowns TRM: 0 0 Thermal event interrupts THR: 0 0 Threshold APIC interrupts MCE: 0 0 Machine check exceptions MCP: 23 23 Machine check polls ERR: 1 MIS: 0 ~ ~ ~ ~ In this file I saw that the serial port and the Intel HDA both are using the same interrupt no 16 and therefore I think the serial port is not responding or is busy. I checked the other serial port /dev/ttyS1 with the same IRQ 16 and executed this command : $ setserial /dev/ttyS1 -a and got the following result: /dev/ttyS1, Line 1, UART: undefined, Port: 0xee00, IRQ: 16 Baud_base: 115200, close_delay: 50, divisor: 0 closing_wait: 3000 Flags: spd_normal skip_test So this was free and I tried to use it. So I tried to switch over to other serial port by running the following command: $ setserial /dev/ttyS1 uart 16550A $ setserial /dev/ttyS1 -a and the result was this: /dev/ttyS1, Line 1, UART: 16550A, Port: 0xee00, IRQ: 16 Baud_base: 115200, close_delay: 50, divisor: 0 closing_wait: 3000 Flags: spd_normal skip_test So I tried to use it and when I did this the system gets shutdown. I tried this with the other two ports also but got nothing working and the system shuts down in each case and the result obtained all the time was same : $ setserial /dev/ttyS0 -a /dev/ttyS0: Device or resource busy So I want to know how I can troubleshoot this problem .......please help me out of this . Thank you