OK, I understand that items altered in Grub at boot time are temporary. But it is confusing to me why the suggested change to the file (permanent) is different ( "pci=nocrs" and "GRUB_CMDLINE_LINUX="8250.nr_uarts=5 pci=nocrs" ). Do they actually have the same result? The "pci=nocrs" is without the uarts=5... My motherboard is fairly new design, I think. Here is some info from LSHW : roger-desktop description: Desktop Computer product: (To be filled by O.E.M.) width: 64 bits capabilities: smbios-2.7 dmi-2.7 vsyscall32 configuration: boot=normal chassis=desktop family=To be filled by O.E.M. sku=To be filled by O.E.M. uuid=876A4EBE-6FBD-11E1-BD11-0011118582DC *-core description: Motherboard product: DZ77BH-55K vendor: Intel Corporation physical id: 0 version: AAG39008-400 serial: BQBH210000JM *-firmware description: BIOS vendor: Intel Corp. physical id: 0 version: BHZ7710H.86A.0057.2012.0208.1904 date: 02/08/2012 size: 64KiB capacity: 6336KiB capabilities: pci upgrade shadowing cdboot bootselect socketedrom edd int13floppy1200 int13floppy720 int13floppy2880 int5printscreen int9keyboard int14serial int17printer acpi usb biosbootspecification *-cpu description: CPU product: Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz *-pci description: Host bridge product: Ivy Bridge DRAM Controller vendor: Intel Corporation physical id: 100 bus info: pci@0000:00:00.0 version: 09 width: 32 bits clock: 33MHz *-pci description: PCI bridge product: Integrated Technology Express, Inc. vendor: Integrated Technology Express, Inc. physical id: 0 bus info: pci@0000:03:00.0 version: 30 width: 32 bits clock: 33MHz capabilities: pci pm subtractive_decode bus_master cap_list resources: ioport:d000(size=4096) memory:f7d00000-f7dfffff *-communication description: Serial controller <------- product: 56K FaxModem Model 5610 vendor: 3Com Corp, Modem Division physical id: 1 bus info: pci@0000:04:01.0 version: 01 width: 32 bits clock: 33MHz capabilities: pm 16550 cap_list configuration: driver=serial latency=0 resources: irq:17 ioport:d000(size=8) <------- *-serial UNCLAIMED <------- ??? description: SMBus product: Panther Point SMBus Controller vendor: Intel Corporation physical id: 1f.3 bus info: pci@0000:00:1f.3 version: 04 width: 64 bits clock: 33MHz configuration: latency=0 resources: memory:f7f35000-f7f350ff ioport:f000(size=32) From DMESG : [ 1.244883] Serial: 8250/16550 driver, 32 ports, IRQ sharing enabled [ 1.380227] serial 0000:04:01.0: PCI INT A -> GSI 17 (level, low) -> IRQ 17 [ 1.401196] 0000:04:01.0: ttyS4 at I/O 0xd000 (irq = 17) is a 16550A [ 13.742194] snd_hda_intel 0000:01:00.1: PCI INT B -> GSI 17 (level, low) -> IRQ 17 also [ 1.072912] ACPI: PCI Interrupt Link [LNKA] (IRQs 3 4 5 6 10 *11 12 14 15) significant that no 17 listed in this or similar lines? /proc/interrupts - WTH minicom open. 17 was NOT mentioned before I opened minicom, or after I closed minicom. CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7 0: 66 0 0 0 0 0 0 0 IR-IO-APIC-edge timer 1: 3 0 0 0 0 0 0 0 IR-IO-APIC-edge i8042 8: 1 0 0 0 0 0 0 0 IR-IO-APIC-edge rtc0 9: 0 0 0 0 0 0 0 0 IR-IO-APIC-fasteoi acpi 12: 4 0 0 0 0 0 0 0 IR-IO-APIC-edge i8042 16: 200002 0 0 0 0 0 0 0 IR-IO-APIC-fasteoi ehci_hcd:usb1 17: 0 0 0 0 0 0 0 0 IR-IO-APIC-fasteoi serial 18: 0 0 0 0 0 0 0 0 IR-IO-APIC-fasteoi firewire_ohci 23: 126894 0 0 0 0 0 0 0 IR-IO-APIC-fasteoi ehci_hcd:usb2 40: 0 0 0 0 0 0 0 0 DMAR_MSI-edge dmar0 42: 62850 0 0 0 0 0 0 0 IR-PCI-MSI-edge ahci 43: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge ahci 44: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 45: 1 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 46: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 47: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 48: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 49: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 50: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 51: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 52: 0 0 0 0 0 0 0 0 IR-PCI-MSI-edge xhci_hcd 53: 16594 0 0 0 0 0 0 0 IR-PCI-MSI-edge eth0 54: 15 0 0 0 0 0 0 0 IR-PCI-MSI-edge mei 55: 466 71 0 0 0 0 0 0 IR-PCI-MSI-edge snd_hda_intel 56: 72300 0 0 0 0 0 0 0 IR-PCI-MSI-edge fglrx[0]@PCI:1:0:0 NMI: 196 123 94 109 25 22 26 34 Non-maskable interrupts LOC: 257044 150281 182654 156581 38352 36071 54757 47418 Local timer interrupts SPU: 0 0 0 0 0 0 0 0 Spurious interrupts PMI: 196 123 94 109 25 22 26 34 Performance monitoring interrupts IWI: 0 0 0 0 0 0 0 0 IRQ work interrupts RES: 405966 392101 359264 421184 32128 34989 47522 47344 Rescheduling interrupts CAL: 706 1049 1166 1187 1172 1232 1144 1188 Function call interrupts TLB: 2826 1937 2048 3408 1009 968 1656 1544 TLB shootdowns TRM: 0 0 0 0 0 0 0 0 Thermal event interrupts THR: 0 0 0 0 0 0 0 0 Threshold APIC interrupts MCE: 0 0 0 0 0 0 0 0 Machine check exceptions MCP: 25 25 25 25 25 25 25 25 Machine check polls ERR: 0 MIS: 0 All this is part of a big learning curve for me. Help in interpreting it is greatly appreciated. I haven't yet tried "GRUB_CMDLINE_LINUX="8250.nr_uarts=5 pci=nocrs" in /etc/default/grub, for fear that I may make the machine unbootable. I haven't yet tried "irqpoll", because the above line seems by my guess to be a better bet, but I don't want to wreck things. Also, is it just a boot that it will be slower? Much slower? From your last paragraph, I have tried calling the modem from another line. It will try to answer, but I don't have another modem to try to connect with. How should I look to see if see kernel complains about "nobody cares" on the terminal? If you mean in something like minicom, there are no result codes or anything else shown - just nothing.