My MB has builtin UARTs (Supermicro X10SAE). I've got an external modem hooked up to /dev/ttyS0. I'm running CentOS7.

  • Without minicom running on /dev/ttyS0:

    • The modem has CTS on.
  • With minicom running on /dev/ttyS0:

    • The modem lights up RTS and DTR along with CTS.
    • Typing text into minicom does NOT light up the TX/RX lights
    • AT does nothing
    • +++ does nothing
    • ATDT does nothing

If I move the modem (on the same box!) to a usb/serial convertor, and connect minicom to /dev/ttyUSB0, the modem responds.

[root ~]#  setserial -a /dev/ttyS0
/dev/ttyS0, Line 0, UART: 16550A, Port: 0x03f8, IRQ: 4
        Baud_base: 115200, close_delay: 50, divisor: 0
        closing_wait: 3000
        Flags: spd_normal skip_test

[root ~]# setserial -a /dev/ttyUSB0 
/dev/ttyUSB0, Line 0, UART: 16654, Port: 0x0000, IRQ: 0
        Baud_base: 460800, close_delay: 0, divisor: 0
        closing_wait: infinte
        Flags: spd_normal

I called the modem while on /dev/ttyS0 and saw a blip on TX for the RING, but it did not show on the screen.

I assume I have the right port -- I even moved it to ttyS1 for comparison. No difference.

I'm stumped! Ideas? I'm beginning to wonder if the MB header to serial jack ribbon cable -- both!? -- are defective somehow.

1 Answer 1


Ah, thanks to a colleague at lunch who pointed out there isn't a single standard for serial headers.

From the X10SAE manual:

eX10SAE Serial Headers

And I see a cable store that sells crossed and straight:

* CROSSED PINOUT * 18" Serial Port Cable DB-9 Male TO 10 PIN IDC Socket

* STRAIGHT PINOUT * 18" Serial Port Cable DB-9 Male TO 10 PIN IDC Socket

I'll try hand rewiring the cables tonight, and if it works, I'll accept this answer.

  • Yup. Confirmed. I needed a crossover serial. When you take them apart the cross over has the ribbon cable alternating up/down on the db9 pins, whereas a straight cable has the ribbon going across the db9 pins across the top first, then the bottom.
    – rrauenza
    Sep 3, 2014 at 4:59
  • Another reference: scantips.com/serial-db9.html
    – rrauenza
    Sep 4, 2014 at 5:50

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