Serial port's modem lights up, but can't communicate

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.