On my desktop (i7-4790K, 32 GB DDR3, Asus H97M-E) I've run out of serial ports I can usefully access via USB serial adapters*. The motherboard does have a real COM port on it, though, and just after I built the system I added an adapter cable/bracket (one of these, IIRC: https://www.canadacomputers.com/product_info.php?cPath=5_1336_1449&item_id=094057). But it's never worked as a port, and I want to make it work. It's enabled in the BIOS and Linux (Ubuntu 19.10) finds it: ttyS0 at I/O 0x3f8 (irq = 4, base_baud = 115200) is a 16550A I'm a member of `dialout`, and the ports seem to check out: $ ls -l /dev/ttyS0 /dev/ttyUSB0 crw-rw---- 1 root dialout 4, 64 May 6 21:54 /dev/ttyS0 crw-rw----+ 1 root dialout 188, 0 May 3 07:50 /dev/ttyUSB0 No data goes in or out. Curiously `stty -F /dev/ttyS0` hangs for a few seconds, but that's the only odd thing I see. Hardware serial ports are kind of ancient history. Anyone remember ancient magic to make 'em work? There's a chance that ASUS don't use the standard serial pin header assignment: I'll trace that if necessary. thanks, Stewart *: you run out of ways to identify the interfaces via /dev/serial/*. Unless I reinvested a fortune in real FTDI adapters with unique serial numbers, I'm kind of stuck. I currently use: 1 real FTDI, 1 knock-off FTDI, a QinHeng HL-340, a Prolific PL2303 and a Silicon Labs CP210. I have another knock-off FTDI and another Prolific, but they identify identically and make device selection problematic.