In olden days, PCs came with a serial card installed. Later on, the serial function was put on the hard-drive interface card. Today, one or two serial ports are usually built into the motherboard (on-board). Most of them (as of 2002) use a 16550 but some use 16650 (32-byte FIFOs). But one may still buy the individual PC serial cards if they need more serial ports. They can be used to connect external serial devices (modems, serial mice, etc...). Only a tiny percentage of retail computer stores carry such cards. But one can purchase them on the Internet. Before getting one for the PCI bus, make sure Linux supports it.
Here's a list of a few popular brands:
- Byte Runner (may order directly, shows prices) http://www.byterunner.com
- SIIG http://www.siig.com/products/io/
- Dolphin http://www.dolphinfast.com/sersol.html
Note: due to address conflicts, you may not be able to use /dev/ttyS3 (tts/3) with a IBM8514 video card (and some others) simultaneously. See Avoiding IO Address Conflicts with Certain Video Boards