Hardware manual

Each serial port has a "file" associated with it in the /dev directory. It isn't really a file but it seems like one.
For example, /dev/ttyS0. Other serial ports are /dev/ttyS1, /dev/ttyS2, etc. But ports on the USB bus, multiport
cards, etc. have different names.
The common specification for the conventional serial port is RS-232 (or RS-232). So it's often called a
"RS-232 serial port". The connector(s) for the serial port are often seen as one or two 9-pin connectors (in
some cases 25-pin) on the back of a PC. But the serial port is more than just connectors. It includes the
associated electronics which must produce signals conforming to the RS-232 specification. See Voltage
Waveshapes. One pin is used to send out data bytes and another to receive data bytes. Another pin is a
common signal ground. The other "useful" pins are used mainly for signalling purposes with a steady negative
voltage meaning "off" and a steady positive voltage meaning "on".
The UART (Universal Asynchronous Receiver-Transmitter) chip does most of the work. Today, the
functionality of this chip is usually built into another chip. See What Are UARTs? These have improved over
time and old models (prior to say 1994) are usually very obsolete.
The serial port was originally designed for connecting external modems to a PC but it's used to connect many
other devices also such as mice, text-terminals, some printers, etc. to a computer. You just plug these devices
into the serial port using the correct cable. Many internal modem cards have a built-in serial port so when you
install one inside your PC it's as if you just installed another serial port in your PC.
2. Quick Help
This repeats more detailed information found elsewhere. If your computer can't seem to find your serial port
and you already know something about hardware resources (addresses like 3F8 and IRQs like 5) then try this:
First, get into the BIOS (often called "setup") when the computer is powered on by pressing certain keys. To
find out what keys to press, watch the screen as your PC starts up. If the words that flash by on the screen too
fast to read, freeze them by holding down the "pause" and "shift" keys at the same time. Then when read, hit
any key to resume (cease pausing) and hold down the key(s) required to enter the BIOS setup. You may have
to try this again since there may be more than one screen which you can freeze with the "pause" key. Also,
look for messages about the serial ports on these frozen screens.
Once in the BIOS menus, try to find menus dealing with the serial port. They could be shown in a menu
dealing with Resources, Plug-and-Play, Peripherals, Ports, etc. Some old BIOSs setups (before 1995 ?) didn't
deal with the serial ports. Make sure the ports you need are not disabled and note how they are configured
(like 3F8 IRQ 4). You may need to change the configuration to prevent conflicts. There could be a shortage of
IRQs if the BIOS has reserved some IRQs that it didn't need to reserve.
For serial ports to be found, either the kernel must have been compiled with serial support, or serial support
must be provided by a module. To check this look in the file /boot/config-2.6... and search for SERIAL. =m
means it's a module and you may check to see the modules that are being used by typing: lsmod.
3. How the Hardware Transfers Bytes
Below is an introduction to the topic, but for a more advanced treatment of it see FIFOs.
Serial HOWTO
1.6 What is a Serial Port? 8