4.2 What is a Serial Port ?
Intro to Serial
The UART serial port (or just "serial port for short" is an I/O (Input/Output) device. Since modems have a serial port between them and the computer, it's necessary to understand the serial port as well as the modem.
Most PC's have one or two serial ports. Each has a 9-pin connector (sometimes 25-pin) on the back of the computer. Computer programs can send data (bytes) to the transmit pin (output) and receive bytes from the receive pin (input). The other pins are for control purposes and ground.
The serial port is much more than just a connector. It converts the data from parallel to serial and changes the electrical representation of the data. Inside the computer, data bits flow in parallel (using many wires at the same time). Serial flow is a stream of bits over a single wire (such as on the transmit or receive pin of the serial connector). For the serial port to create such a flow, it must convert data from parallel (inside the computer) to serial on the transmit pin (and conversely).
Most of the electronics of the serial port is found in a computer chip (or a part of a chip) known as a UART. For more details on UARTs see the section "What are UARTS" in the Serial-HOWTO.
But you may want to finish this section first so that you will hopefully understand how the UART fits into the overall scheme of things.
Pins and Wires
Old PC's used 25 pin connectors but only about 9 pins were actually used so today most connectors are only 9-pin. Each of the 9 pins usually connects to a wire. Besides the two wires used for transmitting and receiving data, another pin (wire) is signal ground. The voltage on any wire is measured with respect to this ground. Thus the minimum number of wires to use for 2-way transmission of data is 3. Except that it has been known to work with no signal ground wire but with degraded performance and sometimes with errors.
There are still more wires which are for control purposes (signalling) only and not for sending bytes. All of these signals could have been shared on a single wire, but instead, there is a separate dedicated wire for every type of signal. Some (or all) of these control wires are called "modem control lines". Modem control wires are either in the asserted state (on) of +12 volts or in the negated state (off) of -12 volts. One of these wires is to signal the computer to stop sending bytes out the serial port cable. Conversely, another wire signals the device attached to the serial port to stop sending bytes to the computer. If the attached device is a modem, other wires may tell the modem to hang up the telephone line or tell the computer that a connection has been made or that the telephone line is ringing (someone is attempting to call in). See the Serial-HOWTO: Pinout and Signals for more details.
Internal Modem Contains Serial Port
For an internal modem there is no 9-pin connector but the behavior is almost exactly as if the above mentioned cable wires existed. Instead of a a 12 volt signal in a wire giving the state of a modem control line, the internal modem may just use a status bit in its own memory (a register) to determine the state of this non-existent "wire". The internal modem's serial port looks just like a real serial port to the computer. It even includes the speed limits that one may set at ordinary serial ports such as 115200 bits/sec.
Unfortunately for Linux, many internal modems today use software (running on the CPU) to do much of the modem's work. Unfortunately, such software may be only available for the MS Windows OS (it hasn't been ported to Linux). Thus you can't use some of these modems with Linux See Software-based Modems (winmodems).
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