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Serial Ports: Difference between revisions
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Serial ports are a legacy communications port which has pretty much been succeeded by [[USB]] and other modern communications technology. However, it is much easier to program than USB, and it is still found in a lot of computers (especially older ones such as the ones the financially limited amateur OS writer might use for [[Testing|testing]]). Also, a lot of phone modems (which are still used to access the Internet quite often, even though broadband is very accessible and affordable) connect via the serial interface. Furthermore, it is possible to use the serial ports for
== Wires, Pins, Connectors and the like ==
The Wikipedia page on [[wikipedia:Serial_port|Serial ports]] has a lot of information, and it is summarised here. The serial interface is very simple. There are actually two kinds of serial port: 25-pin and 9-pin. 25-pin ports are not any better, they just have more pins and are bigger. 9-pin is smaller and is used more often though in the past the 25-pin ones were used more often. The 9-pin ones are called DE-9 (or more commonly, DB-9 even though DE-9 is
Both have the same basic types of pins. A DB-25 has most of the pins as ground pins, whereas a DE-9 has only a few ground pins. There is a transmitting pin (for sending information away) and a receiving pin (for getting information). Some serial ports can have a duplex mode--that is, they can send and receive simultaneously. There are a few other pins, used for hardware handshaking. In the past, there was no duplex mode, so if a computer wanted to send something it had to tell the other device or computer that it was about to transmit, using one of the hardware handshaking pins. The other thing would then use another handshaking pin to tell it to send whatever it wanted to send. Today there is duplex mode, but the handshaking pins are still used.
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If you want to use the serial port for communications, you first have to initialize it. You tell it how fast your connection speed between the other computer or device will be (this is called the baud rate)--you must have the same speed as the other device or computer is setup to use, or you will have problems. It is probably safer to use the slower speeds unless you need the faster speeds for some reason, for example if you are playing a multi-player game over a serial connection. You also need to setup the parity type and the number of bits in a character. Once again, your computer must be setup with the same values for these things as the other computer or device has, or communication will not work.
Once you have setup these things, you still need to setup the interrupt handlers. You can poll the port to see if any new things have arrived, or if it's time to send another character, but this slows things down and will not work very well in most real-time applications or
You use IRQ #4 for COM ports 1 or 3, and IRQ #3 for COM ports 2 or 4 (you can tell which port sent the interrupt when you receive the interrupt). The IRQ handlers check if you are receiving something, and if so they receive the character and handle it somehow, such as placing it into a buffer. They also check if the other side is ready to receive something from you, and if you have something to send, it is sent.
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