Serial Ports

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 broke amateur OS writer/tester might use). 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.

Wires, Pins, Connectors and the like

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 it's technical name) and the 25-pin ones are called DB-25. They plug in to your computer using a female plug (unless your computer is odd and has a female port, in which case your cable will need a male plug).

Both have the same basic types of pins. Most of the pins are not used. Most of the pins are 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.

If you want to connect two computers, you need two things in your cable:

1. The cable needs to have two female plugs so it can plug into both computers.
2. The cable needs to have it's transmit-receive wires and it's handshaking wires switched. This can be done in the cable itself, or as an extension called a Null Modem

For serial devices, you don't need to setup the cable this way. The receiving end of the device has the wires switched and it has a female port, which means you can plug a male plug into it.

Why Use a Serial Port?

During the early stages of kernel development, you might wonder why you would bother writing a serial driver. There are several reasons why you might:

GDB debugging

You can use the serial port to connect to a host computer, and use the GDB debugger to debug your operating system. This involves writing a stub for GDB within your OS. Helpful information might be found at http://developer.apple.com/documentation/DeveloperTools/gdb/gdb/gdb_18.html.

Headless console

You can operate the computer without a monitor, keyboard or mouse and instead use the serial port as a console using a protocol such as TTY or VT100.

External logging

When the system itself is in danger of potentially crashing at times, it's nice to get debugging outputs safe to another computer before the test system triple-faults.

Programming the Serial Communications Port

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 multithreaded environments. In the case of a game, this is not a good idea at all.

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.

Initialization

The baud rate is set by writing a divisor value to the UART. The divisor is the fraction of 115200 that makes the baud rate. For instance, the baud rate 57600 would have a divisor of 2.

The number of bits in a character is variable. Less bits is, of course, faster, but they store less information. If you are only sending ASCII text, you probably only need 7 bits. These days you could consider 8N1 (8 bits, no parity, one stop bit) pretty much the default.

The parity type can be even, odd, or none.

#define PORT 0x3f8   /* COM1 */

void init_serial() {
   outb(PORT + 1, 0x00);    // Disable all interrupts
   outb(PORT + 3, 0x80);    // Enable DLAB (set baud rate divisor)
   outb(PORT + 0, 0x03);    // Set divisor to 3 (lo byte) 38400 baud
   outb(PORT + 1, 0x00);    //                  (hi byte)
   outb(PORT + 3, 0x03);    // 8 bits, no parity, one stop bit
   outb(PORT + 2, 0xC7);    // Enable FIFO, clear them, with 14-byte threshold
   outb(PORT + 4, 0x0B);    // IRQs enabled, RTS/DSR set
}

Receiving data

int serial_received() {
   return inb(PORT + 5) & 1;
}

char read_serial() {
   while (serial_received() == 0);

   return inb(PORT);
}

Sending data

int is_transmit_empty() {
   return inb(PORT + 5) & 0x20;
}

void write_serial(char a) {
   while (is_transmit_empty() == 0);

   outb(PORT,a);
}

Glossary

Baud Rate

is the speed at which the serial line switches between it's two states. This is not equivalent to bps, due to the fact there are start and stop bits. On an 8/N/1 line, 10 baud = 1 byte. Modems are more complex than plain serial lines due to having multiple waveforms, but for the purposes of OSDev this is irrelevant.

baud rate divisor

fastest rate a serial port can run, the number 115200.

stop bits

the NULL bit(s) sent between each character to synchronize the transmitter and the receiver.

UART

for Universal Asynchronous Receiver/Transceiver: the chip that picks a byte a send it bit per bit on the serial line and vice versa.

Related Links

• OSRC's "communication" section