12.14 Serial Ports

The RS-232 serial communication standard is probably the most popular serial communication scheme in the world. Although it suffers from many drawbacks, speed being the primary one, its use is widespread, and there arethousands of devices you can connect to a PC using an RS-232 serial interface. Though the use of the serial port is rapidly being eclipsed by USB use, many devices still use the RS-232 standard.

The original PC system design supports concurrent use of up to four RS-232 compatible devices connected through the COM1:, COM2:, COM3:, and COM4: ports. For those who need to connect additional serial devices, you can buy interface cards that let you add 16 or more serial ports to the PC.

In the early days of the PC, DOS programmers had to directly access the 8250 Serial Communications Chip (SCC) to implement RS-232 communications in their applications. A typical serial communications program would have a serial port ISR that read incoming data from the SCC and wrote outgoing data to the chip, as well as code to initialize the chip and to buffer incoming and outgoing data. Though the serial chip is very simple compared to modern peripheral interfaces, the 8250 is sufficiently complex that many programmers would have difficulty getting their serial communications software working properly. Furthermore, because serial communications was rarely the main purpose of the application being written, few programmers added anything beyond the basic serial communications features needed for their applications.

Fortunately, today's application programmers rarely program the SCC directly. Instead, OSes such as Windows or Linux provide sophisticated serial communications device drivers that application programmers can call. These drivers provide a consistent feature set that all applications can use, and this reduces the learning curve needed to provide serial communication functionality. Another advantage to the OS device driver approach is that it removes the dependency on the 8250 SCC. Applications that use an OS device driver will automatically work with different serial communication chips. In contrast, an application that programs the 8250 directly will not work on a system that uses a USB to RS-232 converter cable. However, if the manufacturer of that USB to RS-232 converter cable provides an appropriate device driver for an OS, applications that do serial communications via that OS will automatically work with the USB/serial device.

An in-depth examination of RS-232 serial communications is beyond the scope of this book. For more information on this topic, consult your OS programmer's guide or pick up one of the many excellent texts devoted specifically to this subject.