Hardware Reference
In-Depth Information
Serial Port Configuration
Each time a serial port receives a character, it has to get the attention of the computer by raising an
IRQ line. Eight-bit ISA bus systems have eight of these lines, and systems with a 16-bit ISA bus have
16 lines. The 8259 interrupt controller chip or equivalent usually handles these requests for attention.
In a standard configuration, COM1 uses IRQ4, and COM2 uses IRQ3. Even on the latest systems, the
default COM port assignments remain the same for compatibility with older software and hardware.
When a serial port is installed in a system, it must be configured to use specific I/O addresses (called
ports
) and interrupts (called
IRQs
). The standard IRQ and I/O port resource settings for serial ports
are shown in
Table 14.14
.
Table 14.14. Standard Serial I/O Port Addresses and Interrupts
Parallel Ports
Parallel ports normally are used for connecting printers to a PC. Because of that they are often
referred to as Line Print Terminal (LPT) ports. Even though that was their sole original purpose,
parallel ports became more useful over the years as a more general-purpose, relatively high-speed
interface between devices (when compared to serial ports). Today, however, USB ports have largely
replaced parallel ports for printing and for interfacing to devices such as scanners and external
drives. I recommend using USB devices in place of parallel devices, but if you support older OSs or
older printers, you might still need to work with parallel ports and devices.
Parallel ports are so named because they have eight lines for sending all the bits that comprise 1 byte
of data simultaneously across the eight wires.
Table 14.15
shows the pinout for a standard PC
parallel port.
Table 14.15. Pinout for a 25-Pin PC-Compatible Parallel Port Connector
