Hardware Reference
In-Depth Information
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 ad-
dresses(called
ports
)andinterrupts(called
IRQs
).ThestandardIRQandI/Oportresource
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 ori-
ginalpurpose,parallelportsbecamemoreusefulovertheyearsasamoregeneral-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 sonamed because they have eight lines forsending all the bits that com-
prise 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
