Omega Vehicle Security OMG-COMM232-PCI - Manual (Page 12)

Technical Description
Omega Engineering COMM+232.PCI Page 8
Technical Description
The Omega Engineering OMG-COMM+232.PCI provides a PCI interface adapter
with 2 asynchronous serial ports providing a versatile interface for modems,
printers and plotters.
The OMG-COMM+232.PCI utilizes the 16550 UART. This chip features
programmable baud rates, data format, interrupt control and a 16-byte input and
output FIFO. Also available as an option is the 16C650 UART that provides a
deeper FIFO (32 bytes) and enhanced clocking features.
Interrupts
A good description of an interrupt and it’s importance to the PC can be found in
the book ‘Peter Norton’s Inside the PC, Premier Edition’:
“ One of the key things that makes a computer different from any other kind of
man-made machine is that computers have the capability to respond to the
unpredictable variety of work that comes to them. The key to this capability is a
feature known as interrupts. The interrupt feature enables the computer to
suspend whatever it is doing and switch to something else in response to an
interruption, such as the press of a key on the keyboard.”
A good analogy of a PC interrupt would be the phone ringing. The phone ‘bell’
is a request for us to stop what we are currently doing and take up another task
(speak to the person on the other end of the line). This is the same process the
PC uses to alert the CPU that a task must be preformed. The CPU upon receiving
an interrupt makes a record of what the processor was doing at the time and
stores this information on the ‘stack’; this allows the processor to resume its
predefined duties after the interrupt is handled, exactly where it left off. Every
main sub-system in the PC has it’s own interrupt, frequently called an IRQ (short
for Interrupt ReQuest).
In these early days of PC’s Omega Engineering decided that the ability to share
IRQs was an important feature for any add-in I/O card. Consider that in the IBM
XT the available IRQs were IRQ0 through IRQ7. Of these interrupts only IRQ2-5
and IRQ7 were actually available for use. This made the IRQ a very valuable
system resource. To make the maximum use of these system resources Omega
Engineering devised an IRQ sharing circuit that allowed more than one port to
use a selected IRQ. This worked fine as a hardware solution but presented the
software designer with a challenge to identify the source of the interrupt. The
software designer frequently used a technique referred to as ‘round robin