User Manual

Technical Description
Sealevel Systems ULTRA COMM+422.PCI Page 10
Technical Description
The Sealevel Systems ULTRA COMM+422.PCI provides a PCI interface
adapter with 4 RS-422/485 asynchronous serial ports for industrial automation
and control applications.
The ULTRA COMM+422.PCI utilizes the 16550 UART. This chip features
programmable baud rates, data format, interrupt control and a 16-byte input and
output FIFO. A full array of advanced UARTS is also available for this card.
Contact Sealevel Systems for more information.
Interrupts
A good description of an interrupt and its 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).