Technical Description - SeaLevel ULTRA COMM+422.PCI User Manual

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 16850 UART. This UART includes 128 byte FIFOs, automatic
hardware/software flow control and the ability to handle much higher data rates than the standard UARTs.
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 performed. 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 its own interrupt, frequently called
an IRQ (short for Interrupt Request).
In the early days of PCs Sealevel 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 Sealevel Systems 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 polling.' This method required the interrupt
service routine to 'poll' or interrogate each UART as to its interrupt pending status. This method of polling
was sufficient for use with slower speed communications, but as modems increased their through put
abilities this method of servicing shared IRQs became inefficient.
© Sealevel Systems, Inc. 7402 Manual | SL9063 7/2021
17