Technical information

What To Do First Serial Solutions
Interrupt Selection._________________
For reliable serial communication each serial port needs its
own interrupt line. In many ways the interrupt is like the bell on
a telephone. When someone is trying to make a call to us our
telephone rings. The bell ringing allows us to stop what we are
doing temporarily, pick up the phone and deal with the person
calling us. After we put the phone down we can carry on where
we previously left off. Imagine what it would be like if
telephones did not have a bell that rang when an incoming call
was received. We would have to constantly pick up the handset
and say "Hello. Anyone there?", just in case we had a caller.
Most of our time would be spent looking for calls that were not
there. Often we would miss incoming calls when we were busy
elsewhere or just forgot to check.
Whenever we attempted to send an outgoing call we would
waste a lot of time waiting for the person on the other end to
pick up the phone and check to see if anyone was there. Most of
the time we would fail to get through and so they would miss
out on vital information we wanted to pass to them.
The interrupt line connected to the serial port allows the
PC to receive data even if the PC is busy doing other things.
The interrupt sets off an alarm in the PC causing it to break off
what it is doing, grab the incoming data, storing it safely for
later, and then continue where it had left off.
If two or more devices are connected to the same interrupt
line then the alarm bell in the PC often does not work correctly.
One device is trying to set the interrupt bell off whilst another
wants it on, the result is unpredictable and data is usually lost.
For reliable serial communication each serial port needs its
own interrupt line. The PC has few only a interrupt lines
available so adding lots of ports can pose problems.
Multiport cards overcome this problem by having special
circuitry on board that prioritises the different ports interrupts
and present a single interrupt to the PC, so using few of the
PC’s precious resources. See below.
Chapter 1 Page 7