Operation Manual
Human-computer interfacing
127
Notes:
Serial and parallel data
Digital data flows in and out of a computer in one of two ways – in serial or in
parallel. On a parallel interface, there is one data channel for each bit of data that
is input. Think of it as having lots of lanes on a motorway. On a serial interface,
there is just one data channel for each direction (Transmit and Receive), which
every single data bit travels along, one after the other. Think of it like a single road.
Parallel interfaces are simpler to design and use, but require more hardware and
data cable to construct (much like the motorway taking up more space than
the road).
Interrupts, polling and multitasking
Now we know the basics of how information (data) flows in and out of a computer,
we have to know how to handle the data that is coming in when it arrives. Consider
this situation: You are working on your computer but, at the same time, you
are also expecting one or more phone calls. Think of the phone call as a source
of input data. There are three ways to be able to deal with both tasks at the
same time:
1. Work on the computer until you are interrupted by the phone ringing. Answer
the phone then continue working on your computer.
2. Work on the computer for a minute. Pick up the phone and check to see if there
is someone on the other end. Put the phone down then do another minute of
work on the computer. Check the phone again. Repeat this all day!
3. Work on the computer yourself and get another person to take phone calls
for you.
The first method is an example of using “interrupts”. When on the phone, you
could write down a note for yourself so that you can prioritise any work that occurs
as a result of the phone call in a timely manner. Using interrupts is the most
efficient use of time with infrequent events. There is, however, a disadvantage to
using interrupts: it requires low-level hardware or software (ringtone on the phone)
to be available.
In serial data signals,
there are two
information streams:
transmit (Tx) and
receive (Rx).
In a parallel signal,
many bits of data
can be sent or received
simultaneously.
SerialParallel