The most rudimentary form of error checking adds a bit that declares that
the sum of the bits in a particular word is either even or odd; at the receiving
end the word is summed again. This scheme works well enough if one of the
bits is garbled along the way. If more than one bit changes from a 0 to a 1, the
word’s evenness or oddness may not change.
So it would seem obvious that a parallel pathway is much, much faster than a
serial road, and that is absolutely true when it comes to short distance com-
muting from device to device on the motherboard. But as computers have
become faster and faster and move larger and larger amounts of data, parallel
circuits run into problems of physics.
Remember that I said that the 8- or 16-bit words arrive at their destination
more or less at the same time?The fact is that over distance and under the
pressure of fast clock speeds, tiny imperfections in the wires of a parallel bus
or cable cause some of the bits to arrive just a tiny sliver of time behind the
others. Engineers call this skew,and the more it occurs, the more words are
rejected. When a word is rejected, it must be re-sent; this data backup can
significantly slow the transmission speed; when there is a problem, a word
is sent, checked, rejected, requested again, sent, and checked. There’s also
the risk of interference between the lines. Either way, the result is a massive
bottleneck on westbound I-95 at the tollbooth. (Oops, I snuck one more auto-
motive metaphor into the chapter — couldn’t be helped.)
Serial ports don’t have the problem of timing arrival of computer words, and
interference between two wires is much more easily controlled. For the first
decade and a half of the PC, parallel and serial ports coexisted on desktops
and laptops; neither was fast enough, but between the two designs you could
attach a printer and a modem and get by. (More on modems in Chapter 15.)
But even if you could get a serial port to communicate with an external device
(not always as simple as you might like because of various standards for word
length, start and stop bits, error checking, and other overhead), there was this
major land mine: Each serial port in a computer needed to lay claim to a partic-
ular set of interrupts (IRQ), memory (DMA) channels, and memory resources.
If you wanted to attach four devices to a computer through serial ports, it was
theoretically possible to do so, but the chances of running into a conflict of one
sort or another was very high.
Certain devices are much more demanding of system resources than others,
sound cards and network cards among them.
Happily, though, while early PC and laptop users tried to sort all of this out
by themselves, engineers were hard at work on other means for computers to
communicate including broadband (cable and DSL phone connections) and
WiFi. And although early computers and laptops could be cajoled into poky
Chapter 16: Breaking Out of the Box: PC Cards, USB, and FireWire 241