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Even Computers Make Mistakes
 Inside your PC, millions of electrical signals,
traveling at nearly the speed of light, are sent and received every second.
Every minute, bit by the billions flow among your computer's CU, RAM, static
RAM (SRAM), adapter cards and peripherals. The complexity of this traffic
is beyond human comprehension.
Ever wonder what would happen if any of this data were somehow garbled during transmission? What would be the consequences if a single electrical pulse were relayed incorrectly?
Its no big deal if a pixel on your screen appears one imperceptible shade
bluer than it should be.
A moment later, the pixel is redrawn as screen
information changes and the error disappears. But a one bit error, could
easily change a $100 transaction to a $1 million one--or reduce it to zero.
If either event happens, while you're balancing your checkbook, it's a
very big deal. Data errors can also make programs crash or behave erratically,
and cause your computer to display almost error message in it's repertoire.
The most common form of data error detection is parity checking.
This technique ensures the integrity of data stored on hard drives
and floppy disks and sent via serial ports, and is often used to detect
errors in RAM. Parity checking requires that each byte of data carry an
extra bit, called the parity bit. The byte's original eight data bits determine
the value of the extra bit whenever new data is stored.
For instance, if the number of l's , found in the original eight bits
is odd (1, 3, 5 or 7), the parity bit will be set to 1. If an even number
of 1's (0, 2, 4, 6 or 8) appears in the original eight bits, the parity
bit will be set to 0. As a result, there is always an even number of l's
in the entire group of nine bits (eight data bits plus the parity bit).
That's why this parity checking technique is called "even parity." Another
technique, "odd parity," manipulates the parity bit to guarantee the number
of l's in a byte is always odd.
You've probably already figured out how the parity bit allows you to
detect errors. If the value of a data bit changes from 0 to 1, or 1 to
0-the value of the parity bit must change, too. If your computer encounters
a byte with a parity bit whose value is inconsistent with those of the
other eight bits, it knows an error has occurred.
Unfortunately, that's all your computer knows. One bit is in error,
but your machine can't determine which of the byte's bits has changed.
Simple even and odd parity checking can also be fooled if more than one
bit changes value. If the net effect of the changes doesn't invalidate
the parity bit's setting, your computer will trust corrupt data.
 To make matters worse, parity checking can actually increase the number of data errors that occur. Parity checking requires 12.5 percent more bits to store and transmit a given amount of data. And the more bits and corresponding circuitry, wires and connectors, the greater the chance that something will go wrong.
This, coupled with the fact that a computer can't recover from an error caught by simple parity checking, indicates this type of checking isn't worth the cost.
Thus the need for more elaborate error-detection schemes. All of them rely on more than one extra bit. Some schemes allow the computer to detect all errors involving 1 or 2 bits, and even correct single-bit errors by identifying the balky bit. Fancier techniques that require even more extra bits can detect and correct more extensive errors.
Unfortunately, all the extra bits make these error-detection and -correction
methods expensive. For now, you'll find effort-correction circuits only
inside your hard drive and in the RAM circuits of certain very expensive
computers. In an attempt to shave costs, manufacturers are removing even
simple parity checking of RAM from many computer models. An extra 12.5
percent of RAM is enough to make a noticeable difference in a PCs final
selling price.
Nobody's Perfect
That unchecked RAM can cause a variety of problems. You might experience
random program crashes, strange messages or images appearing on the screen
or printed pages, or mysterious random hangs. If this sounds like an episode
of This Is Your Life, here are several things you can do (all with
the power off and your computer unplugged):
- If your computer uses SIMMS, remove and then reinsert each unit.
- When you handle SIMMS, keep one hand or arm in constant contact with your
computer's case to prevent static electricity from damaging the RAM.
- If your computer uses RAM chips, firmly press down on the top of each chip
to make sure it's securely inserted in its socket.
- If your computer uses RAM chips, firmly press down on the top of each chip
to make sure it's securely inserted in its socket.
-
Adjust the BIOS settings in CMOS memory that affect RAM access. Increase
the number of RAM wait states as well cache read and write wait states.
- Swap half of your SIMM's or chips with reliable RAM from another computer.
If the problem goes away, 0 of the SIMM's or chips you've just removed
may be bad. To find out which one, reinsert the chips, replacing a reliable
one with a different chip each time u you find the culprit. If the problem
still doesn't go away, the group of SIMM chips you didn't swap may include
a troublemaker Repeat the process, swapping the other half of the memory
group. Continue until you've confirmed that all is working properly.
- Disable your RAM cache, if your computer's BIOS setup program allows you
to do so
- Try each of these suggestions one at time until you've solved the problem
Adapter card culprits
Adapter cards are responsible for surprisingly large number of data
error Besides being complex circuits in the own right, adapter cards act
as a crossroads for many cables and plug into larger sockets. Both factors
are likely sources data error.
In most cases, the data flowing to a from adapter cards isn't checked
for errors. As a result, data miscues involving adapter cards often go
undiagnosed for long time. Symptoms include disk corruption and random
program crashed unreliable modem or printer connections, poor sound from
your sound car stray pixels on your screen and other bizarre behavior.
If one or more of these problems affects your computer, here are sortie
steps that may help:
- Remove and then reattach all cable Remove and then reinsert all adapt cards.
- While the cards are out of the sockets, use a slightly damp soft cloth
tissue to remove lint and dirt from the cards and sockets. You can use
water or alcohol to wet the cloth or tissue. Let everything dry completely
before you reinsert the adapter cards.
- Clean the cards and cable connectors, and apply Stabilant 22 to card, cable
and socket contacts
- Adjust the BIOS settings in CMOS memory that affect your adapter-card bus
speed. All cards will work at 8 MHz or 10 MHZ, but some may not be able
to respond quickly enough to operate reliably at higher bus speeds, such
as 12 MHz or 16 MHz. In most cases, you can reduce the bus speed by increasing
a divisor used to derive the bus clock from the CPU clock.
Handle the cards carefully, making sure they don't suffer damage from static electricity. And be sure to perform these steps only when your computer is off and unplugged. I don't want you to experience any high-voltage-induced data errors of your own!
Keep in Contact
Here are some ways to put the spark back in your system
Electrical contacts occur when signal-carrying metal meets another
piece of metal. In computers, these contacts include places where the edge
of a SIMM meets the prongs of a SIMM socket, and where the pins of a chip
meet the metal receptacles inside a chip socket. Other contacts occur where
cords carrying electrical power plug into hard drives and other internal
components; where cables connect to your drives and adapter cards; and
where the edges of adapter cards meet those 8-, 16- and 32-bit card sockets
along the back of your PC.
Normally, electrical signals jump from one metal surface to another
without a hitch. But if a surface is dirty or corroded, the signal may
become degraded. This can cause a number of problems, from intermittent
data errors to the apparent failure of a drive, adapter card or chip.
If you suspect you're experiencing such a failure to communicate, there
are ways to put the spark back in your system. First, clean the metal surfaces
that meet at the corrupted contact point. The metal fingers on the edges
of adapter cards and some hard drives are easy to clean. just take a rubber
eraser and gently "erase" the contact surface. The white synthetic erasers
sold in art supply stores are best-they don't leave as many crumbs. Avoid
very abrasive erasers, and be careful not to rub too hard or for too long.
You want to remove the dirt and corrosion, not the metal underneath.
You can also clean many contacts with alcohol. Pure isopropyl alcohol
is best because it doesn't leave a residue when it dries, but it's hard
to find. Ordinary rubbing alcohol, the kind you find in any drugstore,
will do in a pinch. If the contact surface is hard to reach or attached
to your motherboard or a card, wet a cotton or foam swab and gently wipe
the contact's surface. Repeat the procedure, using a new swab for each
treatment, until the swab comes out clean. If you're cleaning cables or
other contacts that you can remove from the computer, you may be able to
rinse the contact with alcohol to remove large amounts of debris.
It's not always possible or prudent to clean a contact. For instance,
the contacts inside adapter card sockets are hard to reach.
And the risk of damaging chips by removing them from their sockets,
often outweighs the benefits of cleaning their contacts. But even when
a contact bath is out of the question, there's still something you can
do to improve a connection
A small company named D.W. Electrochemicals (905-508-7500) fax (905-508-7502)
has developed a remarkable liquid called Stabilant 22 that allows even
dirty contacts to perform properly. Stabilant 22 is an organic compound
that allows electricity to flow where it should, not where it shouldn't.
For instance, within your computer, Stabilant enables signals to travel
from one contact surface to another, but not between adjacent pins on a
chip,
Stabilant is a great conductor.
How does Stabilant pull off this trick? The explanation's a bit technical
but for the hard-core techies and terminally curious, here goes:
Normally, Stabilant is an insulator. But in the presence of a large
electric-field-gradient, it becomes an excellent conductor. An electric
field gradient is the "slope" of an electric field. It indicates to what
degree voltage change over distance (voltage difference between two surfaces,
divided by the distance between surfaces). Within your computer, distance
between a pin and a socket is so small that the gradient very large (on
the order of thousands of volts per inch), causing the liquid to become
a conductor. But the distance between adjacent contacts is great enough
to keep the gradient low (on the order of tens volts per inch)-well below
the level Stabilant needs to make t transition from insulator to conductor.
The diluted form of Stabilant 22, called Stabilant 22A, is best for
most computer uses. Apply a drop to the pins of a chip while t is still
in its socket, and the liquid will penetrate the contacts. 0 an eyedropper
or swab to apply Stabilant to contacts inside edge card sockets, cables,
and drive power and cable connectors. You need only a single drop, just
enough to cover the contact surfaces to a depth of 1 or 2 mills, (about
4 to 8 hundredths of an inch).
Contributing Editor Karen Kenworthy of Visual Basic for Applications,
Revealed! (Prma Publishing, 1994) and the manager of WINDOWS Magazine forums
on Amrica Online and CompuServe. Contact Karen's "Power Windows" topic
of these areas or care of the editor at the Windows Magazine addresses,
typically on page 18.
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