Hacking the Xbox

Chapter 2 - Thinking Inside the Box 37
What Are All These Resistors and
Capacitors Doing on a Digital
Circuit Board?
A motif you will notice on many circuit boards is a preponderance
of resistors and capacitors. Capacitors are everywhere
because they help keep noise to a minimum and
stabilize the power supply voltages. They are required because
the copper planes used to distribute power have a
small amount of resistance and inductance. These small
parasitics can cause big problems when a large amount
of current is switched through the power supply. The exact
placement and selection of capacitors is considered a bit
of a black art. If you happen to knock off one of the tiny,
sand-grain sized capacitors on a circuit board while working
on it, chances are that you’ll be able to get away
without replacing it. However, given a defect of this kind,
the most likely problem you will encounter are intermittent
reliability problems.
While capacitors are everywhere to provide local storage
of energy for all the components, the resistors remove excess
energy. Fast signals on a motherboard carry a lot of
energy, and if the energy is not dissipated at the receiver in
a controlled fashion with a resistor, the signal energy will
reflect back to the transmitter and cause problems. The
phenomenon is similar to that of sound in a gymnasium.
When you speak in an empty gymnasium, there is an echo.
If you speak too fast, people will be unable to understand
you because the echo will start interfering with your speech.
However, if you cover the gymnasium walls with foam, the
echo will be absorbed by the foam and you can talk without
interference from your echo.
Resistors are like the acoustic foam you would put on walls
to damp out echoes, so that circuits can talk to each other
at high speeds. Unlike most capacitors, if you happen to
knock off one of these resistors while playing around, you will
have to replace it in order for the circuit to work properly.
These “termination resistors” are often packaged four or
eight to a package, so they almost look like small integrated
circuits. You can distinguish resistor packs from other
components because they are shiny, slightly lumpy, have a
white border, and they will have a reference designator
prefix of “RP” near them. When tracing a signal through a
resistor pack, it is fairly safe to assume that signals flow straight
through, so that a connection on one side goes straight
through to the pin immediately on the other side.
“body” terminal). The most complicated active components are integrated
circuits, such as CPU and memory chips, with hundreds, sometimes
thousands, of leads. Integrated circuits come in a wide variety of
packages, and sometimes the connections are hidden underneath the
package, as is the case in the Ball Grid Array (BGA) package. The