Raspberry Pi A, Руководство пользователя, Страница: 83 / 314

C H A P T E R 4
T R O U B L E S H O O T I N G
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The reading on the voltmeter should be somewhere between 4.8 V and 5 V. If it’s lower than
4.8 V, this indicates that the Pi is not being provided with enough power. Try swapping the
USB adapter for a different model, and check that the label says it can supply 700 mA or
more. A model rated at 1A is recommended, but beware of cheap models—they sometimes
have inaccurate labelling, and fail to supply the promised current. Genuine branded mobile
phone chargers rarely have this problem, but cheap unbranded devices—often sold as
compatible adapters —should be avoided
If your voltmeter reads a negative number, don’t worry: this just means you’ve got the posi-
tive and negative probes in the wrong place. Either swap them around or just ignore the
negative sign when noting your reading.
Display Diagnostics
Although the Pi is designed to work with almost any HDMI, DVI or composite video display
device, it simply may not work as expected when you plug it in. For example, you may find
that your picture is shifted to the side or not fully displayed, or is only visible as a postage-
stamp-sized cut-out in the middle of the screen or in black-and-white—or even missing
entirely.
First, check the type of device to which the Pi is connected. This is especially important when
you’re using the composite RCA connection to plug the Pi into a TV. Different countries use
different standards for TV video, meaning that a Pi configured for one country may not work
in another. This is the usual explanation for a Pi showing black-and-white video. You’ll learn
how to adjust this setting in Chapter 7, “Advanced Raspberry Pi Configuration”.
When you use the HDMI output, the display type is usually automatically detected. If you’re
using an HDMI to DVI adapter to plug the Pi into a computer monitor, this occasionally goes
awry. Common symptoms include snow-like static, missing picture portions or no display at
all. To fix this, note the resolution and refresh rate of your connected display, and then jump
to Chapter 7, “Advanced Raspberry Pi Configuration”, to find out how to set these manually.
Another issue is a too-large or too-small image, either missing portions at the edge of the
screen or sitting in the middle of a large black border. This is caused by a setting known as
overscan , which is used when the Pi is connected to TVs to avoid printing to portions of the
display which may be hidden under a bezel. As with other display-related settings, you will
learn how to adjust—or even completely disable—overscan in Chapter 7.