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PROTOCOL CONSIDERATIONS (CONT.)
The procedure here is protocol-dependent, but to illustrate let's consider the
packet format outlined on the preceding page being sent to a UART. A UART
interprets the start-bit of a byte as a 1-0 transition. When the incoming data is
101010, or hash, it is hard actually to find the start bit. This problem is solved by
the UART sync-byte. The purpose of the sync-byte is to create a high marking
period of at least a byte-length so that the start bit of the following start-byte can
be correctly recognized.
The start-byte is used by the receiving computer or microcontroller to intelligently
identify the beginning of a data packet. The start-byte value should be chosen so
that it does not appear in the data stream. Otherwise, a microntroller may "wake
up" in the middle of a packet and interpret data in the packet as a valid start-byte.
There are many other ways to organize protocol if this proves impractical.
There is always a possibility of bursting errors from interference or changing
signal conditions causing corruption of the data packet, so some form of error
checking should be employed. A simple checksum or CRC could be used. Once
an error is detected the protocol designer may wish to simply discard the corrupt
data or develop a scheme for correcting it or requesting its retransmission.
INTERFERENCE CONSIDERATIONS
It must be recognized that many bands, such as those in which the HP-3
operates, are widely used, and the potential for conflict with other unwanted
sources of RF is very real. All RF products are at risk from interference but its
effects can be minimized by better understanding its characteristics.
Interference can manifest itself in many ways. Low-level interference will
produce noise and hashing on the output and reduce the link's overall range.
Thanks to the capture properties of an FM system, the receiver will still function
when an intended signal is present at a higher level than the interference.
Another type of interference can be caused by higher-powered devices such as
hopping spread-spectrum devices. Since these devices move rapidly from
frequency to frequency they will usually cause short, intense losses of
information. Such errors are referred to as bursting errors and will generally be
dealt with through protocol.
High-level interference is caused by products sharing the same frequency or
from near-band high-power devices. Fortunately, this type of interference is less
common than those mentioned previously, but in severe cases can prevent all
useful function of the affected device. It is in these cases that the frequency
agility offered by the HP-3 is especially useful.
Although technically it is not interference, multipath is also a factor to be
understood. Multipath is a term used to refer to the signal cancellation effects
that occur when RF waves arrive at the receiver in different phase relationships.
This is particularly a factor in interior environments where objects provide many
different reflection paths. Multipath results in lowered transmitter signal levels at
the receiver and thus shorter useful distances for the link.
The receiver's Received Signal Strength Indicator (RSSI) output can be used to
qualify the presence and strength of interference and identify the best channels
for use in a given environment. Refer to the HP-3 receiver guide for more details.
Page 16
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