Reporting Formats:
Illustrations
Parity Checking
These days, the accuracy of many transmissions is verified
using a technique known as
parity
checking
.
In terms of
digital electronics, the term
parity
relates to the methods
used in checking the exactness of a string of binary
numbers. When a parity-checking technique is used, the
communicator performs a calculation on the data it is
sending and embeds the result in the transmission. When
the message is received at the Central Station, the data
and the calculated result are separated so that the receiver
can perform the same calculation on its own – which it
subsequently compares with that which accompanied the
transmission. Although they're beyond the scope of this
discussion, these calculations use
algorithms
that may be
based on the following recognized routines:
• checksum
•
CRC (cyclical redundancy check)
• modolo
15
• Hamming
Codes
In any case,
Parity Checking
offers a distinct advantage
over the
Double-Round Comparison
technique, simply
because the message can be validated in a single round,
resulting in time saved and increased efficiency at the
Central Station. Additional information can be found in the
box at the left and on page B-17.
The following material contains illustrations of each
Reporting Format the CP-220A is capable of processing,
along with background commentary and, when
appropriate, comparisons with other formats.
3x1 Standard Format
Easily the oldest and the most basic of the Digital
Communication formats is known as the
3x1 Standard
Format
. It's a
pulsed
format that reports the subscriber's
Account Number (which can be no more than 3 digits); and
reports an alarm (or trouble, opening/closing, zone bypass,
or restore) condition as represented by a single, final digit.
CP-220A Central Station Receiver Appendix B:
Page B-9
Hook-Up and Installation Manual
Communication Formats
3x1, 4x1, and 4x2 Formats:
With and Without Parity
The most basic reporting formats
used in the security industry (
3x1,
4x1, 3x1 Expanded, 4x1 Expanded,
and
4x2
) will be discussed on the
following pages. Depending on the
control panel (or communicator)
sending these reports, each can be
transmitted either
with
or
without
a
parity
digit. As stated in the text, the
accuracy of the subscriber messages
received by the CP-220A must be
verified before they're acted upon,
and either the
Double Round
(which
uses no parity) or the
Parity
method
is used for this purpose.
The CP-220A must be able to detect
whether the method of verification is
based on
parity
or the
Double Round
comparison. The table on page B-17
explains the manner in which the
receiver identifies the verification
method. See
PARAMETERS 44, 59,
and
60
in
Section
2
for more
information on these formats.