the greatest weight is the most significant bit (MSB).
The bit with the least weight or representing the smallest
value is the least significant bit (LSB). Natural binary
code is used in a system of digital data transmission and
conversion called binary angular measurement
(BAM). Other coding systems such as Gray code and
binary-coded decimal (BCD) are also used by
analog-to-digital converters.
BINARY ANGULAR MEASUREMENT. —
Binary angular measurement words (BAMs) are
standardized binary words used to transfer angular
measurements between shipboard tactical data system
equipments. BAM data words are used to transfer
quantities between digital equipments, from digital
equipments to D/A converters, or from A/D converters
to digital equipments.
BAM data words are specifically designed to
indicate up to 360 degrees of angular values in binary
form, often in steps or increments of as small as
0.009766 degree (the LSB value). Figure 13-3 shows
one example of a BAM word. This 12-bit word (2°-211)
can indicate 360 degrees of angle in steps of 0.088
degree. The LSB is equal to 0.088 degree when set
(ONE), while the MSB is equal to 180 degrees when
set. When all 12 bits are set, a maximum angle of
359.902 degrees is indicated. ZERO or 360 degrees is
indicated when all bits in the BAM data word are clear
(ZEROS).
BAM words are also used to transmit non-angular
values such as range or height. When non-angular
values are being used, the LSB value indicates the
smallest step or increment of the quantity being
transmitted. The MSB value represents half the
maximum value that may be transmitted. The sum of
all bits when set indicates the maximum quantity that
can be transmitted. This corresponds to the 0- to
360-degree capability of common shipboard synchro
systems.
GRAY CODE. —Gray code or reflected binary
code is used in devices where a transition from one
consecutive value to another takes place, such as
angular measurement and encoding. The code is
designed to change from one value to the next with only
one bit change. Table 13-1 shows the relationships
between Gray code, BCD, and natural binary code.
BINARY-CODED DECIMAL (BCD). —BCD
represents decimal values with a 4-bit code, called the
8-4-2-1 code. Each of the 4-bit groupings represents
one decimal digit. BCD encoders allow for immediate
decimal display of the converter output. They are found
in such devices as digital voltmeters and other types of
decimal display devices. Table 13-1 shows the
relationships between BCD, Gray code, and natural
binary code.
SYNCHROS
Up to this point, we have discussed basically
single-phase analog data signals. One of the most
common shipboard analog signals requiring conversion
is the 3-phase or 5-wire synchro signal. Synchros are
used throughout naval ships for the rapid transmission
of analog information between equipments and
stations. They are found in just about every weapon,
communication, underwater detection, and navigation
system in use in the Navy. Numerous kinds of
information involving angular displacement or
ranges of values are transmitted. For the combat
direction system (CDS) equipments to use this
information, the synchro signals must be converted to
their digital equivalent. The following information
provides a limited overview of synchros as they apply
to digital systems and synchro-to-digital (S/D)
conversion.
Figure 13-3.—A 12-bit binary angular measurement (BAM) word.
13-4
