last digit designates whether the message is an A or B
type. In most modes, both an A and a B type of
message are required to transmit all the necessary data
to the aircraft. The remaining data bit time slots
contain the various control commands.
Transmitter Un-key Signal
The last time slot (slot 70) is a 200-µsec period
allotted for transmitter turn-off time and does not
contain any data.
REPLY MESSAGE FORMAT
Reply messages are received during the 18-msec
receive cycle. The reply message contains a total of
56 time slots and occupies a period of 11.2 msec.
This 11.2-msec reply must be received during the
18-msec receive cycle. This allows for a maximum of
4.8 msec for transmission delay.
The reply message consists of a sync preamble,
42-data bit time slots, and a guard interval, as shown
in figure 6-3. The sync preamble is identical to the
control message sync preamble. The information in
the 42-data time slots is divided into groups of digits
that identify the source and type of message, and the
message data. The last time slot is the guard interval
and it allows for transmitter turn-off time.
Figure 6-3.—The Link-4A reply message format.
TEST MESSAGES
During Link-4A operations the controlling station
sends test messages at periodic intervals to the data
terminal set for testing the message processing and
display circuitry of the aircraft being controlled. The
test messages also check the data terminal set and its
interfaces. The two types of test messages are
universal test message (UTM) and monitor control
and reply messages (MCM/MRM).
Universal Test Messages
Universal test messages (UTMs) are Link-4A
control messages that are always addressed to a
particular universal address and contain fixed, specific
information in each data field. The UTMs provide the
controlled aircraft with a means to verify proper
operation of the link.
Monitor Control and Reply Messages
Monitor control messages (MCMs) are Link-4A
control messages that are sent to the data terminal set
from the CDS computer to initiate internal testing of
the data terminal set. After the data terminal set
completes its self-check, the MCM is transmitted with
the universal address. Depending on the equipment
configuration of the aircraft, the MCM will either be
rejected or processed as a UTM.
The monitor reply message (MRM) is sent to the
CDS computer upon the successful processing of the
MCM. The MRM is effectively a return of the MCM
data content which indicates that the internal and
interface tests were successful.
THE LINK-4A SYSTEM COMPONENTS
The Link-4A system consists of the CDS
computer, a data terminal set, a communications
switchboard, and a UHF radio set.
6-4
