Study figure 5-6 again and follow the polling
sequence of the four units in the net. The last report
on the top line is an NCS report and call to PU 04.
This is followed by PU 04s response on the left side
of line two. Next, PU 56 is called and responds with
a picket reply. Upon completion of PU 56s reply, PU
64 is called. After 15 frame times without a response,
PU 64 is called again. PU 64 appears to have
responded to the second call, but the LMS-11 only
recognized the five preamble frames.
By using the link monitor display, the operator or
technician can make sure the connectivity has been
established and that all the correct PUs are being
polled and are responding.
Figure 5-6 also shows several problems that
commonly occur during Link-11 operations. Notice
that PU 64 sometimes responds to the first call-up,
sometimes to the second call-up, and sometimes PU
64 does not respond at all. PU 56 responds all the
time except for the call-up at the end of line two and
beginning of line three. On line six there is a double
response, or echo, from PU 04. If you were to
examine this particular sequence using the frame-by-
frame analysis, you would find the PU 04 was called
again. This indicates the NCS did not receive the
report from PU 04 and repeated the call-up during the
middle of the response.
Figure 5-8.Link-11 messages as displayed by the
LMS-11 link monitor mode.
As shown in figure 5-9, the status display is at the
bottom of each of the LMS-11 display screens. The
status display consists of the status box and two lines
of information just above the status box. The top line,
with the heading XMT-ADDRS: displays the
addresses of all PUs in the order they are being polled.
The operator can monitor the polling in real time. The
displayed addresses change colors to indicate their
status. If the address is yellow, it is currently being
interrogated. The yellow address turns green when
the start code is received. The yellow address turns
red when the PU has been interrogated twice and no
response is received.
The line under the XMT-ADDRS: is used to
display system messages and LMS-11 alerts. Alerts
are displayed on the left side of this line. System
messages are displayed on the right side of the line.
The status box provides the operator with
information about signal processing, link activity, and ,
raw recording of link data. Just below each of the
frame types, a small green box, or light, appears to
indicate the type of frame being processed. These
signal processing status indicators are not displayed in
real time. They are updated approximately every 50
milliseconds. The signal processing indicators are as
Figure 5-7.The link monitor display pattern.