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DIGISONDE-4D
SYSTEM MANUAL
VERSION 1.2.11
5-12 SECTION 5 - SYSTEM SOFTWARE
5:11. Parser and Framer tasks (shown as yellow boxes in Figure 5-2) are responsible for communications
with the DATA Platform. The Parser Task accepts incoming command packets to assure their proper format and
forwards them for execution to the Command Dispatcher. The Framer Task accepts outgoing telemetry data to
format them appropriately for delivery to the DATA Platform.
GPS SYNCHRONIZATION
5:12. All Digisonde-4D operations are kept synchronous with the GPS Universal Time (UT). The DATA plat-
form runs standard Network Time Protocol (NTP) that accepts NTP messages from the GPS receiver over the
RS-422 port to keep the operating system clock matched to UT. Synchronization of the Digisonde
®
circuitry is
done in hardware: each Second-In-The-life (SITL) written in the Timing memory is triggered by the heartbeat 1
PPS signal from the GPS receiver. As for the CONTROL platform, its synchronization involves combination of
both software and hardware solutions. The same Timing memory that is triggered to produce timing signals for
one SITL of the sounder also generates 5 ms external tick interrupt that drives the computer clock of
CONTROL platform. Thus, no drift of the CONTROL platform clock shall be expected unless 5 ms interrupt
becomes unstable. To verify healthy synchronization of the CONTROL platform, and also to initialize its sys-
tem clock after powering up, DESC software periodically analyzes UT stamp
1
from the Periodic Messages
generated by DCART. When the 5 ms interrupt handler of DESC identifies the “golden” interrupt (out of 200
per second) that coincides with the external 1 PPS heartbeat signal from GPS, DESC:
• Checks if no measurement is ongoing and verifies that the latest periodic message from DCART is less
than 1 sec old, in which case:
• Compares its system clock with the (UT stamp from DCART periodic message + 1 sec), and if a mis-
match is observed:
o Ignores 3 cases of the time difference exactly ±1 sec, in which case accidental latency problem
in the handling of the periodic messages is suspected.
o Otherwise, changes the CONTROL platform clock time and generates software event message
TYPE=0x02 with EventID=0009h (System clock changed) into outgoing telemetry stream.
NOTE
During normal operations of the sounder, no software messages “System
clock changed” have to be observed in DCART log window, except for the
first message when CONTROL platform clock is initialized.
5:13. Additional testing of CONTROL platform synchronization is done by DESC packet parser that moni-
tors latency of DCART periodic messages and reports continuous resets of the CONTROL platform system
clock, or mismatches greater than 1 sec, as software error message TYPE=0x03 with ErrorID=0011h (Unstable
clock, cannot set time).
1
DCART Periodic Messages are generated on the round second, typically once a minute, with UT stamp given at 1 sec
resolution.
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