Rockwell Automation 1756-IH16ISOE ControlLogix Sequence of Events Module - Manual (Page 120)

Publication 1756-UM528A-EN-P - April 2004
B-2 Integrating The HiProm GPS Module (1756HP-GPS) into a ControlLogix Sequence of Events Module System
What is the 1756HP-GPS
Module? The 1756HP-GPS module is an integrated GPS product that makes use
of Global Positioning System (GPS) technology to derive accurate time
that is synchronized with the atomic clocks located on the GPS
satellites. The 1756HP-GPS module receives regular transmissions
from GPS satellites that deliver time in the Universal Coordinated
Time (UCT) format.
In addition to receiving UCT time, the 1756HP-GPS module also has
access to time in CST format because the module resides in the
ControlLogix chassis and can obtain the CST from the backplane.
Correlating CST with UCT
As shown in Figure B.1, with every receipt of a UCT value from the
satellite (1), the 1756HP-GPS module captures the CST value from the
ControlLogix backplane at that moment (2), calculates the necessary
offset to account for drift (3) and then stores the values in its on-board
table (4). This correlation table provides a real-time equivalent for any
CST timestamp in the system.
The table holds one hour’s worth of UCT/CST times. Because drift
typically exists between the CST clock and the UCT clock, the
1756HP-GPS module records UCT/CST pairs that allow the module to
calculate a drift compensation between the two clocks to provide
increased synchronization and accuracy.
Figure B.1
1 – 1756HP-GPS module receives
UCT value from satellite.
2 – 1756HP-GPS module captures
CST value from backplane.
CST on backplane
3 – 1756HP-GPS module
calculates an offset for drift
On-board table
UCT CST
4 – 1756HP-GPS module stores
the values in on-board table.
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