Appendix B. Controlling the COM110A Power Supply
B-5
CAUTION
If power is limited and the turn-on time has to be kept very
short, it is critical to ensure that the PC and the datalogger
clocks are not allowed to drift apart, otherwise the PC may try
to call when the COM110A is turned off. Clock
synchronisation can be achieved automatically, during data
collection. Please refer to the PC software manual for details
on how to do this.
The COM110A can also be used with instruction P97 (Initiate
Telecommunications) to make an outgoing call, either to return data to a remote
computer or in the event of an alarm. Instruction 97 is normally controlled by a
program flag (referred to as the Interrupt Disable Flag when programming P97)
which is set low to make P97 initiate communications and call out. The process of
setting this flag is normally controlled by some program control logic before P97
is executed.
To ensure that the COM110A is powered on for the outgoing call, an extra P91
instruction should be included before P97 to set the power control port high if the
interrupt disable flag has been set low. P97 will automatically set the flag high at
the end of a successful call, which the P91 will detect at the next scan and turn the
COM110A off.
It is always best to design a power supply that has plenty of spare capacity to cope
with the power demands of the entire measurement system. If the power supply is
inadequate or faulty and the battery voltage is likely to drop to very low levels you
need to add extra code, as shown above to ensure the COM110A is not turned on
when the battery voltage is low. In particular it should be disabled before the level
where the logger is likely to shutdown (below 9.6 V for the CR510/10X, higher
voltages for the CR23X or CR5000 - see their manuals).
If the voltage falls below this level while the COM110A is already turned on,
since the datalogger program will stop running, there will be no mechanism to
turn the power off. This will cause the battery to discharge sooner rather than later.
If using 12V lead acid batteries, a threshold of 11.5 Volts is advised, below which
the COM110A should not be turned on, as below this level the majority of the
capacity of the battery will have been used.
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