A-1
Appendix A. Controlling the power use of
the COM111 Modem and methods of
resetting the modem
This Appendix describes how the datalogger can be used to control the power supply to a
COM111 modem to minimise power consumption, which also causes a hard reset of the
logger. An example is also shown of how to perform a soft reset of the logger.
A.1 Controlling the power used by a COM111 modem
The power consumption of the COM111 system is much lower than many other
cellular modems, being typically 5 mA while waiting for calls in GSM mode,
20-25 mA in GPRS/3G mode online and <200 mA when on-line. As an option a
power switch (PSW12) or one of the power switches in the wiring panels of the
logger can be used to turn off the power supplied to the COM111 and thereby
minimise power use. Clearly, the COM111 will not answer any incoming calls
when in this state.
The first thing to do when designing the power supply for your COM111 system is
to work out the likely time the modem with be powered on and the length of times
it will be online and transmitting data. Using the figures about you can estimate
the likely daily power use and match that to the battery capacity and solar panel if
used. Take care to check the worst case solar input in the depth of winter.
If the power use is higher that the solar panel can provide you can consider only
switching on the COM111 for, say, one hour per day, during which time a base
station can be synchronised to collect data. The lowest overall power consumption
can be obtained by making the datalogger turn the power on only when it makes
outgoing calls. However, this would prevent you calling the datalogger at will
from a base station to check its status or to load a new program.
Once the total time per day that the datalogger will be turned on in its different
power states is determined, a daily power consumption can be estimated and the
power supply designed accordingly. For most typical remote applications a PS150
and a medium-sized solar panel will suffice. Please refer to Technical Note 12
(Calculating Power Consumption and Solar Panel Size) for further details.
To allow the COM111 to receive calls, program the datalogger to set the relevant
control port high which controls the power switch which will turn the COM111
on. The COM111 will not answer an incoming call when the power control line is
low. Control is normally done using the datalogger code to do turn the power on
or off at specific times, e.g. IF Timeintointerval(a,b,c) followed by a command to
turn the power on or off at the specified time. It is also common only to turn the
power back on if the battery is above a certain level. Loggers with later operating
systems also support the Timeisbetween instruction which is slightly simpler to
code.
The period that the COM111 is switched on needs to be long enough to allow an
incoming call to be started, data collection made and the call completed.
The power can also be switched off/on when using the modem for GPRS/3G
communications but extra steps should be added to the program to shut down the
PPP connection before power-off (use PPPClose) and to re-enable it (PPPOpen),
on power-up.
