Using with Campbell Scientific Dataloggers
15
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
TC35T 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 TC35T 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 TC35T 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
TC35T should not be turned on, as below this level the majority of the capacity of
the battery will have been used.
7.2 Preventing TC35T Connection Problems
There is a very small chance that the TC35T could crash or ‘lock up’, or simply
lose registration with the phone network. Crashes may be the result of electrical
spikes on the power supply or local electrical storms. If the TC35T is powered
continuously, such a crash will often prevent it answering an incoming call again
until it has been reset by disconnecting and then reconnecting the power.
GSM phone modules can also be deregistered from the cellphone network. This
means that they are effectively logged off the network preventing new incoming
calls. The reasons this can happen are that the network may miss the regular "I am
here" messages that the TC35T transmits every few seconds or, in some cases, the
network will timeout the registration due to inactivity. The exact algorithm used to
decide whether to deregister a phone varies from network to network. Problems
are more likely where the connection is marginal or where the local cell is
congested.
To help overcome these problems, the datalogger can be programmed to shut
down the TC35T at regular intervals, not only to save power, but also to ensure
that the TC35T is reset into a known state and to make it re-register itself on the
network. To be sure that the TC35T resets properly, it must be turned off for at
least 30 seconds. How often the TC35T is turned off will depend on the normal
calling schedule, considerations of power consumption and of how long you could
afford to lose communication with the TC35T if it did go offline.
8. Fault Finding
8.1 General
Before placing the TC35T out in the field it is important to check that it is properly
registered on a GSM network. To do this, connect it to a power supply and turn it
on by connecting the power control line to 5V or 12V. After power has been
applied, the small LED in the case should flash on and off rapidly indicating that
the unit is trying to establish a network connection. Within 30 seconds the LED
should switch to flashing more slowly. This indicates that the unit has registered
itself successfully with a GSM network. As a second check connect it to a PC and,
using a terminal emulation program, instruct it to dial a valid telephone number,
e.g.
ATD01509601091
<
ENTER
>. If the TC35T does not cause the remote phone