User's Manual

GlobalTrak ™ GT Lite Operation and Maintenance Manual
ii) Circuits interface to the analog to digital converter (ADC) through a voltage
conditioning circuit, which includes individually selectable voltage divider and
amplifier gain stage.
iii) Circuits are easily accessible on the GT Lite board in order to allow
customization of the circuits for different inputs.
The sensor output is be sampled by the low power controller at a rate of 1 reading per
second, even when the GT Lite is in its lowest power state. The measured value is compared
against a stored upper and lower threshold, and when either is crossed, an event message is
to be immediately generated and sent to the IMB. The sensor is re-armed after the sensor
level returns from the alarm state. At the point where the sensor level crosses the threshold
back to a typical level the sensor alarm is once again active.
All thresholds are user programmable, changeable over-the-air through the GSM network or
locally. Also the thresholds can be disabled, such that no event message is generated.
The circuit enables physical customization of the inputs to adapt to varying sensors. The
basic diagram is shown below. Rectangular blocks represent spaces where SMT 1/8W
resistors can be placed (possibly 0 ohm). Production values are shown in the blocks. The
translation of voltage to the full range of 10-bit digital values at point B is to be included in
subsequent design documentation.
+ 3.3V
DNA
0 Ω
Figure 6
Digital Output
A single digital output circuit is to be wired to the front panel connector. It is a simple 3.3V
digital output, ESD protected, and limited to 100 mA.
External Communications
GSM Cellular
GT Lite uses both Short Message Service (SMS) and General Packet Radio Service (GPRS)
over GSM to communicate with the IMB. The redundant communication allows least cost
routing wherever a GSM network is available.
The GSM transceiver used within the GT Lite must be capable of communicating on any of the
four globally regulated GSM frequency bands, commonly referred to as the 850, 900, 1800
and 1900 MHz bands, making it usable across the globe.
0 Ω
DNA
0 Ω
B
A
Analog Ground
Prepared: R. Straz Date: July 25, 2007 Page 19 of
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