Sierra Wireless DART 200 CDPD Modem - Manual (Page 91)

PN1197-00 Revision 1.0
7 Vehicle Installation
Considerations
Electrical transients
When a Data Access Radio Transceiver (DART) 200 is installed in a
vehicle it is usually powered from the vehicle’s electrical system.
Transients or temporary outages that can occur during starts or other
vehicle operations, can cause the DART 200 to reset the power. These
transients can also impact the attached Mobile Application Subsystem
(MAS).
There are several options available to minimize or eliminate vehicle
power disturbances, including filters, cut off switches, and universal
power supplies. The best solution; however, is to compare the needs of
the application and equipment to the cost of the proposed solution. Try
alternatives during testing to avoid problems during application roll out.
The simplest power backup system is a 12-volt battery; large enough to
supply the modem’s maximum transmit current, with a blocking diode to
prevent discharging the battery into the vehicle’s electrical system. The
battery floats on the vehicle’s 12 volt DC power system, constantly being
recharged, and used only during periods of transient voltage drop. The
diode prevents the vehicle’s electrical system from drawing power from
the backup battery during these voltage drops.
The diode needs to allow at least 2.5 amps of current in the forward
direction, and withstand a reverse voltage of at least 20 volts. Higher
rated diodes, available at a nominal cost from electronic stores, dealers,
and catalogs, provide a better safety factor.
Application considerations
Resets can occur during or between communications with the remote
system. The most serious situation occurs if the reset happens during
communications, because the modem switches from online to command
mode. If the reset occurs between communications, the impact can be
less critical because many applications take the modem out of online
mode during this time period anyway.