GE Security 940 User Manual

Configuring the reader 21
trade-off between read range and cabling distance is common to
all proximity badge readers. The power selection switches on the
94x/97x readers allow the optimum power setting to be selected
to suit individual installations.
For example: On the high power setting, giving the greatest
badge read range, the reader typically requires 200mA of supply
current from the microcontroller. If there is 1,000 feet of 22-
AWG cable between the reader and the microcontroller, the total
reader power and power return path is 2,000 feet. Since 22-AWG
cable has a typical resistance of 16 ohms per 1,000 feet, the total
resistance in the reader’s power and power return wire is 32
ohms. By Ohms Law (V=IR), it follows that the total voltage
dropped in the reader power and power return wires will be 6.4V
(6.4V = 200mA x 32 Ohms). Therefore, the reader supply voltage
will drop from 12V at the microcontroller to 5.6V (12V - 6.4V) at
the reader. Such a supply voltage is too low for the reader to
function reliably.
If the low power setting is selected, the badge read range is
reduced. However, the reader now typically requires only 75mA
of supply current; therefore, the voltage drop in the power and
power return wires is much less. In this case, the reader supply
voltage will only be reduced to 9.6V; high enough for reliable
operation.

GE Security 940 Specifications

General

Brand	GE Security
Model	940
Category	Security System
Language	English