ORACLE SERIES 2E, 2EC, 4E, 4EC, 4H
Operations Manual
Eberle Design Inc.
Page 17
Section 4
Theory of Operation
4.1 GENERAL
The ORACLE Loop Monitor works on the principle of measuring the period of a pre-
determined number of cycles from an oscillator whose frequency is directly related to the
inductance of the loop to which the unit is connected. The measurement is done by
counting the number of cycles of a stable crystal reference that occur during the period to
be measured. This scheme provides a high resolution measurement in a short period of
time. Consecutive measurements can be compared with each other to determine whether
or not the frequency of the loop oscillator has changed sufficiently to indicate the presence
of a vehicle.
The ORACLE circuitry can be broken down into four major blocks. The loop oscillator,
digital processing circuitry, switch controls and outputs/indicators. The oscillator input
incorporates a transformer to isolate the loop from the internal oscillator circuitry and a
surge protector is used to protect the unit from transient damage. The loop oscillator
frequency may be modified by switching capacitors in or out of the oscillator circuit in order
to affect crosstalk avoidance. The output signal from the oscillator is fed through a squaring
circuit and provided to the digital processing section.
The microcontroller uses a high speed crystal clock count to calculate the loop inductance,
frequency and percentage of inductance change. The values are displayed on the front
panel LCD. The microcontroller also processes the toggle switch selections and stores the
operating parameters in non-volatile memory. The operational mode determines whether
the detection CALLS are to be Presence or Pulse and the sensitivity setting determines the
number of loop oscillator cycle “blocks” to use in the period measurement process. The
microcontroller then begins the measurement process by energizing the loop oscillator for
the channel to be measured.
During period measurement the microcontroller counts the number of cycles of the crystal
based reference frequency. When the period ends, the count is compared with a stored
reference and detection decisions are made. Small changes which occur over a relatively
long period of time are considered due to the changing environment, and the stored
reference is modified accordingly by the microcontroller.
Detection CALLS are made via the optically isolated transistor. These isolate the traffic
controller input from the ORACLE circuitry. Output CALL and loop fault monitor status are
indicated by both the front panel LCD and high intensity LEDs.
4.2 TROUBLE ANALYSIS
The following should be used to troubleshoot both the Oracle Loop Monitor and the loop
installation.
4.2.1 LCD OR LED NOT LIT - DETECTOR DOES NOT OPERATE OR HAVE POWER.
Power supply fault
: The ORACLE detectors require a 10.8 to 28.8 VDC nominal supply.
The detector consumes approximately 50 mA per channel. The ORACLE will operate at a
voltage as low as 10.8 Vdc. Supply voltages below this may result in the unit entering a
reset state. In this case, the unit will appear to be non-functional.
4.2.2 FAULT AND DETECT LEDS FLASHING
Loop or lead-in wiring has a fault:
Check the type of fault being indicated on the LCD or by
the number of flashes on the Fault LED. The type of fault may assist in locating the
problem. A single flash followed by a pause indicates an open circuit loop or loop