The operation of the Electronic Digital Tachometer is straight forward. After power is supplied to the Tachometer, the engine is started, and the self-tests are performed, the default display of engine RPM appears on the display. The default display is insured via the use of internal timers that will restore the display to the current RPM even in the event that one of the panel buttons becomes stuck or defective.

Internally, two independent tachometers watch the pulses received from each magneto. Each tachometer is accurate to less than 1 RPM and can be individually enabled/disabled via buttons on the face of the Tachometer.

Engine operation ranges are indicated on the large green, yellow and red LEDs, see Figure 3, items D, E, and F.

Three small LED magneto system alert indicator lights are located within the "Status" area on the upper left corner of the Tachometer face, see Figure 3, items A, B, and C. The left and right red alert indicator lights, when illuminated, indicate, because of a loss of ignition signal to the tachometer, a possible malfunction of the respective left or right magneto ignition system.

While performing a magneto check during engine run-up, the red alert indicator lights will illuminate, thus identifying the grounding of the respective right of left magneto systems.

Between the left and right red magneto ignition system alert indicators is a yellow "RPM Synchronization" indicator. This small yellow indicator is illuminated when there is a difference of more than 80 RPM between the right and left tachometers. This indicator also may flicker during extreme RPM excursions of the engine.

There are three panel buttons, see Figure 3, items I, J, and K. Each button has two modes of operation:

• Press-and-hold,
• Press-and-release.

Press-and-hold button operations instruct the Tachometer to perform a specific operation when a button is pressed and held for more than 2/3 of a second.

Press-and-hold button operations are placarded on the face of the Tachometer above each button.

Similarly, press-and-release button operation instructs the Tachometer to perform a specific operation when a button is pressed and released in less than 2/3 of a second. Press-and-release button operations are placarded on the face of the Tachometer below each button.

PRESS AND HOLD OPERATIONS:

The left button, K, upon depression, will cause the Tachometer to display the non-fractional portion (0000.) of the current accumulated engine hours. When the button is released, the fractional part of the engine hour (.00) is displayed for a short period of time. The clock is started whenever the engine RPM exceeds 800 RPM and is recorded in real hours.

The right button, I, upon depression, will cause the Tachometer to display the current contents of the RPM trap. This trap records the highest engine RPM achieved before the button was pressed.

The middle button, J, upon depression, clears the RPM trap. During depression of the switch, the RPM trap is zeroed. When the button is released, the trap will record the current engine RPM.

PRESS AND RELEASE OPERATIONS:

During normal operation, the Tachometer presents the average of the left and right internal tachometers on the display. However, a mechanism exists to mask either tachometer from the display, leaving the remaining tachometer to display its RPM.

A masked tachometer is indicated by the regular flashing of the right or left signal loss status indicator LEDs. This feature is handy when attempting to determine magneto/ignition problems.

Quickly pressing and releasing the left button, K, causes the Tachometer to mask or un-mask the left internal tachometer.

Quickly pressing and releasing the right button, I, causes the Tachometer to mask or un-mask the right internal tachometer.

An internal interlock prevents masking both internal tachometers at the same time, therefore preventing total loss of RPM indication.

If the tachometer is masked, pressing the button will un-mask it and allow its RPM to show on the display, and conversely, if the tachometer is un-masked, pressing the button will mask it from display.

Quickly pressing and releasing the center button, J, causes the Tachometer to alternately dim or brighten the LED indicators.

The LED indicators, see Figure 3, items A through F, are bright enough to overcome daylight washout conditions. However, during night operations the large green, yellow, and small red and yellow LEDs are dim-able. The large red LED still operates at full intensity to maximize the possibility of gaining pilot attention during excursion into restricted RPM ranges.

The Tachometer is fully electronic and uses timing information from the primary leads ("P-Leads") of both the left-hand and right-hand magneto ignition systems, operating the Tachometer's internal left and right tachometers, to determine engine RPM instead of a rotating cable driving a magnetic slip-clutch analog type display;

The Tachometer uses super-bright LED indicators to indicate normal range engine operation (Green LED), cautionary range operations (Yellow LED), and do-not-exceed or restricted range RPM (Red LED) as substitutes for the ranges normally painted on the tachometer dial.

The primary display consists of four 1/2" high characters on a back-lit Liquid Crystal Display (LCD), easily and clearly visible in daylight and night flying.

Diagnostic features available include: alert indication of loss of magneto signal, indication that both magnetos are reporting different RPM, and the ability to mask RPM from either magneto.

Magneto test, via the ignition switch, is indicated by the illumination of the grounded magneto system's alert light and the display of the amount of RPM that the engine has slowed. This is indicated as a negative number on the display (number is preceded by a leading hyphen or minus sign);

LED indicators are dim-able (except the restricted or red-light indicator) to reduce pilot annoyance during night flying;

A specific engine hour is pre-set at the factory to accommodate tachometer changes on non-zero-time engines. Engine time may be changed only by Horizon Instruments, Inc., Repair Facility.

A) Right Magneto System Alert Indicator

B) RPM Sync Loss Indicator

C) Left Magneto System Alert Indicator

D) Normal Operating Green RPM Range Indicator

E) Cautionary Yellow RPM Range Indicator

F) Restricted Red RPM Range Indicator

G) Limitations Placard

H) 4 Digit LCD Display

I) Right switch for Overspeed Trap display and right tachometer enable/disable

J) Middle switch for Overspeed Trap Clear and LED indicator dimming

K) Left switch for engine hours display and left tachometer enable/disable.