ENGINE AR-PRIM 20R6498 - Caterpillar

The process is repeated through two more inverter-exclusive OR groups as shown below. From the third exclusive OR, the pulses are fed both through an inverter and a binary counter (Q8) with its 2 and 4 outputs fed to a NAND. The first five pulses pass immediately through the inverter and are counted in the binary counter. The sixth pulse passes through the inverter, and after a short delay, causes the counter to change level. This places the output of the NAND low and the output of the inverter high for the next two pulses. These cancel algebraically and are not transmitted. The final output is as shown below. These pulses are fed to a NAND (Q9) which serves as counter control. If the NAND is open, the pulses are fed to the counters (Q12, Q15, Q18 and Q21).16. LATCH CONTROL As covered under the topic "GALLONS PER HOUR AND HORSEPOWER READINGS", the first JK Flip-Flop (Q26) feeds a signal to the tachometer circuit's Latch Control (monostable multivibrator Q25) once a second (every 2000 counts - the crystal oscillator clock, after the decade dividers, is running at 2000 Hz). This signal activates the latch control (Q25) which in turn activates the latches and also signals the counter control.17. COUNTER CONTROL (17A), COUNTERS (17B) The NAND which serves as a counter control (Q9) turns the counters on and off. The rpm count (for example: 2400) is fed from the multiplier circuit through the counters. When the latch control is triggered once each second by the Latch Control, the counters are stopped.18. LATCHES (18A), DECODER-DRIVERS (18B), RPM DISPLAY LIGHTS (18C) The latches receive the output of the counters and become latched when signaled by the Latch Control once each second. The stored count corresponds to engine rpm, and the value is decoded and displayed. In the Engine Speed Verify mode, the one millisecond pulse from the counter Q36 is fed to the tachometer input of the voltage comparator (Q4). Due to the multiplication factor of six present in the RPM circuitry, the one millisecond pulse is displayed as 6000 rpm.Power Board Circuit Functions
POWER SUPPLY The power source consists of two or three 6 volt gelatin electrolyte batteries connected in parallel. The batteries are coupled to the power supply through switch (S2). The battery voltage is supplied directly to power the readouts. A Zener (CR14) is used to provide a regulated 5 volts for the logic circuitry. A DC-to-DC converter is used to provide a positive and negative 15 volts for the Operational Amplifiers and the Digital-to-Analog Converter. CHARGING CIRCUIT The Horsepower Meter is designed to charge from a fully charged 12 volt battery. A minimum of 11 volts is necessary to operate the charging circuit. The GEL-CELL batteries will not be fully charged unless the charging source is at least 11.5 volts. Q41, Q42 and Q43 make up a current limiting voltage regulator which controls the charging current supplied to the batteries. Reverse polarity protection is provided by diode (CR11).