Garmin GAD 42 Maintenance Manual - Page 12

7.20 Valid High Inputs All valid high inputs are located on the CPU board, and are identical to the HDG VALID HI input described here. When the HDG_VALID_HI input pin is connected to aircraft power voltage over 10 V, a diode conducts base current to a transistor through a resistor. This turns on the transistor, and the output normally connected to Vcc through a resistor is shorted to ground. When this output is grounded, and the corresponding valid low output is active (high), a logic high is generated by a digital IC. The CPU reads this as a logic high from the data bus through a discrete input latch. When the HDG_VALID_HI input is open, grounded, or connected to a voltage less than 10 V, no base current can flow, and the transistor is turned off. This results in a logic low being read by the CPU. A diode provides transient voltage protection for the transistor. A capacitor and resistor form a low-pass filter to reduce high-frequency noise at the input. 7.21 Control Outputs All control outputs are located on the CPU board, and are identical to the output described here. When the DIST_IND_OUT latch output from the CPU data bus is at logic high, gate voltage is applied through a resistor to a power FET. This turns the FET on, and DIST_IND_CONTROL is grounded through a resistor. If the current through the resistor exceeds about 0.5 A, enough voltage is developed across a second resistor to turn on Q581. This shorts the gate voltage applied to the FET to ground, turning it off for over-current protection. A zener provides transient protection for the FET. 8.0 ASSEMBLY DRAWING Figure 2, page 13, shows an assembly drawing (with part numbers) for the GAD 42. GAD 42 MAINTENANCE MANUAL P/N 190-00159-01 Page 11 Rev. A