HYDRA

Service Manual

amplifier. A1VR2 is the reference for the positive supply. A1R14 provides the current to bias the reference zener. A1C4 is the output filter, and A1C9 provides frequency compensation of the regulator circuit. Transistor A1Q1 and resistor A1R13 make up the current-limit circuit.

When the voltage across A1R13 increases enough to turn on A1Q1, output current is limited by removing the base drive to A1Q2.

The -5.4 volt regulator operates like the +5.3 volt regulator, except that the NPN transistors in the positive supply are PNP transistors in the negative supply, and the PNP transistors in the positive supply are NPN transistors in the negative supply. If a VDD- to-VSS short circuit occurs, diode A1CR4 ensures that current limit occurs at the limit set for the -5.4V dc or +5.3V dc supply, whichever is lower.

2-31. Power Fail Detection

The power fail detection circuit generates a signal to warn the Microprocessor that the power supply is going down. Comparator A1U24 compares the divided-down raw supply voltage and the band-gap generated reference voltage. When the raw supply voltage is greater than about 8V dc, the output of A1U24 is "high" and when the raw supply falls below 8V dc, the output goes "low". Resistors A1R39 and A1R41 make up the divider, and resistor A1R43 provides bias for the band-gap reference. Resistor A1R42 is a pull up resistor for the comparator output, and resistor A1R45 provides positive feedback to provide the comparator with some hysteresis.

2-32. Digital Kernel

The Digital Kernel is composed of the following eight functional circuit blocks: the Microprocessor, the ROM (Read-Only Memory), the NVRAM/Clock (Nonvolatile Random Access Memory and Real-Time Clock), the EEPROM (Electrically Erasable Programmable Read-Only Memory), the Counter/Timer, the RS-232 Interface, and the Option Interface.

2-33. Microprocessor

The Microprocessor uses an eight-bit data bus and a sixteen-bit address bus to access memory locations in the ROM (A1U8), the NVRAM/Clock (A1U3), the Counter/Timer (A1U2), the Digital I/O Registers (A1U13, A1U16, A1U26), the Memory PCA (A6), and the IEEE-488 PCA (A5).

The Microprocessor oscillator operates at a 4.9152-MHz frequency determined by crystal A1Y1. The A1U4-68 system clock signal (the Microprocessor oscillator frequency divided by four) is a square wave with a frequency of 1.2288 MHz. This system clock also determines the memory cycle time of 0.813 microseconds. The system clock is also used by the Display Assembly and the IEEE-488 option assembly after being damped by series resistor A1R19 to minimize the EMI generated by this signal’s sharp edges.

When the address bus is stable, the Microprocessor enables either the reading of memory (by driving RD*, A1U4-67, low) or writing of memory (by driving WR*, A1U4-66, low.)

The Microprocessor uses a three-wire synchronous communication interface to store and retrieve instrument communication configuration and calibration information in the EEPROM (A1U1). See the EEPROM description for more detailed information.

The Microprocessor communicates to the Display Controller using another synchronous, three-wire communication interface described in detail in the Display Controller Theory of Operation in this section.

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Fluke 2635A, 2625A service manual Power Fail Detection
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2635A, 2625A specifications

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