Fluke 5725A Break-DetectCircuitry, 4-31, Power Up and Reset Circuitry, 4-32, Watchdog Timer, 4-33

Theory of Operation 4

Detailed Circuit Description

Break-Detect Circuitry 4-31.

The 6303Y divides the 4.9152-MHz crystal frequency by four to create the 1.2288-MHz ECLK. This signal serves as the clock for the 74HC4020 counter (U506), which is configured as a divide-by-16384. The RECEIVE line is usually high, except for the brief interval when characters are being transmitted. As long as the low state is less than 13.34 ms, RECEIVE holds the CLR of the 74HC4020 TRUE. However, if communication between the 5725A Amplifier and the 5700A Calibrator fails, the 5700A attempts to reset the 5725A by sending a break (holding RECEIVE low for greater than 26.7 ms). This action causes the 74HC4020 counter output (BREAK) to go high. Signal BREAK is NOR’ed with CABLEOFF from the High Voltage Sense assembly (A6); either signal causes a NON-MASKABLE INTERRUPT to the 6303Y. The 5700A also uses BREAK to reset the 5725A.

Power Up and Reset Circuitry 4-32.

Reset and glitch-detection circuitry primarily consists of U508, a TL7705A. This chip detects if the power supply falls below 4.5V, if a reset input from a momentary contact switch occurs, or if there is an output from the watchdog timer. Any combination of these conditions resets the board via RESET* for 130 ms.

Watchdog Timer 4-33.

Watchdog circuitry contains a 4538 Dual Monostable Multivibrator. The first multivibrator (U509A) is configured as a 300 ms retriggerable one-shot that can fire a second 3 ms one-shot (U509B). Control line STROBE 2 of the 6303 (bit 1, port 6) retriggers the first one-shot every 50 ms and refreshes the high voltage relay driver. If STROBE 2 is stuck high or low during operation, the first one-shot is no longer retriggered, and its output transitions low. The second one-shot, configured to trigger on a falling edge, now sends a pulse via RESIN* to the TL7705A reset IC U508. This action forces the reset circuit to pull a hard reset on the 6303Y for 130 ms.

The first trigger to the 4538 occurs when the 7705A RESET* line makes a transition from low to high. During normal operation, STROBE 2 prevents another reset by retriggering the first one-shot well before the 300 ms timeout. The watchdog fires only if the CPU is executing erroneous code which does not activate STROBE 2.

Current Amplifier Assembly (A2) 4-34.

The Current Amplifier assembly enables the 5725A to extend the current output range of the 5700A Calibrator to ±11A dc or 11A ac. The 5700A drives the Current Amplifier assembly input through the 5725A interface cable. There are no user input terminals. Output current is available at binding posts on the 5725A front or rear panel. Selection of front or rear 5725A binding posts is done by unplugging one cable and plugging in another cable on the Current Amplifier assembly inside the 5725A chassis. Additionally, relays on the Current Amplifier assembly can route the 5700A current source to the 5725A output terminals, if so selected at the 5700A front panel (or by remote command).

The Current Amplifier assembly has five distinct sections: the error amplifier, output, control, monitor, and power supply. Each section is first briefly described, then described in detail in the following paragraphs. Refer to Figure 4-3, a block diagram of the Current Amplifier assembly, to see the interrelationship of the following function blocks.

4-19