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The input voltage or signal is applied to the non-inverting input of the differential amplifier, U17A. The output voltage of U17A is divided by four (4) for compensation of the DAC’s full range output of ±3 V.
Now that the flow of the circuitry has been established, it can be observed how the circuitry works during normal operation. For this example the signal -CH1 will be tied to ground. A +5 VDC digital supply line will be monitored for voltage surges in excess 0.25 V.
The DAC is loaded with the binary serial data word, 01101000. This provides a trigger level that is 1.313 V. The +5V input signal +CH1, is applied to U17A’s non-inverting input. The output, BUFCH1, of this amplifier is divided by 4, thus BUFCH1 is equal to +1.25V. This is compared with the reference voltage, TRIGLEV1, of 1.313V. Since TRIGLEV1 is higher than BUFCH1 the comparator’s output, COMPCH1, remains at +5V. When the voltage on +CH1 exceeds +5.25V BUFCH1 will then be higher than TRIGLEV1. This will drive the comparator to saturation and COMPCH1 will equal 0 V.
INTERRUPT GENERATION
All interrupt generation circuitry is contained within U3 the control FPGA. For the first part of the interrupt generation section, Channel 1 (COMPCH1) polarity is normal. The input channel signal inversion will be examined during the second part of this section. It will be assumed that Channel 1 is the only activated channel and all others are masked out.
The command to specify the debounce time and input polarity is received in the data and command buffers and subsequently transferred to the control FPGA at U3. The input signal COMPCH1 has been compared with the reference voltage TRIGLEV1, as previously discussed, and is routed to the debounce circuitry inside U3 (see Figure 5-3). The debounce circuitry will not allow COMPCH1 to pass through, unless it is low, for longer than the specified amount of time. This circuitry is very useful in blocking out transients from generating false interrupt requests. When the specified time limit has elapsed, and COMPCH1 is still active, it will then be compared with the programmed value in the mask register. Since COMPCH1 is not masked out it is then OR’ed with the remaining unmasked channels. The first channel (COMPCH1) to pass through the debounce circuitry and mask register will latch into an internal register called “First Latch Register”. This register is available to the user for determination of the interrupting channel. COMPCH1 is used to clock an internal latch that in turn drives the “LATIRQOUT” signal on the front panel connector. COMPCH1 is also used in the pulse generation circuitry that generates the Interrupt Request (IRQOUT) pulse that is 500 ns wide.
When signal inversion is selected the interrupt will be generated when COMPCH1 is less than the reference voltage TRIGLEV1. The debounce circuitry and the mask register use this signal INV to determine polarity (see Figure 5-3). INV determines whether COMPCH1 is treated as an active low for normal and active high for an inverted signal.
