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Detailed introduction
When continuous reading of time is required, counter A can be called to count time while counter B records external events. This provides counter and display preset timer functionality. In some applications, it is necessary to measure the time required to count a preset number of events. For such applications, Counter A combined with a preset blind counter can be used as a preset counter, while Counter B records time at 0.01 second intervals. In measurements that require correction for dead time of the detector and its associated electronics, the Gate A input can be switched to gate the clock on and off with a time resolution of 100 ns. A positive logic signal that defines the system live time is connected to the gate A input. This configuration provides a live time clock (Counter A) and a counter (B).
The MN X 10P option provides maximum flexibility in setting presets. The M and N values provide two-digit precision, while P has the option of decimal. Preset values can be selected from 0.01 to 990,000 seconds, 0.01 to 990,000 minutes, or 1 to 99,000,000 counts.
The basic model 994 includes an 8-digit decimal LED display that allows the entire contents of counter A or B to be read out instantly, even in a dimly lit room. Readout and control capabilities can be expanded by adding field-installable options.
The full functionality of CCNIM (Computer Controlled NIM) can be obtained by adding the RS-232-C option. This plug-in board allows computer control of all functions normally selectable from the front panel, including starting and stopping counting, readings, resets, setting presets, selecting displayed counters, and selecting the desired time base. To eliminate unintended operational interference, the computer can be in remote mode with all front panel controls disabled. With the CCNIM option, the computer can read the A and B counts, preset values, and which counter is being displayed. The CCNIM option can directly drive printers with RS-232-C ports.
The inputs to Counters A and B can be individually selected as positive or negative sensing inputs by changing the input polarity jumpers on the counter printed circuit board (PWB). The negative input mode is designed to accept NIM standard fast negative logic pulses with a fixed threshold of -250 mV over a 50Ω input impedance. The negative input can handle count rates up to 100 MHz. Positive input mode can accept count rates up to 25 MHz on 1000Ω input impedance. To increase flexibility in positive input mode, both counters contain precision discriminators. The discriminator threshold can be set from +100 mV to +9.5 V using the front-panel, 25-turn trimpot. The threshold can be adjusted to suit the amplitude of a specific logic pulse source or used as a precision integral discriminator on analog pulses. For the latter application, the discriminator's TTL logic output is a test point on the front panel. These outputs can be used to trigger the oscilloscope when viewing analog signals at the oscilloscope's counter input. The oscilloscope trace will show the signal counted by the Type 994, allowing the threshold to be adjusted accordingly.
All frequently used functions are conveniently accessible via the front panel. Count, stop and reset functions can be manually controlled via three buttons. When the Model 994 is enabled for counting, the door LED will illuminate. Time base button selects 0.01 second, 0.01 minute or external time base. In external mode, the preset counter counts events at the counter A input. The "Display" button switches the display to display the contents of counter A, the preset stop value, or the contents of counter B. To change a preset, you must first select the preset mode via the Display button. The Preset Select button can then be used to select M, N or P for adjustments. Use the "Preset Advanced" button to change the value of M, N or P. There are LED indicators on the display to indicate whether M, N, or P has been selected, to warn when an overflow occurs in Counter A or Counter B, and to indicate when the computer in remote mode should disable the front panel controls. The Dwell knob adjusts the display's dwell time from 1 second to 10 seconds when the Model 994 is in automatic recirculation mode.
The counting function of the entire module can be disabled by holding the "enable" input below +1.5 V using an external signal source. The door LED will then turn off. If counting mode is activated, an open circuit at the Enable input or >+3 V allows the instrument to start counting. The interval output of another ORTEC timer can perform this function to synchronize the Model 994's count with another timer. The interval outputs of all ORTEC timers provide nominal +5 V when counting and <+0.5 V when counting is disabled.
The A and B counter inputs can be independently gated via the Gate A and Gate B inputs on the rear panel. The interface connector for the RS-232-C option is located on the rear panel. Each counter has a dedicated rear panel output for signal overflow. Counting these overflows on another counter extends the counting capacity of the Model 994.
The Model 994 is powered by ±12 V and +6 V supplies from the standard NIM power box. For power boxes that do not include a +6 V power supply, the internal +6 V power supply option is available. This option is field installable and draws power from the 117 V AC power cord in the power box.
