994 CCNIM Dual Counter and Timer

• Two 8-decade counters and a timer with the configuration flexibility to serve a variety of measurement needs
• IEEE-488 and RS-232-C options provide CCNIM capability with full computer control and readout
• Can directly drive printers having RS-232-C or IEEE-488 ports
• All options are field-installable
• An 8-decade LED display provides instantaneous readout of the entire counter capacity, even in dimly lighted rooms
• All commonly used controls are easily accessible on the front panel
• 100-MHz counting rate capability
• Preset time or counts set with the precision of a two-digit and decade selection

The ORTEC Model 994 Dual Counter and Timer incorporates two eight-decade counters and a blind preset timer. Considerable functional flexibility is designed into the instrument, allowing it to be configured for a variety of measurement tasks. Typically, it can be used as two counters recording separate events under the control of the preset blind timer. When continuous readout of the time is needed, Counter A can be diverted to count the time while Counter B records external events. This provides the function of a counter and a displayed preset timer. In some applications, the time taken to count a preset number of events must be measured. For this application, Counter A, coupled with the preset blind counter, can be used as a preset counter while Counter B records the time in 0.01-second intervals. In measurements where it is important to correct for the dead time of the detector and its associated electronics, the Gate A input can be switched to also gate the time clock On and Off with a 100-ns time resolution. 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).

Excellent flexibility in setting the preset value is offered by the MN X 10^P selection. The M and N values provide two-digit precision, while P selects the decade. Presets can be chosen in the ranges of 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- decade LED display that offers instantaneous visual readout of the full contents of Counter A or B, even in a dimly lighted room. By adding field-installable options, considerably enhanced readout and control capabilities can be incorporated.

The full power of CCNIM (Computer-Controlled NIM) can be obtained by adding the IEEE-488 option or the RS-232-C option. These plug-in boards allow computer control of all functions normally selectable from the front panel, including start and stop count, readout, reset, setting the preset value, selecting the displayed counter, and selecting the desired time base. To eliminate accidental operator interference, the computer can disable all front-panel controls in the Remote mode. Computer readout with either of the two CCNIM options includes A and B counts, the preset value, and which counter is being displayed. The IEEE-488 option also reads the overflow status for both counters. Implementation of the IEEE-488 interface in the Model 994 is compatible with the Standard NIM Digital Bus.* The CCNIM options can directly drive printers having RS-232-C or IEEE-488 ports.

The inputs to Counters A and B are individually selectable as either positive or negative sensing inputs by changing the Input Polarity Jumpers on the counter printed wiring board (PWB). The negative input mode is designed to accept NIM-standard, fast-negative logic pulses with a fixed threshold of –250 mV on a 50-W input impedance. The negative inputs can handle counting rates up to 100 MHz. The positive input mode can accept counting rates up to 25 MHz on a 1000-W input impedance. To enhance the flexibility of the positive input mode, precision discriminators are included on both counters. The discriminator thresholds are variable over the range from +100 mV to +9.5 V using front-panel, 25-turn trimpots. The thresholds can be adjusted to suit the amplitude of a specific source of logic pulses or used as precision integral discriminators on analog pulses. For the latter application, the TTL logic outputs of the discriminators are provided as test points on the front panel. These outputs can be used to trigger an oscilloscope while viewing the analog signal at the counter input on the oscilloscope. The oscilloscope trace will show the signals that are being counted by the Model 994, thus permitting a very selective adjustment of the threshold.

All the commonly used functions are conveniently accessible on the front panel. Manual control of the Count, Stop, and Reset functions is via three push-buttons. The Gate LED is illuminated when the Model 994 is enabled to count. Selection of the 0.01 second, 0.01 minute, or external time base is made by the Time Base push-button. In the external mode, the preset counter counts the events at the counter A input. The Display push-button switches the display to show the contents of Counter A, the preset stop value, or the contents of Counter B. To change the preset value, the Preset mode must first be selected with the Display push-button. Subsequently, the Preset Select push-button is used to choose M, N, or P for adjustment. Changing the value of M, N, or P is accomplished with the Preset Advance push-button. The display contains LED flags to indicate whether M, N, or P has been selected, to warn when overflows have occurred in Counter A or Counter B, and to advise when the front-panel controls are disabled by the computer in the Remote mode. When the Model 994 is used in the automatic recycle mode, the Dwell knob adjusts the dwell time of the display from 1 to 10 seconds.

The counting function of the entire module can be disabled by holding the Enable input below +1.5 V using an external signal source. This condition also turns Off the Gate LED. Open circuit or >+3 V at the Enable input allows the instrument to count, if the Count mode has been activated. The Interval output of another ORTEC timer can perform this function to synchronize the Model 994 counting with the other timer. The Interval outputs on all ORTEC timers provide nominally +5 V when counting and <+0.5 V when counting is inhibited.

Independent gating of the A and B Counter inputs can be achieved with the Gate A and Gate B inputs on the rear panel. Interface connectors for the IEEE-488, RS-232-C, and print loop options are also located on the rear panel. Each counter has a rear-panel output dedicated to signaling overflows. Counting these overflows on another counter extends the counting capacity of the Model 994.

The Model 994 derives its power from the ±12 V and +6 V supplies in a standard NIM bin with power supply. For bins that do not contain a +6 V supply, an Internal +6 V Supply option is available. This option is field-installable and derives its power from the 117 V ac lines in the bin.

*Please refer to "Standard NIM Digital Bus (NIM/488)," DOE/ER-0457T, U.S. NIM committee, May 1990; Standard NIM Instrumentation System, NTIS, U.S. Department of Commerce, Springfield, Virginia 22161.

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