Standard computer interfaces built in to the Model 974A include the RS-232-C and 20-mA loop. The command format adheres to the Standard NIM Digital Bus, an easy-to-use language for programming NIM instruments. All front-panel functions (with the exception of Display Test) may be remotely controlled via a computer or dumb-terminal. In addition, several functions not accessible via the front panel are accessible with the computer interfaces. The front panel may be locked out by the ENABLE_REMOTE command from a computer.
The input polarity to each of the four counting channels may be independently set using internal connectors. The maximum count rate for negative signals is 100 MHz; the maximum count rate for positive signals is 25 MHz. Each counting channel can be independently, externally gated through the Gate 1, 2, 3, and 4 inputs. All four counting channels may be simultaneously, externally gated through the Master Gate input.
The architecture of the ORTEC Model 974A Quad 100-MHz Counter/ Timer is designed for maximum flexibility. Counter channel 1 acts as the gate controller for counter channels 1, 2, 3, and 4. When counting is started, the input gates to counting channels 1, 2, 3, and 4 are opened. When the accumulated counts in counting channel 1 equal the selected preset value (selected and displayed on the front panel), the counting interval is terminated and the input gates to counting channels 1, 2, 3, and 4 are closed. If the Dwell control is turned fully counter-clockwise to the Off position, no new counting cycle will be initiated. If, however, the Dwell control is set for a chosen dwell time, a new counting cycle will be automatically initiated after the end of the chosen dwell time and automatic reset. Since the input to counting channel 1 can be selected as the internal 0.1-second time base, the internal 1-minute time base, or an external source, channel 1 can act as a presettable timer or a presettable counter.
The Event counter, which is accessible only through one of the computer interfaces, is primarily intended as a means of labeling printouts or listings of counting cycles. After the Model 974A receives an ENABLE_EVENT_AUTO command, the Event counter will increment one count at the end of each preset counting interval. The contents of the Event counter, along with the contents of each of the four 100-MHz counting channels, are output during the implementation of a SHOW_COUNTS command. This results in an integer labeling of the counting cycle printout. The Event counter may be used alternately as a counter of external pulses by using the ENABLE_EVENT_ EXTERNAL command. The maximum count rate input to the Event counter must be <4 kHz; its capacity is 8 decades. The ENABLE_EVENT_PRESET command allows the Event counter to control the total number of counting cycles in a given counting run. After an Event preset value is set via computer interface, the Event counter will allow continuous counter interval recycling until the accumulated value equals the preset value.