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System 8400 Updates: Hardware Trigger |
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The PSI 8400 has provision for an electronic signal (or hardware trigger) to initiate data acquisition. This feature is provided so that the user may control when data acquisition begins with a high degree of certainty. When hardware trigger is used, data acquisition begins within 6 microseconds after the trigger event. The hardware trigger may be used in two ways:
An example of when ATRIG might be used: The user is testing rotating machinery, and wishes data acquisition to begin at 0 degrees rotation, and each additional data point to be at 10 degree intervals. Thirty-seven ( 37 ) data points are desired - I.E. start at exactly zero degrees rotation, and take data including the return to zero. The user will supply equipment that will generate a pulse at each rotation point that the user desires to acquire data. 37 triggers will be supplied to the 8400, and the 8400 will acquire and return 37 measurement sets of data, one for each trigger event. An example of when ITRIG might be used:The user is testing in a "blow-down wind tunnel," and does not wish to begin data acquisition until the model is injected into the flow stream. One trigger event will be generated as the model is injected. The user then wants the 8400 to acquire data at exactly 100 Hz, for 240 seconds. After the single trigger event, data acquisition will be "internally triggered" by the 8400 internal clock. The Trigger Signal: The trigger signal may be either TTL ( 5 volt logic ) or open collector ( 12 volt ) logic, selected by jumpers on the PC-117 card in the 8400 SP chassis. The default is TTL ( 5 volt logic). Please note that the hardware detects the change in the level of the signal. Strictly speaking, the hardware does not detect the "edge" of the signal. The 8400 SP does a certain amount of signal conditioning, so that all data acquisition units in the 8400 SP chassis see a common "clean and sharp" trigger signal. On average, data acquisition begins about 3.8 microseconds after the signal changes from 5v to 0v, but all units will begin data acquisition within 6 microseconds (worst case) of the trigger event. In the default setup, the "Trigger Line" is held at +5 volts by a "pull up resistor" internal to the 8400. The simplest "trigger" may be implemented by simply connecting a "momentary closure, de-bounced switch" between the trigger input and trigger ground, with up to 5 meters of cable. Activating the switch will create a trigger event. If the user wishes to have an external electronic device create the trigger event, a simple "one-shot" that generates a square wave pulse of at least 50 micro-seconds is all that is required. If the trigger cable is less than 10 meters, TTL logic is sufficient. If the trigger cable is longer than 20 meters, open collector drivers are required. TTL is often sufficient for cables in the 10 meter to 20 meter range (depending on cable quality and electronic noise at the installation), but when in doubt, we recommend that open collector be used. |
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