Campbell Scientific CR3000 CR3000 Micrologger - Page 321
Low-Level ac P1 - P4, 1.5.1.3 Switch Closure P1 - P4, 1.5.2.1 High Frequency Mode
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Section 8. Operation 8.1.5.1.2 Low-Level ac (P1 - P4) Rotating magnetic-pickup sensors commonly generate ac output voltages ranging from thousandths of Volts at low rotational speeds to several volts at high rotational speeds. Pulse channels contain internal signal-conditioning hardware for measuring low-level ac-output sensors. When configured for low-level ac, P1 through P4 measure signals ranging from 20-mV RMS (±28 mV peak) to 14-V RMS (±20 V peak). Internal ac coupling is incorporated in the low-level ac hardware to eliminate dc offset voltages of up to ±0.5 Vdc. 8.1.5.1.3 Switch Closure (P1 - P4) Switch-closure mode measures switch closure events, such as occur with a common tipping bucket rain gage. An internal 100-kΩ pull-up resistor pulls an input to 5 Vdc with the switch open, whereas a switch closure to ground pulls the input to 0 V. An internal 3.3-ms time-constant RC-debounce filter eliminates multiple counts from a single switch closure event. 8.1.5.2 Pulse Input on Digital I/O Channels C1 - C8 Digital I/O channels C1 - C8 can be used to measure pulse inputs between -8.0 and +16 Vdc. Low frequency mode ( +16 Vdc is to be measured. Under no circumstances should voltages greater than ±50 V be connected to channels C1 - C8. 8.1.5.2.1 High Frequency Mode Digital I/O channels have a small 25-ns input RC-filter time constant between the terminal block and the CMOS input buffer, which allows for higher-frequency pulse counting (up to 400 kHz) when compared with pulse-input channels P1 - P4 (250 kHz maximum). Switch-closure mode is a special case edge-count function. Because of signal conditioning for debounce, 150 Hz is the maximum input frequency at which switch closures can be measured on digital I/O channels. 321