Campbell-scientific CR800 and CR850 Measurement and Control Systems Uživatelský manuál stáhnout pdf (Strana 300)

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Section 8. Operation

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Figure 94: Pulse-input channels

8.1.5.1.1 High-frequency Pulse (P1 - P2)

High-frequency pulse inputs are routed to an inverting CMOS input buffer with

input hysteresis. The CMOS input buffer is an output zero level with its input ≥

2.2 V, and an output one level with its input ≤ 0.9 V. When a pulse channel is

configured for high-frequency pulse, an internal 100-kΩ pull-up resistor to 5 Vdc

on the P1 or P2 input is automatically employed. This pull-up resistor

accommodates open-collector (open-drain) output devices for high-frequency

input.

8.1.5.1.2 Low-Level ac (P1 - P2)

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 P2 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 - P2)

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.

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