Section 8. Operation
300
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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