Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 317

Section 8. Operation 8.1.5.3 Pulse Measurement Tips • The PulseCount() instruction, whether measuring pulse inputs on pulse channels (P1 through P2) or on digital I/O channels (C1 - C8), uses dedicated 24-bit counters to accumulate all counts over the user-specified scan interval. The resolution of pulse counters is one count or 1 Hz. Counters are read at the beginning of each scan and then cleared. Counters will overflow if accumulated counts exceed 16,777,216, resulting in erroneous measurements. • Counts are the preferred PulseCount() output option when measuring the number of tips from a tipping bucket rain gage or the number of times a door opens. Many pulse output sensors, such as anemometers and flow meters, are calibrated in terms of frequency (Hz (p. 456) ) so are usually measured using the PulseCount() frequency option. • Accuracy of PulseCount() is limited by a small scan-interval error of ±(3 ppm of scan interval + 10 µs), plus the measurement resolution error of ±1 / (scan interval). The sum is essentially ±1 / (scan interval). • Use the LLAC4 (p. 560) module to convert non-TTL level signals, including low-level ac signals, to TTL levels for input into digital I/O channels C1 - C8. • When digital I/O channels C1 - C8 measure switch-closure inputs, pull-up resistors may be required. Figure Connecting Switch Closures to Digital I/O (p. 318) show how pull-up resistors can be incorporated into a wiring scheme. • As shown in figure Connecting Switch Closures to Digital I/O (p. 318), digital I/O inputs, with regard to the 6.2-V Zener diode, have an input resistance of 100 kΩ with input voltages < 6.2 Vdc. For input voltages ≥ 6.2 Vdc, the inputs have an input resistance of only 220 Ω. FIGURE. Connecting Switch Closures to Digital I/O Figure 100: Connecting switch closures to digital I/O 317