C Terminals
• Maximum input frequency <1 kHz
• CRBasic instruction: TimerIO()
• Rising or falling edges of a square-wave signal are detected:
o Rising edge — transition from <1.5 Vdc to >3.5 Vdc.
o Falling edge — transition from >3.5 Vdc to <1.5 Vdc.
• Edge-timing resolution is approximately 540 ns.
8.1.3.6 Edge Counting
Edge counts can be measured on C terminals.
o
C Terminals
• Maximum input frequency 400 kHz
• CRBasic instruction: TimerIO()
• Rising or falling edges of a square-wave signal are detected:
o Rising edge — transition from <1.5 Vdc to >3.5 Vdc.
o Falling edge — transition from >3.5 Vdc to <1.5 Vdc.
8.1.3.7 Pulse Measurement Tips
Basic connection of pulse-output sensors is illustrated in table Switch Closures
and Open Collectors
(p. 356, p. 357)
The PulseCount() instruction, whether measuring pulse inputs on P or C
terminals, uses dedicated 24-bit counters to accumulate all counts over the
programmed 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. 517) ) so are usually measured using
the PulseCount() frequency-output 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. 646) module to convert non-TTL-level signals, including
low-level ac signals, to TTL levels for input into C terminals.
• As shown in the table Switch Closures and Open Collectors on C Terminals
(p. 357),
C terminals, 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, C terminals have an input resistance of only 220 Ω.
356
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