Section 8. Operation
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 Timer Input on I/O NAN Conditions
• NAN is the result of a TimerIO() measurement if one of the following
occurs:
o Timeout expires
o The signal frequency is too fast (> 3 KHz). When a C terminal
experiences a too fast frequency, the CR800 operating system
disables the interrupt that is capturing the precise time until the next
scan is serviced. This is done so that the CR800 processor does not
get occupied by excessive interrupts. A small RC filter retrofitted
to the sensor switch should fix the problem.
8.1.3.8 Pulse Measurement Tips
Basic connection of pulse-output sensors is illustrated in table Switch Closures
and Open Collectors
(p. 378, p. 378).
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
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