OMC-042 / 043 / 044 Data loggers
Users
’ manual
page 36
Digital Sensors- pulse input
This section describes the operation of digital sensors, NOT to be confused with serial sensors, which
also is a digital technique. A digital input is an input which measures the digital value of the input signal.
This is a discrete value “0” or “1”. Examples of digital sensors are: switches, rain gauges and digital pulse
devices. Digital inputs have the advantage that they are not that depended on their sample rate as analog
sensors are. This is because they are interrupt-driven. There are two different modes for sampling the
status of a digital input: counting events (pulses) or checking status. The first is called “digital pulse” and
the second “digital alarm”. Below, a menu of the settings of a digital pulse input is shown.
Sample interval
This is exactly the same feature as in other menu’s, although there is something interesting to notice
about the sample interval of a digital input (pulse sensor). With analog inputs, the sample rate defines
how many times an input is sampled. So, if your sample interval is very long, you may miss some events.
But with digital inputs, this is NOT the case. The digital inputs are interrupt driven, and respond
immediately on an event. So, even when your sample rate for the digital input is very low, no events are
missing. This is an important difference! So, what is the use of the sample interval in respect to digital
inputs? It defines the update rate of the counter value connected to that port.
Example: Suppose a user connects a wind speed sensor (anemometer) to the digital input, and this
sensor generates a pulse with a frequency of 13 Hz. So every second, the sensor has generated 13
pulses. Now the sample interval was set to 5 seconds. What happens is this: The data logger will count
all pulses, and update the counter value every 5 seconds. So, the user has every 5 seconds a new data
value.
Port mode
This defines the electrical behavior of the input (pull up / pull down)
Pull up
In this mode, a resistor is connected (from the input) to the positive rail of the internal power supply. So, in
order to create an event, the external device must pull the input to the ground. So, use this mode when
you have
a “negative switching” sensor (a device that switches to ground and has no voltage to drive the
port, e.g. a tipping bucket rain gauge).
Digital pulse sensor
[0] Exit
[1] Name >> Digital pulse
[2] Sample interval >> Data log interval
[3] Port mode >> Port 2; Internal pull up
[4] Register mode >> Pulse (low frequency)
[5] Units per pulse >> 1
[6] Register value >> 0 Pulses
[7] Register reset >> Off
[8] Log each counter change >> Off
[9] Counter (unit) >> Counter
[A] Quantity (unit) >> Not used
[B] Mean rate (unit/h) >> Not used
[C] Max rate (unit/h) >> Not used
[D] Min rate (unit/h) >> Not used
[R] Remove
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