Campbell Scientific SR50-L CSC Ultrasonic Distance Sensor - Manual (Page 16)

SR50 Sonic Ranging Sensor
10
3.3 Pulse Train Output
This option is most commonly used with 21X or CR10s without SDI-12. The
SR50 sensor can also output a pulse train to represent the distance to target
value. The pulse train option outputs a series of 5 Volt pulses on the SDI-
12/Data Bus with each pulse representing a distance to target unit. Several
options are available in the type of pulse train that is output. The pulse
frequency can be selected as either 1000 Hz or 100 Hz. Selectable units are
2.5 mm/pulse, 1cm/pulse, or 0.1 inch/pulse.
Because the 21X datalogger does not have SDI-12 capabilities, the SR50’s
pulse train output is measured using the datalogger’s P3 Pulse Count
instruction. Configure the pulse channel as “high frequency” with option 00.
All CR10X, CR510, and CR23X dataloggers have SDI-12 capabilities.
Typically the SR50 is measured with the SDI-12 output format with CR10
dataloggers. If your CR10 does not have SDI-12 capabilities, the SR50’s pulse
train output is measured using the datalogger’s P3 Pulse Count instruction.
Configure the CR10 pulse channel as “low level ac” with option 01.
To place the sensor in Pulse Train mode, select the desired option by setting
the internal Address/Option jumpers (see Figure 8) according to the list in
Table 2. Only addresses 0 to 5 are applicable for the Pulse Train output. The
sensor initiates the measurement when the Sensor Enable Line is pulsed high
to +5 Volts. The pulse width should be a minimum of 5 ms and a maximum of
250 ms in width. When the SR50 detects that the Sensor Enable Line is high,
it makes a measurement. When complete, the SR50 outputs the appropriate
pulse train on the SDI-12 Bus. The SR50 will output the number of pulses
based on the closest target that was detected. If the SR50 is unable to detect
any targets, the sensor will output a single pulse to indicate that no reading was
obtained.
Please note that it is more critical to consider the time it takes to make a
measure and transmit the result with this option. After the SR50 detects the
pulse on the Sensor Enable Line, the measurement begins and can take up to 3
seconds before the pulse train begins. The time required to transmit the pulse
train varies significantly depending on the pulse option selected and the
distance to target. For example, if a target was 5 meters from the sensor and
the 1000 Hz, 1 cm/pulse option was selected, the pulse train would require 0.5
seconds to transmit. If the 100 Hz, 2.5mm/pulse option were selected for the
same distance, the pulse train would require 20 seconds.
It is also important not to leave the Sensor Enable Line high for too long. If
the sensor completes the measurement, and detects that the Sensor Enable Line
is still high it will immediately initiate another measurement and output a
second pulse train. This may cause a second pulse train to be counted and
added with the first pulse train creating an error in the measurement recorded.