How can I configure the CTR 1 GATE to output a pulse (10us long) and then immediatel​y take a pulse width measuremen​t?

Question

I'm using the BNC-2120 DAQ and LabVIEW for interfacing with an ultrasonic position sensor.  The sensor is the "PING))) ultrasonic range finder."  It measures the distance from PING))) to some object directly in front of it.  It has 3 pins (5V, ground, and a signal pin).  The 5V and ground are easily taken care of with the 5V and digital ground outputs on the DAQ.  The signal (SIG) pin works in the following way:
1) Send a 10us, 5V pulse to SIG.  This triggers PING))).
2) Wait 200us.  PING))) takes a distance measurement.
3) SIG outputs a square wave with a specific pulse width. 
The pulse width varies with the distance of the object away from PING))).  I've tried using CTR 1 OUT to generate the pulse, and then using CTR 1 GATE to measure the pulse width.  However, the measurement is stuck measuring 0V because CTR 1 OUT is on the same pin!  So PING))) tries to output a pulse, but cannot due to the fact that CTR 1 OUT forces the SIG voltage to be 0V.  So I need to use just one counter I/O line to trigger and measure.  How can I do this?

Answer 1

Hi Matttastica, 
What DAQ
card are you using? The reason I ask is that the PFI lines on some cards can be
used as PFI lines and digital I/O lines, while on others they can only be used
as PFI lines. The pinouts and diagrams for our cards can be found at www.ni.com/manuals. You can accomplish
what you are looking to do if your PFI line is a DIO lines as well.
I would
suggest doing two tasks, one for counter output and one for counter input.
(Note that both these are not going to be the same line). The counter output
will be used to do a single pulse generation, while the counter input will be
used to read back the period. 
This will
work because, at first, we are going to route the line from the counter output
to the counter input line by using a ‘DAQmx connect terminals’ (basically
making the input line an output line for a moment). First, setup the tasks and
the DAQmx connect, start the tasks and have the counter output do a ‘wait until
done’ to ensure the pulse is sent. After this is done, do a DAQmx disconnect
terminals (turning the input back from an output to an input), and then do a
DAQmx read for your counter input (period measurement). You may look at the
shipping examples for pulse generation and period measurement in LabVIEW (Help »
Find Examples... » Search tab … search for examples)
One note is
that since this is software timed, it may not be fast enough to meet your 200us
timing. If this ends up being the case, please look at the 6552, as it can do
per clock cycle direction change very fast.
David L.
Systems Engineering
National Instruments