I want to count pulses with negative polarity using an NI9361. The signal is coming from a BNC cable, which is lead into a BNC breakout adapter. If I connect the signal pin to DI+ and the ground to GND my measurement program does not count at all.
I guess thats because it is supposed to measure a positive signal.
My next attempt: using the differential measuring mode by connecting the signal to DI- and the ground to DI+. Counting is fine, as long as only one device is simultaneously measured. As I ideally want to perform 8 parallel measurements I tried two devices at once (one is the final measurement application, one a wavetect pulse generator). That's where I ran into the following phenomenon:
If the pulse generator is connected to ctr0 and nothing else is connected, it counts just fine (I additionally verified this using an ortec 874 timer/counter module). As soon as I connect the second device, the count rate of ctr0 goes up by about the count rate of ctr1. If I disconnect the second device, the count rate goes back down again. I suppose this happens due to some disturbance occurs from connecting the ground to DI+?
How can I measure some negative input signal without running into these problems?
To measure a negative input signal you should wire your device as shown in Figure 5 below.
Something to keep in mind is that the COM terminal is the floating ground reference for the input comparator (Figure 1).
All channels share the same COM reference so it sounds like there might be an interconnection of ground to a signal line.
If you're able to share a simplified wiring diagram of what terminals you're connecting to both of your devices that can help the forum troubleshoot.
So I can proceed like that even If my device is actually not intendet to be a differential device? (As I just have a negative pulse against ground)
My wiring is pretty simply explained:
SIG (Dev 1) -> DI0-
GND (Dev 1) -> DI0+
SIG (Dev 2) -> DI1-
GND (Dev 2) -> DI2+
My device is supplied with 2x 18-24 V, as well as a high voltage of about 1200 V if that helps by any chance.
What is COM of the 9361 connected to? Also how is the 1200V related to this measurement setup?
The 9361 should not be used with such dangerous voltage levels.
The COM is not connected at all. Should it be connected to the ground of the supply?
The high voltage is only used to power a part of the measurement setup. The signal put into the 9361 has voltages of around 5 Volts.
Relooking through the specifications, you'll need a logic level translator/inverter to convert your negative pulse to a positive one.
The 9361 DI+ and DI- input ranges are +0 to +5V, they have overvoltage protection up to +/-30V but we should keep the input signals within the operating range.
Once you have converted your negative pulse to a positive pulse, you will need to connect your COM to the ground reference of your positive pulse circuit.