MATLAB: Unable to observe output from NI PCIe-6323 board in oscilloscope when using Real-Time Windows Target (SLDRT IN R2014a)

desktopdigitalfrequencynioutputpcie-6323pwmreal timesimulink

I am using Simulink Desktop Real Time to interact with a NI PCIe-6323 board. My aim is to generate PWM signals in Simulink and send it to the board in order to control a motor. I am currently generating the PWM signal in my Simulink model and sending it to the board using the Digital Output block (which is configured for the NI PCIe-6323 board). I have an oscilloscope connected to the board as well. When I run the model, I observe no output in the oscilloscope.

However, when I comment out the Digital Output block and use an Analog Output block, I see some output in the oscilloscope. Why is the Digital Output block not working for me?

Also, I'm trying to connect to the PFI pins on the NI board.

Best Answer

For your current workflow, it is recommended to use the Frequency Output block, as this will output a hardware-generated, stable PWM up to the maximum hardware clock frequency of the board. This cannot be achieved by generating the PWM signal by software and using Digital Output.

To set up the block, please note that the Frequency Output block accepts frequency (in Hertz) and duty (in the range 0 to 1) as either parameters or inputs. In addition, please set the counter channel to PWM generator.

As far as pin settings are concerned, please note that Simulink follows the standard NI pin assignment for counter I/O, as described in the NI documentation. The correct pins in this case would, therefore, be:

PWM channel 1 - CTR0OUT - PFI 12 - pin 2

PWM channel 2 - CTR1OUT - PFI 13 - pin 40

PWM channel 3 - CTR2OUT - PFI 14 - pin 1

PWM channel 4 - CTR3OUT - PFI 15 - pin 39

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MATLAB: Is PFI6 the default HwDigitalTriggerSource for the National Instruments USB-621x device if PFI<4..7> are strictly digital output lines

Since there is no direct way through the NI-DAQmx API to determine whether a PFI port is configured for input or output, it is not possible to maintain a list of valid terminals for the various National Instruments boards supported by the Data Acquisition Toolbox. The PFI6 port was traditionally the terminal used by E series devices for ANALOGOUTPUT hardware triggering, and thus was chosen to be the default.

Please consult your NI board documentation to determine whether PFI6 is a digital pin configured for output only. If so, set the 'HwDigitalTriggerSource' property of your ANALOGOUTPUT object to an appropriate pin that is configured for input.

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The two digital and 8 PFI I/O checkboxes allow you to choose if that particular digital pin should be input (when unchecked) or output (when checked). This does not apply to Analog Input and Analog Output which cannot change signal direction, therefore there are no checkboxes for them.

There's no need to map analog I/O (or other I/O) in any way. Hardware pins are hardwired to hardware channels and these are hardwired to Real Time Windows channel numbers. The only pitfall can be that while hardware channels are numbered from 0, RTWin channels are numbered from 1.

For example:

    pin  1  = hardware channel AI0 = RTWin channel 1 
    pin 21 = hardware channel AI1 = RTWin channel 2 
    pin 22 = hardware channel AI2 = RTWin channel 3

A pin diagram for the NI PCI-6221 hardware can be found at the following link:

 <http://sine.ni.com/ds/app/doc/p/id/ds-15/lang/en>

Important points in note while using an Analog I/O blocks in your models if you see an unexpected output from this block on a Simulink scope:

1. A first probable reason can be due to a cross-talk caused by unconnected analog input pins or something like that.

2. Ensure that the ground is connected correctly.

3. Does the single-ended/differential setting match your wiring (i.e. in the model check what the board options are set to: single-ended/differential) ?

4. What happens if the AI 0 pin is short-circuited to ground - does the Scope show zero in this case ?

5. Does Analog Output work correctly? Are the analog output voltages seen on the right pins? This is used to double check that the connector pin numbers are not swapped and correctly understood.

Best Answer

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