MATLAB: Hi/Low analog output to USB

analogData Acquisition Toolboxusb

Hi,

I am trying to use MatLAB script as an on/off switch. In simple words, I wish to set a constant output of ~0V (low) or ~5V (high) at the USB port whenever I need to. How could I do this in a most simple and inexpensive way?

Thank you

Best Answer

This cannot be done directly with USB. USB is a shared serial bus, and only digital signals can be carried on it. If you were to use USB you would need to have a USB-interfaced device on the other end that decoded the serial commands to turn a switch on or off.

One important question for this purpose is what kind of rate of change do you need for this purpose? Another question is what precision you need in the timing. A third question is how much control you need over the pulse widths.

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MATLAB: Acquiring data from NI Daq and serial port simultaneously

The NiDaq card probably has some buffering on board, so if you were to start the NiDaq data collection and then were to do some data collection from the serial card, and then were to receive the buffered data from the NiDaq card, then the data will have been sampled simultaneously.

Acquiring the data from the two devices simultaneously is much more difficult, and is not possible with the hardware found on most systems. Acquiring data from the serial port normally requires one processor interrupt per byte, and you cannot have two different processor interrupts happening simultaneously. The work around here is to use a high speed serial card that has its own local interrupts and buffers the serial data and DMA's it to main memory: in theory multiple DMA's can happen simultaneously (but users do not often have any control about which DMA''s are going on.) This approach presumes, of course, that the NiDaq card also has DMA, which I have not researched.

Now, acquiring data from two sources simultaneously has nothing to do with getting synchronized samples from the two source. You would have a hope of getting synchronized samples from two data acquisition cards (or channels) by using the sample trigger signal for both, but Alas, since serial ports are asynchronous beasts, getting synchronized information through one of them is near hopeless.

When I see a question like this, unless the author has been very specific, I tend to think that what the author really wants is for the two signals to be time-stamped so that the information from the two sources can be brought in to alignment. Many NiDaq cards are able to time-stamp the data they acquire with a fairly decent precision. Unfortunately, time-stamping serial port data is very approximate at best... unless you can convince the device generating the serial data to construct time-stamps and send them as part of the data, and the clocks on the NiDaq and the remote serial device need to be aligned.

My recommendation would be to avoid this kind of problem if possible. It is almost always better to use multiple channels on the data acquisition card (NiDaq) if you need synchronization.

MATLAB: How to understand the mapping of analog I/O channels while using the NI PCI-6221 37 DAQ card with analog input block in Real Time Windows Target 3.6 (R2010b)

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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MATLAB: Acquiring data from NI Daq and serial port simultaneously

MATLAB: How to understand the mapping of analog I/O channels while using the NI PCI-6221 37 DAQ card with analog input block in Real Time Windows Target 3.6 (R2010b)

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