Problems with DIO using USB-6008

[wissels]

Hello,
I'm trying to use a USB-6008 DAQ to externally control a stpper motor connected to a chopper drive.  The USB-6008 needs to send the chopper drive state, direction and enable commands (or direction and single-step commands) to the chopper drive.  This is done through TTL connections on the chopper drive. 

As a first order approximation, I used the USB-600x Interactive Control vi to manually control the signals sent to the drive through pins 17-26.  I used some of the lines to send the direction, enable, state (low/high), and half/full step commands to the chopper drive.  I also read in a few lines including the clock and frequency at which the motor returned to its "home" state (or when the motor had completed one revolution.)  This seemed to work fine, until I tried measuring the clock on the chopper drive using an oscilloscope (and next using a LabView vi which crashed repeatedly ).

Now when using the USB-6008, when writing a high signal to the lines  to have an oscillating pulse of about 0.2V.  All of the input pins also exhibit this behavior. The reference pin (pin 30 +2.5V) gives a stable +2.5V while pin 31 (ref +5.0V) has the same behavior as the pins pulled high.  When writing a low signal to the lines, however, the signal is a stable 0V.  Also, when I try to control the chopper drive manually (using on-board switches), the motor vibrates with a frequency similar to that found in the DIO lines.

I'm worried that I damaged the USB-6008 in some way and any suggestions would be appreciated.
Stephanie

[Doug M]

Stephanie,
Do you mean to say that when you write out a logic high signal on your DIO lines and measure the voltage with a DMM, you are reading only .2V? Or what do you mean by oscillate? Have you tried measuring the output of the DIO lines while not connected to a load?

Doug M
Applications Engineer
National Instruments

[wissels]

Hi Doug,

Do you mean to say that when you write out a logic high signal on your DIO lines and measure the voltage with a DMM, you are reading only .2V?
Yes.  I was using the example vi (USB-600x Interactive Control.vi) to send the logic high signal, so I doubt it was a buggy program. 

Or what do you mean by oscillate?
Sorry.  I wasn't very clear in my first post. The signal remains ~0.2V, but seems to have periodic inductive jumps in voltage.  I don't know right now what the frequency or amplitude of these jumps is.  (These jumps are visible on the power supply of the chopper drive in that the current and voltage being pulled jumps as well). 

Have you tried measuring the output of the DIO lines while not connected to a load?
Yes.  The lines exhibit the same behavior described in the first thread I posted. When the chopper drive and motor are run manually (ie. via the on-board switches of the chopper drive instead of the USB 6008) , the motor behaves normally. 

Thanks,
Stephanie

[Doug M]

Stephanie,
If you have completely disconnected your stepper driver from the 6008 and it is reading .2V when in the high state, it is likely the device is damaged.  The digital lines on the 6008 are by default configured in an open collector state and can only source .6 mA of current at 5V.  You can connect an external pullup resistor that can increase the output current for a channel to 8.5mA.  Trying to draw more current than this could damage the device.  Trying to sink more current than this could also damage the device.  See pages 15-17, 21 of the USB-6008/6009 user manual available here.

Doug M
Applications Engineer
National Instruments

[Sergey(PSU)]

to Doug M

Can you describe, please, or give a link about pull up resistor. Because, I need to drive a relay and the current from DO is just a bit not enough(it should be anough according to relay specification).

I have tried to drive a relay from AO and it works. I am just not sure if I will need AO for other purposes.

Thanks. Sergey.

[Doug M]

Sergey,
The pages in the manual that I described in my last post are especially good to read.  The pullup resistor is described on page 16.  Basically you are adding in a resistor from the digital line to the 5V pin.  The manual describes how to determine the value of the pullup resistor.

[Sergey(PSU)]

To Doug M

Thanks.

[wissels]

Thanks, Doug.

[AleDa]

I worte a massage about open-collector and USB 6008

http://forums.ni.com/ni/board/message?board.id=250&message.id=46479&query.id=53995#M46479

but nobody answered...

may you help me?

Thanks.