Are all DIOs on an USB-6009 shorted?

[Gurdas]

Hi!,

We see a strange behaviour on our USB-6009.

The signal connected to any digital input (say 5V) is visible on all the DIOs!! This does not happen once we have launched our program because the channels get configured. This happens when our program is closed and the USB device is connected to the PC. The PC could be on or off, makes no difference.

What happens due to this is that some of the other DO get 5V and they start acting strange (because we have relays firing on those channels).

One suggestion is to insert a 10K pull-down resistance between any of the DIO and the DGND. I am yet to try that.

But before I do that I would like to know what is happening here and whteher this is expected behviour? I have never seen this before!

Thanks,

[Wayne.C]

Spec for 6009 says following about DIO:

The default configuration of the NI USB-6008/6009 DIO ports is open collector, allowing 5 V operation, with an onboard 4.7 k

Ω pull-up resistor.

I don''t think a pulldown resistor will fix your problem.  I'm guessing that you currently have one side of your relay coils connected to common and are switching +DC to the other side of the coils.  ( sourcing )  You should consider modifying your external circuit so one side of relay coils is connected to +DC and use the DO to switch a low level to other side of coil.  ( sinking ) 

What you want is to use DO as a sinking output, not a sourcing.  That way when the DIO pins are unconfigured and pulled high, it will not cause your relays to be energized.

[Gurdas]

Centerbolt,

Thanks for that input. Pls note that we keep switching the relays on and off at various points in the application. Which means we send both +5VDC and 0VDC to the relays, depending on whether we want  them to go off or on.

In light of this, does your suggestion still hold good?

We also observe that the DC signal on PFI pin is spread out into all the 12 DIOs. Is this expected or should we send the device back to NI for repair/repalcement?

Thanks.

[Wayne.C]

If one side of relay coil is permanently connected to +5VDC and the DO is high (+5VDC ), there will be 0V potential across coil and relay will not actuate.  When DO is low, then there will be potential across coil and relay will actuate.  Make sure you are using flyback diodes across your relay coils to protect the DO's from damage.

To make this work you would need to change your wiring to relay coils and invert the sense of your outputs.  In sinking configuration, all outputs are active low.

If your DO pins are still in the default open collector configuration, I would expect to read +5VDC on all your DO when device is unconfigured.  Remember each output has an internal 4.7K pullup to +5VDC.  I don't believe your device is damaged because it is operating correctly when your program is up and running.

[Gurdas]

Please help me understand the ideal operation of this device for below condition.

(State 1)

The USB-6009 is disconnected from the PC. There are no voltages/wires coming to it. What should my multimeter show when I measure the DIO pins with respect to DGND?

(State2)

The USB-6009 is disconnected from the PC. There is a +5VDC coming to PFI. What should my multimeter show when I measure the DIO pins with respect to DGND?

(State 3)

The USB-6009 is connected to the PC and PC is booted up but app is not running. There are no voltages/wires coming to it. What should my multimeter show when I measure the DIO pins with respect to DGND?

(State 4)

The USB-6009 is connected to the PC and PC is booted up but app is not running. There is a +5VDC coming to PFI. What should my multimeter show when I measure the DIO pins with respect to DGND?

I just had a discussion with the engineer integrating this device. He said that with the device on its own (lying free on the table or connected to PC but app not running), whenever there is +5VDC on the PFI, he reads +2.5VDC (approx.) on the DIOs. And the +2.5V and +5V reference signals dimish by the value on the DIO. That is they become 0VDC and +2.5VDC respectively!!

Thanks,

[Wayne.C]

When you took the voltage readings, was the output wiring to the relays disconnected?

[elecengineer]

I am having similar problems with my 6009. There is very strange behavior on the digital I/O lines. I can write a high or low value to the P0.X lines but as soon as I read with nothing connected it comes back high. If I put a meter between a digital output line and a ground line, there is 1 mA of current flowing. Some devices that require high-impedance such as my ADC on the board that needs it's output connected to a digital I/O could potentially fail because of this.
Any fix?


Message Edited by elecengineer on 02-05-2008 09:33 AM

[Hani R]

Hi elecengineer,

What do you mean when you say "as I read with nothing connected it comes back high."  Are you toggling between high and low while taking the measurement or are you not changing the state at all.  Correct me if I'm wrong, but what I understand is that you are measuring the voltage out of the DIO pins when they are in their default state.  The following KnowledgeBase article, KB 44EBIR4G: Can I Use a Pull-down Resistor on My USB-6008/6009 Digital Outputs to Obtain a Logic Lo..., mentions that the default state will be high (as you've experienced).

Some external circuitry may be required (voltage follower for example) to act as a buffer to your ADC.  The alternative is to use a DAQ card that has programmable DIO start up states (mentioned in the article). 

[elecengineer]

Thanks for the info. I've found some work-arounds and stumbled upon the article you mentioned about the default start-up states. I appreciate the follow-up.