USB 6008 power up inconsistent

[VeeJay]

Hi,

Now that I have read enough about the DAQ 6008 and know that the DIOs power up to 5V when it is connected to the USB. And, knowing that , I have worked around it. But, here is my problem. very random behavior occurs in that few channels dont power up and I disconnect the DAQ and connect it back; they do. This random behavior is affecting my system. Should I trust the internal pull up or should I myself pull the voltage up to 5V using an external 4.7k resistor?

Thanks!

V

[EricS.]

Hi VeeJay,

I just looked through the USB-6008 user manual and specifications (found here) on page 23 it states:

At system startup and reset, the hardware sets all DIO lines to

high-impedance inputs. The DAQ device does not drive the signal high

or low. Each line has a weak pull-up resistor connected to it. 

This means that the default power up start would have each of the digital lines set for input and pulled up to 5V internally.  You shouldn't need an external pull up, though you can see on page 22 you can definitely add one.

My question is what ports are not at 5V when you plug the 6008 in the first time? Also, is it possible that some of the lines you have connected to the digital ports are driving the lines low?  Do you notice this behavior when you don't have anything connected to the 6008?

[VeeJay]

Thanks  Eric. The lines power up but some lines are random. It gets me to disconnect the DAQ from the USB and then connect it back. I have become so paranoid that everytime I connect the DAQ, I use a voltmeter and check the voltages. My circuit setup needs the lines to be powered up; I have worked around the pull up state of the DAQ 6008. But, the random behaviour concerns me. So, I added external resistors to make sure  the lines pull up to 5V. When the DAQ isnt connected to anything, everything seems fine. When I connect it to the system I use, random behavior is observed.

Vj

[EricS.]

Since everything powers up as expected when the DAQ isn't connected to anything, it means something that your connecting is pulling it low, even if not intentionally.  If you want to look into that more, what are those signals coming from? Do you know what logic level they will be at when the system starts up?

But, it's good to hear the external pull-up resistors are enough to get rid of the random behavior. 

[VeeJay]

well, The output of the DAQ goes first to an inverter 7404IC to bring the high states to low. And then to an opamp. So, I dont see how these components would bring the state to low. Any thoughts?

THanks!

🙂 Vj

[RavensFan]

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VeeJay wrote:

well, The output of the DAQ goes first to an inverter 7404IC to bring the high states to low. And then to an opamp. So, I dont see how these components would bring the state to low. Any thoughts?

THanks!

🙂 Vj

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Huh?  You have the output going to an inverter that brings the high states to low, and you don't see how it would bring the output to low?

[VeeJay]

Ok. Maybe I wasnt clear. The OUtput of the inverter is low.  When the DAQ pulls up to 5V, the inverter brings it to 0V. I know that. But, in some cases, the few DAQ channels dont power up to 5V, which implies the output of the inverter is 5V and that screws up my system.

VJ

[johnsold]

VJ,

As has been pointed out the USB-6008 has all digital lines set to be inputs at power up.  Since you sometimes see high outputs from your inverter,  something in your circuit most likely is causing the behavior.  TTL devices require substantial current to ground (~ 1 mA) to bring the inputs low.

When you have a device which cannot be specified to provide the output you need at power up, it is best to provide power valid logic to protect your system.  One way of doing this is to control the power to the external system from one of the outputs of the USB-6008.  A solid state relay driven by the USB-6008 turns on the power to the rest of the system.

If that is not feasible, another way is to use two flip-flops in a debounce circuit.  It would require two pulses from the USB-6008 to enable the other outputs.

Lynn