USB 6008 Pull Down Resistor

[ajmartin]

Hello,

I have a USB 6008 reading a 5V signal on one of DIO ports. The 5V is either there or the circuit is open. SInce the 6008 has internal pull ups an open looks like a true TTL signal. I am thinking about using a 850 Ohm pull down on the I/O line.  I figure that should waste about 5.8ma when the 5V is present.  The 6008 spec for the line is 8.5ma.  The 5V is coming from a mechanical pressure switch that is activated when a vacuum is applied , so the switch does not consume any current.. It looks to me like this should work. But............ Does any one see a problem with this?

Thanks,

Alan

[apok]

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@ajmartin wrote:

Hello,

I have a USB 6008 reading a 5V signal on one of DIO ports. The 5V is either there or the circuit is open. SInce the 6008 has internal pull ups an open looks like a true TTL signal. I am thinking about using a 850 Ohm pull down on the I/O line.  I figure that should waste about 5.8ma when the 5V is present.  The 6008 spec for the line is 8.5ma.  The 5V is coming from a mechanical pressure switch that is activated when a vacuum is applied , so the switch does not consume any current.. It looks to me like this should work. But............ Does any one see a problem with this?

Thanks,

Alan

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what did you designate the IO as... and why do you need a pull down resistor?

[johnsold]

apok,

He indicated that the device he is monitoring has an open switch which connects to +5 V internally, hence the need for a pull down.

Alan,

I think your solution should work. That produces a low voltage of 0.768 V which is less than the 0.8 V Vil max specification.

Lynn

[SnowMule · Accepted solution]

Connect the switch to Gnd and invert the bool out of the DAQmx Read?

[apok]

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@johnsold wrote:

apok,

He indicated that the device he is monitoring has an open switch which connects to +5 V internally, hence the need for a pull down.

Alan,

I think your solution should work. That produces a low voltage of 0.768 V which is less than the 0.8 V Vil max specification.

Lynn

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thanks lynn for clearing that up.....agreed!

[ajmartin]

Apok,

The I/O line is configured as a digital input on the 6008. 

The 5V signal connected to the 6008 is either there or not. If the signal is not there the 6008 pulls the line high.

DAQmx Read will always see this as a high. If the 5V is there DAQmx Read sees a HIGH if the 5V signal is not there DAQmx sees the line in as a HIGH.

So when the signal is not there I need the line to be low.  I could use a transistor driven by the 5V signal or I could use a relay driven by the 5V signal, either of these then connected to the 6008 line would provide a LOW signal to the 6008 if the 5V goes away.  I was just contemplating if there was a simpler way of getting the 6008 not to float HIGH when there is no input signal on the digital I/O port when configured to acquire a digital signal.

I hope this is more clear?

Thanks Alan

[ajmartin]

I wish I thought of that!  My first thought was a transistor but that would also require a few resistors and I really wanted to keep this as simple as possible. Then I though of pulling down the line when the circuit was open, that would take one resistor per line. How ever using your suggestion , no external extra parts.

I just tried it and it works fine.

Thanks to every one that replied!

Alan