Is it safe to use the 5v supply on DAQ 6009 (via a relay) in a digital I/O line?

[Cordova]

I would like to record the time of an external event via a relay trigger to a digital I/O line on a USB-6009 DAQ.  Would it be safe to just use the 5V power supply on the DAQ itself as the current supplied to the digital input line when the relay circuit is triggered?  Thanks for your assistance.

[AnalogKid2DigitalMan]

If I understand correctly, you want to pass the 5V through the relay contacts (not to power the coil) and route that signal into the digital input. That should work just fine. BTW, from what i saw yesterday researching an answer to another post, I think the 5V supply is limited to 200mA, well above your needs to just trigger a digital input.

Be aware that mechnical contacts can bounce open and closed a few times before settling, so if you just look for the first closure occurentce you should be OK.

[Cordova]

Yes, the 5V would be routed through the relay contacts.  I was just concerned that 200 mA would be too much.  Many thanks.

[AnalogKid2DigitalMan]

The input will only draw as much current as it needs, probably less than 1mA.

Have fun!

[Buechsenschuetz]

I am not familiar with the input circuit of that board. But in most cases digital inputs are tied to +5V via a pull-up resistor. To get a low state at such an input line it has to be connected to GND or has to be driven by a TTL compatible driver circuit. In other words, in your case routing +5V to the input may not change its state - it will be in the high state with nothing connected and in the high state when +5V are applied.

I would suggest checking the input circuitry and probably connect the relay contacts such that they connect the input line to GND when not active and will route +5V (or leave them open, i.e. tied to high level by the internal pull-up resistor) to the input line when active.

[Cordova]

Buechsenschuetz-That's very pertinent information- does that mean that the default state of a binary input line is 1?  Perhaps I can just wire the input line to the ground (as you suggest) through the relay contact, and register the external event when the contact closes and the voltage at the input line transitions to zero.  In that case, I would not need to integrate the 5V power line at all..  More generally, I was worried about damaging the system by potentially short-circuiting it through direct connections to the ground.  Thanks!

[AnalogKid2DigitalMan]

I looked at the manual

http://www.ni.com/pdf/manuals/371303e.pdf

There is a 4.7K pullup to +5 onboard the device. So I think Buechsenschuetz is correct, another 5 stars for him! Indeed I would try using the relay to pull it to ground and monitor what state it is reading. Either way, an input connected to +5 or GND, you will not damage anything.

Let us know how it works out for you.

[Buechsenschuetz]

As mentioned I do not know this board in detail but on other NI boards the default input state is 1. From a hardware designer's point of view (which is mine) this makes sense, since a TTL input left open may be recognized as an irregular state and is very sensitive to noise on the input.

As stated by the original poster, usually it is sufficient to connect circuitry which pulls the input down, there is no need to feed +5V into the input.

BTW in many cases it is safer to use a 0 input value as 'active'. In case the input line breaks or is open, the input will be regarded as 'inactive'.