How to create a contact closure for equipment trigger using DAQ

[stlchem]

I need to create a contact close to trigger a piece of equipment at a point in my test. I am using a USB-6001 and LabView. I previously was able to do this with a USB-6008 (and 6009) by wiring the equipment leads to a digital output pin and ground. Switching that DIO pin would allow the equipment to see a contact close. With some later USB-6008 and now with the USB-6001 the system sees closed contact all the time when wired this way, so constantly sees a trigger using that same approach. Is there a straightforward way to wire and program this? 

If there is a better board to post this too please let me know. 

Thanks

[JB]

This seems to be the explanation :

• USB-6009 (page 4) : Pull-up resistor 4.7 kΩ to 5 V
• USB-6001 (page 4) : Pull-down resistor 47.5 kΩ to DGND

If further help is needed, please indicate the details of the equipment input connected to the DO of the DAQ.

[stlchem]

JB

Thanks for the reply. That may explain it to you, but not to me. 🙂

It is not clear that the effective resistance would be so different. But I am a chemist and not an engineer so most of this is over my head. I am not sure how high the resistance needs to be to appear as a open circuit to the equipment. 

The equipment is a old titration Dosimat (essentially a automatic syringe that acts as a precise burette). It is triggered by shorting the inputs on a couple of banana plugs. I have no specs of other details on the Dosimat.  I suspect I was just lucky that it used to work. The approach was based on the previous approach of earlier folks. My backup is to add another relay which I was trying to avoid but this could be a very cheap relay to just act as a contact closure. 

If anyone has a simple approach let me know but I will start searching for a cheap relay. 

Thanks again

[santo_13]

If you're looking for a cheap USB controlled relay, this is a good option - https://numato.com/product/1-channel-usb-powered-relay-module/

[JB · Accepted solution]

It would be useful to know the input specifications of your equipment to define a suitable circuitry. In the absence, since it worked with a USB-6009, I suggest you connect a resistor (1k to 10k depending on what you have available, 1k seems better) between the digital output and the 5V of the USB-6001.

[stlchem]

Thanks again JB,

I agree, but I cannot find a manual for this equipment that has any such detail. It is very old equipment.

However, here is a new twist. I was going to try your idea and some others I considered but found that all other DAQ from the 6008/9 and 6001 that I still have on hand perform as desired. It seems the two that have  a problem are out in the field and are the only ones that fail. One is a 6008 and the other a 6001. For now I will switch them out with ones that are working and will test these ideas on the problem ones, but I likely will move to using a added relay in the future just for assurance. Hell, its not my money 😁

[JB]

Even if you have marked your question as solved, this information may be useful to you and save you the trouble of using a relay.

Excerpt from the NI USB-6001 manual:

Output driver type   Each channel individually programmable as open collector or active drive

How is the output configured? I would use the open collector type.

Please see here for more details and here for the specifications of USB-6001/8/9.

[stlchem]

I really appreciate your comments. I will definitely follow up with looking into this once I get a non-working DAQ in hand (should have one later today). If nothing else I should end up a little more educated on these issues. If I learn anything useful I will post it again.

These are currently set up as default outputs. I will play a little with the options. 

[stlchem]

I have been out for some time, so I am only recently come back to this. I have been able to play with the instruments and it appears that the added resistor is all that is needed. As I mentioned the DAQ USB that did not work on one instrument did work on another. These instruments were of different ages and had some differences. It appears that the older (problem one) system has a slightly lower internal resistance, and the newer USB DAQ also must have a slightly lower impedance so that when it turned off there was still enough leakage current to trigger the instrument. The addition of a 1K resistor worked, at least in the one case I tried.

Thank you again JB for your help

[JB]

I'm glad to hear that your problem is solved... and in such a simple and inexpensive way.