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I am using a National Instruments USB-6000 DAQ to control some equipment that accepts TTL inputs.

Specifically, I have to pull a pin (normally 5V) to low. As I understand it, to do this, I can use my DAQ's digital output configured as open collector operation. Is this correct so far?

Furthermore, I'm confused as to the use of a pull-up resistor in this case. Looking at this example, there is a pull-up resistor on the path to Vcc. In my case, there is already 5V on the pin by default. So do I still need a pull-up resistor? I can only think that I would need one in series to limit the current going into my DAQ.

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In my case, there is already 5V on the pin by default. So do I still need a pull-up resistor?

No. If that is TTL input pin, and it measures +5 V, then it already has a pull-up resistor to +5 V. An open-collector (or open-drain) driver is appropriate in that case.

It would be helpful to check with the user manual of the actual device which has that input, as that may (should!) give more details of how its inputs should be driven.

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  • \$\begingroup\$ So, during a low signal, my DAQ provides a path from the pin to ground. But that's ground with respect to the DAQ. Does my DAQ and the equipment I'm controlling have to be connected to the same ground level? Couldn't my DAQ's ground not be low enough for the pin to actually register as low? \$\endgroup\$ – echl Aug 31 '16 at 20:26
  • \$\begingroup\$ @echl - Hi - "Does my DAQ and the equipment I'm controlling have to be connected to the same ground level?" - Yes. That is true of normal (non-isolated) connections between an input and an output - there needs to be a "common" reference level, which is usually the shared "ground" connection between them. Is there a problem for you to do that? You haven't given any details about the device to which your DAQ output is connecting. It would help if you can supply details about that device e.g. datasheet, web page link etc. \$\endgroup\$ – SamGibson Aug 31 '16 at 20:34
  • \$\begingroup\$ This is the instrument So I just connect my DAQ's ground to the instrument's ground (page 10 of the pdf I linked for the pinout), right? \$\endgroup\$ – echl Aug 31 '16 at 23:50
  • \$\begingroup\$ Thanks for that instruction manual. On that 25-pin connector, the manual says that the two inputs are "TTL compatible". Having a pull-up on those inputs makes sense, since the normal state for those two signals ("HOLD" and "MAN") is logic high. Therefore yes, I would just connect the Gnd signal reference from your DAQ output, to pin 13 of that 25-pin connector, which they specify is the signal Gnd reference pin for that ventilator device. If you have any doubts, I recommend that you ask the manufacturer of the ventilator about their interface, as they know their equipment better than us! :-) \$\endgroup\$ – SamGibson Sep 1 '16 at 18:44

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