This is my second time encountering this issue with an NI USB DAQ (previously with a USB-6008 or 6009 if I recall correctly) device, so I figured I'd get opinions, maybe I'm off base and this is normal. I recently bought an NI USB-6501 because of it's claim to implement digital outputs which can optionally be set as Open Collector. However, the outputs are not strictly Open Collector, as they already have an internal pullup resistor to 5V.
A bit of quick research indicates this practice of installing pullup resistors on Open Collector outputs is not entirely rare, but why? The entire point of an open collector output is to allow me to select the pullup resistor value and the voltage to which it is pulled to, so by installing a 4.7k pullup to 5V, internally, the extent to which this output can be used has been limited.
My use case was to drive an HP RF switch which required the grounding of a selected lead in order to select which RF channel was to be switched through, and all other lines are to be left floating (and presumably be pulled up to +24V internally, inside the RF switch). I recall it didn't work and I had to use a relay board instead (this 4.7k pullup to +5V was likely not allowing the logic line to reach a high enough level to be interpreted as a logic high, inside the RF switch).
So I guess my question is, why is an open collector output implemented with a pullup resistor already implemented? Does NI have any module which does not have a pullup, or one which can optionally be disconnected?
azuther wrote: Does NI have any module which does not have a pullup, or one which can optionally be disconnected?
You should probably be looking at the industrial DIO, especially if you are talking 24V. You are lucky if you did not break your previous DAQ when applying those voltages to something expecting TTL. So look at the 651X series DIO.
Yeah, I realized after-the-fact that what I had attempted to do had exceeded the voltage rating of that USB DAQ by a factor of 5. That said, the question remains, does NI offer any truly open-collector devices, and how do I know if, when they say "open collector", they don't actually mean open collector?
Did you look at the 651X series DIO like I stated in my previous post. The digital outputs are usually relays for those. If there is a pull up, it is supplied by you.
Yes I looked at the specs for them, and it calls the outputs "sinking" outputs which I believe to be synonymous with open-drain/open-collector, but how do I know that they don't have a pullup internal, as they do on the two USB DAQ devices I've used which call themselves "open-collector".
You could look at page 18 of the PCI-6528 manual listed here to determine if it meets your needs: http://www.ni.com/documentation/en/digital-io-device/latest/pci-6528/specs/