Digital I/O

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USB-6000 DIN-mount digital output isolator/relay modules

Does anyone know of a good isolated digital output module that's compatible with the USB-6000, USB-6001, USB-6002, and USB-6003 series cards?


I have been trying to install these in some industrial machinery designs, and they don't just play nicely with the 24V DC, 4-20mA, and 120V AC signals that are common in these scenarios.  I have been using Grayhill, Phoenix Contact, and Crydom optoisolated digital IO modules (for example: with the X-series cards with good results in these applications, but the X-series cards provide 5V I/O at 16 or 24 mA, with up to +/- 20V protection circuits.  I can't find any isolators that work with the USB-6002's measly 4mA and 3.3V. Even the modules that say 3V only work with at least 10 mA, because they need to light an indicator LED AND the optoisolator!  And when 10mA at 3.3V is provided, the output current is insufficient to drive many higher-current solenoids and so on.


I can use a logic-level FET like a FQP30N06L or a transistor array like a ULN2003A, but these are all PCB-level components and should have extra protection diodes and resistors, and need a PCB mount point (which could be the 779511-01 NI USB-6000 Series Prototyping Accessory, at a very reasonable $37), and they require the wiring team (who are used to using LOTS of screw and crimp terminals) to break out the soldering irons, which they don't like to do. Also, if they need replacement - because the electrically noisy systems present inside some of these machines WILL destroy an un-protected MOSFET - it's not going to be acceptable to ship one of those while it might be OK to ship something that can be installed with a screwdriver.

 Has anyone found a digital isolation option that (1) mounts on DIN rail or a panel, (2) drives an amp or more at 24V with 3.3V at 4 mA input, and (3) is rugged enough to survive an industrial machine environment?

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You drive a hard bargain.


I was looking for something similar to do 5V-24V and 24V-5V logic level conversion/isolation and during that search I came across these:


Close to your needs, but I think the USB output drive will still be lacking. I have not looked at your USB series specs, do they offer an open collector output to increase drive levels? I thin the USB-6008 and 6009 offer this.


BTW- if anyone needs level conversion/isolation as I mentioned above, this is what I settled on. A very nice solution indeed!

Input drive current is on the order of 5mA for the 5V inputs and 3mA for the 24V inputs are what I measured since they were not on the spec sheet.




"It’s the questions that drive us.”
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Thanks for the reply!  Those look very similar to the Phoenix Contact modules.  Unfortunately, they only offer 5V signalling (turning on at 4V) and draw 6-9mA, which makes them inapplicable for these IO purposes.


The USB-6000, USB-6001, USB-6002, and USB-6003 only output 3.3V at 4mA.  They do have an open collector output, but with a 4.7k pullup resistor, they can only drive high input impedance modules.  To work with a 4.7k pullup to 5V and an input requirement of 80% of 5V=4V (like the listed modules), the isolator can only draw 0.2 mA, requiring an input impedance of at least 18k.


18k is very easy for a FET input circuit - for example, many NI cards themselves have input impedances over 20k.  It just doesn't make any sense to me that all of these digital IO modules have input impedances of 1k ohm and less!  I guess the current from the input can be used to drive the LED, but I would much prefer to provide a 5V or 24V power connection and let the LED be lit by the output side.  

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Update: After getting a PM about this message, I thought I'd update the post.


I ended up using a ULN2003 Darlington array to drive my optoisolators.  

When I specifically asked NI support about it, they said this series was intended more for educational use rather than industrial applications.

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