Digital I/O

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Low-cost way to detect 60 channels of 24 AC?

Hey Folks,


I'm working on an applicatoin where we want to simply detect if 60 channels are on or off. They are 24V AC signals goin out to solenoids, and we just want to see if the lines are turned on or off. I've been told that my best options are either using a high channel count Digital card like a PXI-6511 (this is a PXI-based system, by the way),  and rectifying my signals with $0.50 rectifiers, OR by using SCXI (which we don't want to do because of the cost). 


Any other creative ideas?


-Matt S.

LabVIEW Integration Engineer with experience in LabVIEW Real-Time, LabVIEW FPGA, DAQ, Machine Vision, as well as C/C++. CLAD, working on CLD and CLA.
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I would say your simplest, and cheapest solution is going to be the digital I/O route with the rectifiers.  If you just plain don't like rectifiers, you could read in analog input and do something with triggering on each line, but that is going to require a much more expensive card(s).  Not to mention require more coding on your part.
Doug Farrell
Solutions Marketing - Automotive
National Instruments

National Instruments Automotive Solutions
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One thing you need to watch out for are the large voltage spikes generated when a solenoid coil is de-energized.  If this were mine I would use some sort of optical isolation between rectified 24V coil voltages and digital inputs.  At the bare minimum I would install RC snubbers at each solenoid coil to prevent the spikes.


There are isolated modules commercially available from Crydom and others that accept a 24V AC input signal and put out a 5V logic signal.  Cost may exceed your budget but implementation would be clean and easy.

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