You can also make use of Kepware's UCON driver to create a custom interface. After which forward that data to the Splunk instance via the IDF in real time. In that way, there will be no need to store any data in your server since all of them are properly indexed inside Splunk itself. This also gives you the added benefit of displaying the data in real time on a dashboard.
Alternatively, you can also load up the Modbus library on your Arudino, assign the values to the correct registers and communicate with Kepware as the Modbus Master. That way, you can avoid developing the UCON driver and just grab the off the shelf Modbus drivers.
Link for Modbus library: https://code.google.com/p/simple-modbus/
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