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The PLC controlling our refrigeration plant is a PLC 5/40. Years ago I wrote a VB6 GUI program using INGEAR's ABCTL driver; It still works perfectly under XP, but, needing Windows 10, I had to get their NET.ABLINK driver and rewrite in C# (at which I'm no guru). The program's basic function is to make one read of 5 ints, one read of 2 ints, three reads of 8 bits of DI, three reads of 8 bits of DO, assigning the read values to screen elements after each read. These calls are in a Timer event that allows plenty of time for the I/O. There are thread.sleep calls between the reads as well, to give them more than enough time to complete. (I also put some test code nonsense in Button events). My C# program works initially, but after 1 to 10 minutes it begins to spit error messages about startIndex, which I can't even find in my solution. If I let it run it it throws an error for incorrect data types, as though the PLC is responding to an int read with bit data. I attach a screen shot of the failure, as well as the form code (to open in Notepad or code editor). Thanks for any tips you can provide! RefrigForm.cs RefrigForm.cs.txt RefrigForm.cs