Some limit in the PLC I imagine:

"<Snip> I opened the RSLinx Event Log and DDE Client Diagnostics to watch what was happening during the code running. It showed me that I am limited to 115 Maximum Data Points so the 800 I was trying to do at once will not be possible. <Snip>"


from http://www.plctalk.net/qanda/showthread.php?t=37614