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Using multiple PCI-6509 cards in one PC

I have 2 PCI-6509 DAQ cards in a single PC. Both these cards are being controlled by one task.
 
The problem is that pin 34 floats from 0V to 1.3V in about 30 seconds, when a LabWindows/CVI program drives the output low. The problem occurs with both cards no matter which PCI slot that they are in, and only if both cards are initialized and active at the same time. The problem always occurs to the second device of the two. The problem only occurs when the DAQ cards are connected to the cable and to an NI output block. The voltages are being read at the block pin out. When the voltages are read directly at the card connector, no problem is seen. It does not matter which cable or output block of the two is used as the problem is always on the one set-up as the second device.
 
When just one card is in the PC, it all works fine.
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