Hi Michael,
It depends on your performance goals. I would try running the application on the NI 6115 alone, and see if the single point AO and counters are fast enough for what you need. If they are, then you only need one device. If they're not, then using the 6259 for AO and counters might help.
Single point AO on the 6115 should bypass the device's deep onboard memory, but I would still expect single point AO on the 6259 to be faster. The 6259 supports memory mapped I/O for on-demand AI and AO, and it is enabled by default. The 6115 does not, so it has extra overhead from calling into the kernel when you perform an on-demand AO write.
The 6259's counters may be somewhat faster than the 6115's counters, due to differences between the NI-STC2 ASIC on the 6259 and the older DAQ-STC on the 6115. However, this is speculation, because I don't have any benchmarks that are comparable between the two boards.
Brad
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Brad Keryan
NI R&D
