Hi Norm,
I think the most direct replacement (while still getting a modern board) would be the PCI-6023E. The PCI-6024E will let you have analog outputs, and the PCI-6025E will let you have all of that plus a lot more digital I/O.
If you are planning to start using analog output at high rates, check out the non low-cost E series boards since you will get 3 DMA channels instead of 1. The DMA's are necessary to maintain consistent high speed rates when making a combination of input, output, or counters.
Hope this helps.
Best,
John N
Applications Engineer
National Instruments
