The 6509 has 96 DIO lines. Will that be enough for you? There is a USB, PCI, PCIe, and PXI version available.
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The easiest and cheapest answer answer for increased channel count in an I2C/SPI application would be to simply buy multiple USB-8451's. The alternative would be to use a High Speed DIO card or FPGA but that would require that you program all I2C or SPI communication yourself. You would have to build your clock and data waveforms yourself and the hardware is more expensive than buying multiple 8451s.
