Serial Comm. on Digital I/O ports in xPC Target

I feel like I may be missing something obvious here. I would like to achieve some sort of digital communication over the digital I/O ports that I have on one of two NI cards. Ideally I'd like to do SPI, I2C, and UART, but if I can get any sort of I/O going then I can happily bridge to another protocol on a microcontroller. Should I be using bit/byte packing blocks (which seem to output a static value and not change at some sample rate), a FIFO buffer, etc? Or do I need to define a matlab function myself based off of the xPC target time in order to bit-bang I/O? Thanks in advance.