Scott
I did mean I2S but it depends on what factors are involved and SPI is of course often suitable.
I've used both SPI and I2S for proprietary high speed communication between processors (for example - including over optical connections which allow long distances) and personally I prefer I2S for it because the inbuilt frame synchronisation makes it easier to handle detection/loss of synchronisation in noisy environments or when the two processor can be powered independently (and so one suddenly stops or resets) [I never actually used I2S for audio, although that is of course where it originated]. SPI can do it using chip select framing but the HW doesn't coordinate it as I2S HW does, which puts a fiddly SW layer on top to do it, which tends to be more SPI controller specific.
USB is still not used much in industrial communication and is quite succeptible to noise in some environments. It is popular as connection to a configuration tool (preferred over UART) but RS485 etc. is still practical for the actual communication between devices. Ethernet of course has various advantages in many instances, which is my preference in the majority of cases.
Regards
Mark
