Protocols
From Steak Wiki
SPY, IIC, YOUART, etc...
Somewhere there's a wiki with example waveforms of various protocols. I think it was the sigrok wiki which has example pictures for 'some' protocols (e.g. https://sigrok.org/wiki/Protocol_decoder:Ook), but there may also be a way to generate waveforms.
Protocol List
PCI-E
Extensions with PCI-E
This protocol allows for extension (think IDC Ribbon cables from computers circa 1990's) cords to be used supposedly. Shielded ones work better.
SPI
Basically a shift register(?)
SPI Slave mode vs. Master mode for Timing
Nuts and volts Issue 4, page 61. Master mode was unable to be used as a B&W Composite Video output, but Slave mode was. This used a pic18 to generate a constant stream of Pixel data at 1.75MHz.