Classic parallel (ATA, PCI, parallel ports) is going away and not really coming back. The newer interfaces like 4xPCIe and USB are multiple serial interfaces b

Classic parallel (ATA, PCI, parallel ports) is going away and not really coming back. The newer interfaces like 4xPCIe and USB are multiple serial interfaces bonded together. The difference is all the bits in a true parallel interface operate synchronously. All the bits get set; the clock pulses to signal data is ready; all the bits update again. The problem with this approach is you have to wait for all the bits to settle before you can read. It's no problem at low frequencies, but when you get into hundreds of MHz or more, it becomes a real challenge to get everything time-aligned. 4xPCIe channels operate on 4 independent clocks. They have the same nominal rate, but they don't have to be precisely aligned. Each channel is transmitting a single frame at a time - it's not splitting them into 4-bit packets and sending 1 bit down each channel. I'm hard-pressed to think of a new technology in the last decade that implements splitting a byte into bits and sending each down a separate line.