Hi.
They are some kind of sync interface that start from:
(in system)
2-wire :
like i2c ( one is the master other are slave)
clock , data(bidirectional)
4-wire:
like spi ( one is the master other are slave)
clock, data-in,data-out,rst
6-wire:
data-in, clock-in, sync-in
data-out, clock-out, sync-out
(out-system)
other wire interfaces.
like: t1-t3, e1-e3,isdn ,usb, etc...
as you see all the interface have the clock singnal and data for minimum
interface the other is for the specific protocol .
for more info tell more for your needs.
Best regards. 8)