I wanted to add my two bucks worth (gotta love inflation) to this discussion. I have implemented various systems to use a bi-directional bus structure to transfer information from one place to another. Several implementations have actually used multiple handshaking signals and on others it has been reduced to a bare minimum.

I made a system cable design for XTreeLink that sent an 8-bit wide protocol using just one handshake line each way. That went from PC to PC. There was also a version of an XTreeLink cable that moved 4-bit wide data using a single handshake line each way. That used a system called "fully interlocked handshake" to move a data element across the bus. The Master had initial control over the direction by definition but then the details of the higher level protocol system soreted out the details of how direction change-overs worked.

In another system I implemented a transfer system that was 1-bit wide. This used a very minimalistic hardware implementation that was kind of clever. The setup was used to let a PC, being the master, use its parallel port to communicate with a microcontroller. The microcontroller had no spare I/O pins support the communications link so I devised a scheme to use one Row and 1 Column position if the 3x3 keybard matrix of the target product. The product actually only had 8 switches in the keypad matrix and so the PC had an adapter that could close a logical switch at the 9th key matrix position. This "Test Key" input to the microcontroller, when detected at the next keypad scan, told it that the host PC wanted to do some communications. After a SYNC handshake process the ROW and COL lines of the keypad matrix became a bi-duplex serial bus. Here is a PDF link to a picture of how the basic timing of the system worked.

http://www.carouseldesign.com/TEST_COMM...TIMING.pdf

If your interested in learning a bit more about how the 1-bit wide system worked download the ZIP from below and look at the document, schematic info and so forth.

http://www.carouseldesign.com/TEST_COMM...T_COMM.ZIP

Michael Karas