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Buffers instead of Muxes

Multiplexers are usually employed to convey data from different sources into a single destination. Figure 1 shows an hypothetical example in which one of four registers (R1, R2, R3, R4) is selected to input a destination register R0.


If the sources and destination are placed in different boards as shown in figure 1, wiring could get expensive. For example, if each data path is 16-bits, then 64 wires would be required to interconnect the two boards.


Figure 2 illustrates a different solution. The MUX in BOARD 2 has been replaced with four 3-State buffers placed in BOARD 1. Now the selection is done in the source, not in the destination. An external BUS has been used to interconnect the two (and possibly others) boards. Consequently, only 16 wires are required.

This technique not only saves wires (and connector's pins) but it also allows for a more flexible and modular design. For example, a second 4-registers board could be added at a later time without having to modify neither BOARD 1 nor BOARD 2.