Technical Q&A HW93

The "green" PCI bus

Q: I understand the PCI bus is "green." What does this mean?

A: Green means "energy saving." Reflective waves use less energy than non-reflective waves. For instance, the PCI bus uses a "reflective wave" model. This is a model that does not terminate a transmission line in its characteristic impedance; in this case, the transmission line is the PCI bus. For instance, if a transmission line is shorted the wave will reflect, but invert and the sum of the convolution will be zero. If the transmission line is terminated in the line's characteristic impedance, then there will be no reflection which is the case for non-"green" lines. However, if the transmission line is left unterminated the wave will reflect but by 3dB as it convolves on itself, hence "green." The purpose is to use half of the signal to drive the line and therefore less current.