USB Mass Storage and USB-Attached SCSI Are Both SCSI

I should have used "and/or".

> the PCI bus is not terminated.

I'd say PCI, arguably, is still terminated. It just uses series-termination instead of parallel-termination. In a parallel-terminated line, when the signal hits the receiver, the termination resistor to ground absorbs the energy, an ideal textbook scenario. In a series-terminated line, there's a resistor in series of the transmitter, which is the termination of the line. When the signal hits the receiver, it reflects back to the transmitter via the series resistor again, and the energy is absorbed.

Quote High Speed Digital Design by digital guru Howard Johnson,

> 1. The driving waveform is cut in half by the series-termination resistor before it begins propagating down the line.

> 2. The driving signal propagates at half-intensity to the end of the line.

> 3. At the far end, the signal reflection coefficient is +1. The reflected signal is half-intensity. The half-sized reflection plus the original incoming half-sized signal together bring the signal at the receiving end to a full level.

> 4. The reflected signal (half-sized) propagates back alone the line toward the source, where it damps out at the source termination.

Note the step (1) and step (4), when omitted, gives an impression that Reflected Wave Switching has no termination at all, which is not how it actually works. The termination resistor is chosen in a way that, when the resistor is combined with the driver's internal impedance, they approximately match the characteristic impedance of the transmission line, thus ensuring the termination is optimal.

> Reflected Wave Switching

It's the most common method to terminate a point-to-point digital interconnect, usually people just call it "series termination". I just finished designing a circuit board yesterday and the data interface was terminated using this technique. Reflected Wave Switching was just another confusing term for that ("confusing" because, while it's a fairly descriptive name, but in many books with a voluminous discussion on serial/parallel termination and whether the signal is reflected, the term Reflected Wave Switching is often not explicitly mentioned. On the other hand, we have computer books that mention Reflected Wave Switching in PCI, but without a discussion of how series termination really works).

The problem of series-termination on a shared bus is, first, there's no clearly-defined transmitter or receiver, a signal can come from either end from multiple devices. Also, the impedance of the bus is affected by the number of cards and the physical locations of the slots on a backplane. It's simply impossible to select an optimal series termination resistance to achieve an ideal series termination. Using series-termination in a shared bus is a hack, but PCI managed to successfully engineer a "good enough" compromise solution for PCI's data rate.

One can say PCI doesn't have termination since many drivers feature on-chip series termination. But it's true for other interfaces as well, so I still like to imagine PCI's "Reflected-Wave Switching" as a compromised form of series termination, as described by Howard Johnson [0].

[0] http://www.sigcon.com/Pubs/news/3_3.htm

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Ultimately, it was the transition to point-to-point serial lines that ended all the termination headaches.