What are Serial Buses

Well a serial bus is a bus with a single data line, or a pair of data lines in the case of a differential set. How come PCI express uses a pair of receive lines and a pair of transmit lines, I guess you could say it's still a serial bus but with simultaneous transmit and receive capability. But what about the JTAG and SMbus interfaces that get glued onto the PC expansion bus? Is it still a serial interface. The 1x PCIe bus uses 36 lines, including power and grounds, that's a lot of pins for a serial bus.

Now what about the 16x PCIe interface? PCIe 16x has 16 differential pairs going in each direction [16 Tx and 16 Rx], and it still has the JTAG and SMbus interfaces. That's 164 traces for PCIe once you count all the ground lines. At least some serial buses like CanBus use one net [pair], or a differential pair but it still requires a 9-pin D-sub connector.

How is that different from an 8-bit, 16-bit, 32-bit or 64-bit parallel bus? The PCI bus only uses 188 pins to develop a 64-bit bus, that's only 20 more nets than PCIe and you get a lot more control pins.

So you get your requirement to interface two boards together and your told that one of the boards is I/O constrained, do you walk into a design review and say you serial bus implementation requires 164 nets?

The first digital bus I ever designed to was the GPIB interface, the old byte serial, bit parallel interface. So I really already know what the difference between a serial bus and parallel bus is; A Serial bus transmits a byte sequentially one bit at a time, while a parallel bus sends the entire byte out at once over individual lines. There is no limitation on the number of pins for a serial bus, but I would assume the normal assumption would be less than a dozen nets at most.

The point is a new page was added covering the Serial Wire Debug Interface, one of the true serial buses. Well maybe not, looks like it needs power and ground and resides on a ten pin header. Maybe IC to IC communication on the same PCB only requires just two nets, I'll have to do a bit more research on the topic.

Of course any copper wire parallel bus will be lighter weight than a hydraulic control system. With less lines, a serial bus should be lighter still, requiring less wires than a parallel interface. Better yet a network designed to operate over an Optical fiber would come with the lowest weight penalty.
With the cost of putting metal into space being about $10,000/pound the best design would be the safest, most reliably and the lowest weight. The same is true for any Avionic Bus Design. The Protocol may not set the maximum weight of the bus but an interface's protocol sets the minimum safety requirements of an interface and monitors and adds to a buses reliably.


Graphic; Comparison of different size NASA rockets: Saturn V, Space Shuttle, Ares I, Ares IV, and Ares V.