CAN sounds the most applicable in this case. The distances inside a house can be handled by CAN at 500 kbits/s, which sounds like plenty of bandwidth for your needs. The last node can be a off the shelf USB to CAN interface. That allows software in the computer to send CAN messages and see all the messages on the bus. The rest is software if you want to present this to the outside world as a TCP server or something.
CAN is the only communications means you mentioned that is actually a bus, except for rolling your own with I/O lines. All the others are point to point, including ethernet. Ethernet can be made to logically look like a bus with switches, but individual connections are still point to point and getting the logical bus topology will be expensive. The firmware overhead on each processor is also considerably more than CAN.
The nice part about CAN is that the lowest few protocol layers are handled in the hardware. For example, multiple nodes can try to transmit at the same time, but the hardware takes care of detecting and dealing with collisions. The hardware takes care of sending and receiving whole packets, including CRC checksum generation and validation.
Your reasons for avoiding PICs don't make any sense. There are many designs for programmers out there for building your own. One is my LProg, with the schematic available from the bottom of that page. However, building your own won't be cost effective unless you value your time at pennies/hour. It's also about more than just the programmer. You'll need something that aids with debugging. The Microchip PicKit 2 or 3 are very low cost programmers and debuggers. Although I have no personal experience with them, I hear of others using them routinely.
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I see some recommendations for RS-485, but that is not a good idea compared to CAN. RS-485 is a electrical-only standard. It is a differential bus, so does allow for multiple nodes and has good noise immunity. However, CAN has all that too, plus a lot more. CAN is also usually implemented as a differential bus. Some argue that RS-485 is simple to interface to electrically. This is true, but so is CAN. Either way a single chip does it. In the case of CAN, the MCP2551 is a good example.
So CAN and RS-485 have pretty much the same advantages electrically. The big advantage of CAN is above that layer. With RS-485 there is nothing above that layer. You are on your own. It is possible to design a protocol that deals with bus arbitration, packet verification, timeouts, retries, etc, but to actually get this right is a lot more tricky than most people realize.
The CAN protocol defines packets, checksums, collision handling, retries, etc. Not only is it already there and thought out and tested, but the really big advantage is that it is implemented directly in silicon on many microcontrollers. The firmware interfaces to the CAN peripheral at the level of sending and receiving packets. For sending, the hardware does the colllision detection, backoff, retry, and CRC checksum generation. For receiving, it does the packet detection, clock skew adjusting, and CRC checksum validation. Yes the CAN peripheral will take more firmware to drive than a UART such as is often used with RS-485, but it takes a lot less code overall since the silicon handles so much of the low level protocol details.
In short, RS-485 is from a bygone era and makes little sense for new systems today. The main issue seems to be people who used RS-485 in the past clinging to it and thinking CAN is "complicated" somehow. The low levels of CAN are complicated, but so is any competent RS-485 implementation. Note that several well known protocols based on RS-485 have been replaced by newer versions based on CAN. NMEA2000 is one example of such a newer CAN-based standard. There is another automotive standard J-J1708 (based on RS-485) that is pretty much obsolete now with the CAN-based OBD-II and J-1939.
For longer distances, and greater immunity to electrical noise, differential signalling is often used.
For example RS-422
RS-422 drivers are relatively low-cost (start at under £2). They can be connected directly to the UART for point-to-point communication.
There are also specific drivers to extend the range of I2C e.g. NXP datasheet or TI datasheet.
These should work well over twisted pair cables, e.g. Ethernet twisted pairs.
Best Answer
They symbol rate is 25 Gbaud. But you will find that the 3-dB bandwidth of the channel needed to achieve that is significantly less than 25 GHz. It's probably between 12 and 19 GHz, but I'm not familiar with this specific standard.
I don't know what's been commercialized, but you can be sure that no physical medium gets accepted into 802.3 until at least 3 or 4 companies (including both implementers and potential customers) have convinced themselves that the technology is not only achievable, but also will reduce costs relative to previously defined media.
On the other hand, they have been wrong in the past (or at least, they've defined standards that were superseded by even newer technologies before they reached a wide market)
Notice that this medium is limited to 3 m link lengths. Whereas 40 GBase-T was defined for up to 30 m, and media intended for actual LAN applications are generally defined for 100 m or more.
Most degradations in transmission lines scale with link length, so reducing the length allows us to achieve a higher bandwidth over a given cable geometry.
As mentioned in comments, the Ethernet standards for 100 Gbps and up also generally require substantial equalization at both the source and receiver. This is even more true of the short-distance copper media, intended for links within servers or across backplanes, than for the longer distance fiber media intended for links between servers or even across campuses or cities.