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.
Best Answer
Given your available options, it appears that you do have some pins available:
So it looks like the consensus is 2 pins if you want bidirectional communication, or possibly 3 for SPI. Given equal pin counts, I'd go for the UART if there's one available on both chips. Assuming a 1:1 connection, you can just toss data across without regard for timing or collisions, and the hardware is really easy.
As for the stack overhead, there are many different protocols that can operate from a UART, but it's probably best in this case to define your own. From the comms' perspective, you're just spewing out bytes and receiving them at the other end. The hardware will sync itself to each byte automatically, but you still have to know which byte is what. You'll have that problem regardless of which option you choose.
If you're clever with a UART (and I'm about to give you the answer), you can connect the two TX pins together with two resistors in series, then have a comparator on each end that drives the RX pin based on the local TX and the center tap of the two resistors. This allows full-duplex on one wire. See below for a schematic.
For ESD, add a series resistor inside both cases for each pin and put some clamping diodes on the outboard side of the resistor. There are diodes specially made for this.
simulate this circuit – Schematic created using CircuitLab
Here's how it works:
So in all cases, the receiving comparator output equals the sending TX.
If you don't like the hardware complexity inside the box, then you can use a discrete pin for each direction and keep only the "high" and "low" resistors and ESD diodes - a separate copy for each pin.