If I wanted to put just an absurd number of MCU devices on a single bus, like, 100s, and I was okay with 1-10Kib/s total data rate, and wanted the protocol to be daisy chainable, what would I use?
Pref given to low-connector counts, and protocols that can be done with COTS parts
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I2C might be an option, though 7 bits might be sorta limiting. I'd have to check how common 10-bit I2C is possible.
RS-485 is possible, but getting big chains (more than 32?) seems to be a thing some chips discourage.
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I2C isn't that hard to bit-bang at that rate (EXTI ISR for RX), would give you some room to extend the protocol. Single ended RS485, i.e. single pull-down bus on 9600 baud UART. But you'd need a protocol to manage. There is also 1-Wire style PWM (1 bit UART).
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The advantage of a PWM style bus is that it is straightforward to write an enumeration state machine based on the wire-AND of the bus.