### The 400 pF wall

I2C was never meant to leave the board. SDA and SCL are open-drain, held high by
a pull-up resistor, so every edge is an RC charge curve — the resistor against
whatever capacitance is hanging off the bus. The spec caps that capacitance at
400 pF, and a metre of ribbon cable eats most of it. Past that the rise time
misses the timing budget and the bus stops working, or worse, works until the
room gets noisy.

The other two failure modes are quieter. Single-ended signals referenced to a
local ground have no common-mode rejection, so a long run acts as an antenna.
And two boards several metres apart rarely share a ground potential; the offset
shifts the logic thresholds until a high stops reading as a high.

You can paper over the first problem with stronger pull-ups. You cannot fix the
other two without changing what is on the wire.