[likeabatterycar]

> then you have to use something else like a data diode

From what I read ARINC 429 is a one-way bus making this completely redundant and unnecessary.

[gte525u]

ARINC 429 definitely one way to do it. It has a fixed baud rate - there is no feedback to the transmitter. Typically, each "bus" contains a single or small subset of messages (called labels) over it.