David Taylor  wrote:
> RPi 4B, 4 GB.  Fast command-line compiles etc.  Noted that the serial
> port changed from ttyAMA0 to ttyS0.

What is the primary and secondary uart changed between the pi 1/2 and 3/4.
ttyS0 was and is the mini-uart, you might not want to use that, or at least
it requires fixing the cpu with core_freq=250. See
https://www.raspberrypi.org/documentation/configuration/uart.md

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