Pi3 -> Pi3 communication over GPIO

[jefferee]

So I'm using a Pi3, with multiple FTDI USB/RS-232 adapters attached, and having issues with the USB bus periodically locking up. Can't really diagnose what's going on, because when the USB locks up, I can't communicate with the Pi over ethernet, or connect a keyboard.

Is there a good way to connect another Pi3 over the serial UART pins? I've edited my console settings as recommended  here, but no joy. Even from a clean install the -sh process appears to be locking down /dev/ttyS0 and /dev/ttyAMA0.

PiCore version is 8.1.4.

Any ideas? Or maybe I should set up a cron job to save dmesg output to the SD?

[jefferee]

What I ended up doing, for anyone (probably me) googling this later:

Checked mmcblk0p1/config.txt on both Pis, to ensure both had the lines

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enable_uart=1to enable the serial console, and

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dtoverlay=pi3-disable-btto disable the bluetooth (this line is specific to Pi 3). I did this so that the console can be run through the (better) /dev/ttyAMA0 interface, rather than /dev/ttyS0.
In 8.1.4, both lines were included in config.txt by default.

Modified mmcblk0p1/cmdline3.txt on both Pis:
Changed console entries on the server to

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console=ttyAMA0,115200 console=tty1and removed all

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console=entries on the client.


Modified /opt/bootlocal.sh, to change terminal startup.
Commented out the stock serial terminal startup line

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/usr/sbin/startserialtty &on both machines.
On the server only, added the line

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/opt/serialtty.sh &(we'll get to that script next).

On the server, created a script /opt/serialtty.sh

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#!/bin/sh

# Start serial terminal on Raspberry Pi
while :
do
  /sbin/getty -L /dev/ttyAMA0 115200 vt100
done
This forces the console to use /dev/ttyAMA0, so it assumes bluetooth is disabled.

...et voila!