Thanks for all your answers.
The best tip came from Sun Support Germany.
The problem is the keyboard:
o if no keyboard is attached (neither SUN keyboard nor an ASCII
terminal on ttya), the CPU will not boot.
o if the machine is running and you disconnect the keyboard, the CPU crashes
because it detects a BREAK signal.
We have made a special serial connector:
The CPU should not see the BREAK signal, that is why 14ohms are added on the
CPU side between the Reception pin (3) and the -5V pin (25).
We can now power on our SUN without any monitor nor ascii terminal on ttya,
provided this connector is on ttya. We can connect and disconnect the terminal
as we want, the SUN runs normally.
This was tested on a SUN IPC running SunOS4.1.1.
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