On Wed, 8 Apr 1998, Charles M. Hannum wrote:

> The zs chip in the SS1 (and possibly SS1+ and SS2) is latching the
> interrupt line.  There's a window between when zscnputc() is called
> and when the chip finishes sending the console character, where the
> interrupt may be delivered but there will be no TX bit set in RR3.
> This causes the `crazy interrupt'.

There has been a problem in the sun3/3x console code which I believe may
be related to this problem, since they share the zs code with the SPARC. 

If you use a VT style terminal as your console, reverse question marks
will appear on the screen quite, mostly at the ends of prompts from the
shell.  I used an oscilliscope to examine the signals actually being sent
to the terminal and it I found that these reverse question marks
correspond to characters with improperly formed stop bits.  We suspect
the problem is some sort of corruption caused by improper interrupt
handling.

I can send you a small GIF of the waveform if you like.

-J