I've been running NetBSD on an old box as a server for a while now
with a serial console. The problem is if I turn off the machine I'm
using as a terminal while the serial cable is in it, the server dropps
into the kernel debugger. 

I can't make up my mind if this is a hardware or software problem. The
serial cable I'm using doesn't use hardware flow control. I'm guessing
when I turn of the terminal it spits out rubbish which confuses the
UART on the server. This happens with any machine I use as a terminal.

Thanks