5.3 Ultra 1E install hangs at serial probe

[N0OCT at sbcglobal.net]

Subject says it all.  The machine is definitely an Ultra 1E, and 
I am installing 5.3 from CDROM, and monitoring things via the 
port.  Things go fine until FreeBSD probes the serial port.  If 
I'm attached to ttya, it hangs there.  If I'm attached to ttyb, it 
hangs there.  If I pull off the serial cable before the serial 
probe and then reattach it, the machine is still hung.

Was gibt's?  Is there something really stupid I'm missing, or has
anyone else experienced this?