I'd like to use PPP at 460,800 bps across a null modem cable to another
machine, but pppd won't allow me to choose this speed and gives me a
"tcsetattr: Invalid argument" error message when I try anything above
115,200.

I'd like to know what it is that is imposing this limit.  Is this a
limitation of my hardware, its driver, pppd, or do I just need to
reconfigure something to allow operation at these speeds?

I'm using NetBSD 1.3 with the GENERIC kernel.  The system is a few months
old Pentium II machine, and as such I assumed, but know of no way to
verify, that the serial ports would be capable of more than the one in my
four year old system, which can go up to 115,200 bps.  The kernel
identifies my port as a ns16550a if this is any help.

-- 
Mark Sawle                    mls26@cam.ac.uk
Robinson College, Cambridge   mlsawle@locutus.demon.co.uk