> Digging deeper, we've measured the time
> between when the card raises the interrupt line and when the driver accesses
> the registers on the serial card to handle the interrupt is sometimes really
> long when compiling and reading/writing to the com ports (up to about
> 7ms!!!).

How are you measuring that?

I've sorted out interrupt latency problems in the past by using a logic
analyser to trigger when the IRQ line is active for more than (say) 5ms
and using the 'trigger out' to generate an NMI and hence either enter
the debugger or take a system dump.

Alternatively look at the stack trace when the interrupt finally happens.
This will probably be close to an splx() call unless the problem is
caused by another interrupt.

	David

-- 
David Laight: david@l8s.co.uk