Using a SUN to sense an external signal

[Robert Cohen]

I want to interface an external signal to a SUN (4/370, but the same
applies to any other SUN), so I am converting this TTL signal to RS232
levels (+/- 9V in this case), and am feeding the signal into pin 8
(Carrier Detect) on serial port ttya.  I want a program execute a command
immediatly after this signal goes high.  So far, the fastest way I've
found is to set up ttya using ioctl(), and then executing the following
loop:

     do {
        ioctl(Fp, TIOCMGET, &i)
     } while(!(i & TIOCM_CAR));
      ...

  where Fp is the file number returned by opening
/dev/ttya using: Fp=open("/dev/ttya", O_RDWR|FNBIO).

The commands that need to be executed after pin 8 is asserted appear after
this loop.  My goal is to have as little delay as possible between the
time the line is asserted and the the post-loop code is executed.

Here is the problem:  This delay is affected by the system load, even if
no swapping is occurring.  Sometimes this delay is too long for my
application.

Questions:
  1:  Is there a faster way to write the above loop?
  2:  Is there a faster, perhaps different way to sense an external
      signal?
  3:  If this delay cannot be made shorter, a long delay
      is acceptable if it is guaranteed to be almost fixed. Can
      this type of precision be achieved (+/- 30 microseconds)?

                       Thanks in advance,
                             cohen at ecse.rpi.edu
                             Bob Cohen
                             Image Processing Laboratory
                             Rensselaer Polytechnic Instute
                             Troy, New York 12180