How to execute matlab program using external trigger via serial port

I need to get idea about execution of matlab program using external trigger via serial port.

Thanks.

[Walter Roberson]

Is it a true serial port or a USB virtual serial port?

What is your target latency (between actual transmission of the signal, and MATLAB starting to process the signal) ?

Are you able to hook the signal up to RTS or DTR ?

[Nuwan Dassanayake]

I need to work with true serial port. In my task I am planing to run matlab program using external switch.

[Walter Roberson]

Do you want to trigger by sending a byte of data, or by changing the level on a line? Are you able to hook the signal up to RTS or DTR ?

Is it acceptable to require that the MATLAB program already be running and waiting for the trigger, or is it required that MATLAB not start up until the trigger is received?