Hello,
I'm about to buy a Duemillanuove and drive it with PD but I'm worried about the speed.
I need to control the digital output with a precision of 5 millisecond.
Is it possible to reach that speed with Windows + PD + usb ....?
If not, what is the maximum speed?
Also, do you know a fast low power relay board to connect to the interface? something with electronic switches like transistors or Reeds or optocoupler ....
Thanks very much
Dyno
I'm about to buy a Duemillanuove and drive it with PD
PD = Police Department? Prozac Derivative? Poopy Diaper? Pastel Daisies?
LOL ;D
No PD is "Pure Data"
Is it possible to reach that speed with Windows + PD + usb ....?
The bit you can't control is the latency in the PC to USB. Windows is probably your limiting factor here.
There is an inherent delay while the serial data gets to the arduino due to the baud rate that you set. Basically 10 time the baud rate is the byte rate so to get bytes at 5mS intervals you have to be going at least 2000 baud which is quite slow.
What are you going to be doing with the Arduino? What is it going to control?
There are a number of factors that influence overall speed. There is a data transfer speed - how fast can you stream data to the Arduino. The transfer rate is up to 115,200 baud. That assumes that the PC is sending and that the Arduino is reading the data. If the Arduino needs to reply, the transfer rate is cut in half.
If the Arduino has to do anything other than watch the data come in, the speed will go down. How much depends on what the Arduino has to do.
So, perhaps you could divulge a few more secrets.
Thanks guys 
My project is to trigger a camera running at 40 frame/s
and synchronise a sequence of spot light (on/off 0.9 A) every frame.
I don't need any input for the moment.
I tried with a DMX interface but it's too slow.
So that's the reason why I'm looking for a fast interface.
Can the switches for the light handle so speedy on/off ?
switches add delay...
D.
Not if I'm using electronic switches I guess.
I am assuming that the spotlights are LED lights, otherwise you can turn them on an off at that speed but the light won't come on and off at that speed. This is due to the thermal lag in the filaments.
Also you might upset people with the strobe like nature of the light, fit inducing and all that.
yes you're right it's LED lights
It'll be used for photo experiment only, not in public.
Are you tapping into the camera's timecode signal to sync with the picture? cauze the camera does not start the microsecond you push the button.
It's hard to strobe exactly in the same place of the frame, specially if you are looking to capture something like bullet freezeing multiple exposures (strobes) on single frame.
There is capturing bullet project on the Arduino Forum:
Arduino Forum ? General ? Exhibition ? Photographing a Speeding Bullet
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1260724268
D.
Thanks, it's a very nice project 
Unfortunately I don't have any Time code but It should work considering that the sequence will be very short (1 sec).
the lights will stay on about 25 ms and the shutter speed will be arround only 1/1000.There's nothing about MicroSecond scale.
Video or camera?
Do not mix to short shutter time and strobe.
The point of the strobe is to freeze the motion in longer shuttertime frame. Multiple strobes pr. frame gives then bullet-time apperence, multiple sharp bullets or moving objects pr. frame. Vishnu type pictures if waving the hands
if the strobe is very short and the shuttertime is shorter, then you risk not having the strobe lit during the time the camera is having the shutter open. 1/1000 in video samples the picture for 1/1000 sec an holds it for 1/30 or 1/25 sec (othewise it shows video 40 times slower during playback) so if the strobe is not lit during the sampling then you have black frame 
D.