Dear Elisabetta,
The trigger signal of the scanner is way too short to be detected by you
hardware. This is due to the limited sampling frequency of you hardware. You
need to extend the trigger signal so that it can be sampled adequately by
you hardware (serial port/parallel port/something else). I've done this
using a flipflop in feedback. Every time a trigger pulse is generated, the
flipflop changes its output state. Several flipflops in cascade can act as a
signal divider.
It requires some electronics skills. If you want to, I will make a easy
circuit scheme and post it, but it still needs to be done by a technician.
Greetings
Pieter Vandemaele
----- Original Message -----
From: "Elisabetta Pagani" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, August 21, 2002 11:28 AM
Subject: siemens trigger signal
> Dear SPMers,
>
> I am having problems to synchronize a sequence of visual stimuli,
projected
> with a laptop, with the scanner acquisition during an event-realted fmri
> experiment.
> I am using Presentation as software for the stimulus generation; this
> software can control the stimuls presentation timing by reading on the
> parallel port a trigger signal from the scanner. Our acquisitions are made
> with a Siemens Vision and I have simply connected the trigger port,
located
> under the bed of the scanner, with the parallel port of the PC.
> No signal was detected by Presentation during the acquisition.
> I am wondering if someone who has experience in this kind of experiment
> could help me in solving this problem with some suggestion.
>
> Thank you in advance,
>
> Elisabetta Pagani
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