detecting TTL pulses

[Anonymous]

I am trying to detect a 90 microsecond TTL pulse (generated from a
motion capture system) for handshaking purposes in LabVIEW. Any help
is much appreciated!

The set-up is: We have a DSR pulse that indicates to the motion
capture system to start recording. As confirmation, the motion capture
system used to send back a DTR pulse, that LabVIEW detected until now.
However, there was much time-variability in when we received the DTR
pulse back, and that was unacceptable.

So, now we are using the DSR pulse to signal "start recording" and a
90uS TTL pulse to indicate confirmation of recording.

However, big problem I am running into is: the code that was detecting
the DTR pulse until now, is not able to detect this TTL pulse (inputs
are wired correctly... and the pulses look OK on the scope being high
for 90uS and low for 25ms, then again going high for 90uS.)

I need to just detect the very first transition from low to high in
the TTL pulse train to determine the exact time-stamp (to at least 1
millisecond resolution) indicating the motion-capture system has
started recording.

I am entirely new to this, so it would be a LOT of help if someone
could point to examples or pointers on how to do it.

Thanks.

[JB]

Which board and which input do you use to read this pulse ?

[johnsold]

When you say DSR and DTR, it sounds like you are talking about an RS-232 signal. RS-232 is defined for positive and negative signals, not TTL levels. If you are sending TTL to a circuit designed to work with RS-232, it may not work.

As JB wrote, please give specifics about your signal source and the equipment you are using to read it.

Lynn