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How can I trigger an instrument with the rising edge of an output signal?

Hello,
 
I am generating an output square signal that has high and low levels from a few ms to several seconds ( with one of the two outputs I have on the 6221 card ). This signal is used to heat a system and it works fine. I am also using an HP 34401A to measure a resistance that changes with the temperature.This instrument is set to  take measurements every 100ms. My problem is to trigger this instrument : since I don't need to acquire the data all trough the test, I would like to be able to start it, at any time I want, on a rising edge of the heater signal. It doesn't have to be synchronized accuratly with when I want to start the acquisiton but it has to start on a rising edge.
 
To sum up, I don't know how to "get" the rising edge of the output signal of the heater in order to send a digital or numeric ( 0-5V ) trigger to the HP instrument....
 
would anybody know how to do this?
 
I have attached the part of my program that generates the square wave signal for the heater
 
I am using labv 8.0 and a 6221 E card
 
Any help would be great,
 
regards
 
Marc
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Since the 34401 has an external trigger input, why don't you just physically wire the signal to that? You may need to buffer the signa in order to drive both the heater and trigger input.
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