SR400 Triggered Gating

Hi,

I used a LabView program to collect the data from SR400 through GPIB.

Now basically what I need is that we send in TTL signal to SR400 and only let the photon signals within the timing duration we want pass through. 

Hence we want to use the high(low) voltage to open the gate and use the low(high) voltage to close it, which means we use the extenal signal to control the gate width.

However, the gate width is determined internally. 

Would you please tell me how can I let the photon signals within the timing duration we want pass through?

Thanks!