output overshooting and undershooting problem of PCI-6534

Hi,

I'm using the digital outputs from PCI-6534 to control varies devices, like shutters,CCD camera. But actually the output is not a clean TTL signal, several snapshootes are attached below.

I also try to terminate each output channel with schottky diode following the same scheme in the manual. It does not work somehow.

What could be the reason for this problem? Does this indicate the hardware itself is broken?

Any help would be appreciated

Cheers,

Yibo