From your experience Andrew, would you say that the USB-6009s can realistically handle being clocked by an external TTL compliant source at 40 kHz continuously? - No, the 6009s can not handle being clocked by an external TTL source at all, regardless of the rate. these devices only support hardware timing from the their own internal sample clock, so the best syncronization you can get is to start them at the same time using a digital trigger.
Cheers,
Andrew S
