Spent most of last week on a two camera shoot, using an XDCam as the primary camera, and an EX-3 as the 2nd unit. Most of the interviews, we jam synced the TC between the two cameras with a BNC cable left in place during the interview, which worked fine.

I was surprised, though, so see how fast the TC swam apart between the two units when not cabled together--often coming seconds apart from each other in just an hour or two...and 15+ seconds apart overnight while on free run.

I didn't expect a total lock...but that's a lot of TC drift. The EX-3 was largely running faster, if I recall.

Seems like quite a bit of difference.
Were both cameras constantly powered and on during the period that this much drift occurred? Typically if running on battery power we plan to re-jam time code every time a battery is changed, since most cameras drift when disconnected from power, and many drift as well when powered down.

Did both cameras have the same drop-frame time code setting? Cabled together for the entire time You were shooting would force them to be the same. However, if one camera had it's time-code set to "drop-frame" and the other camera's time-code was set to "no drop-frame" you would get the problem you describe. If you disconnect the cable from both cameras after the initial jam-synch...they would drift apart over the time periods you describe and that would explain the difference as well.

It's also recommended that you genlock them if you want perfect sync.

Hmmmm....drop/non drop might be an issue.

On the EX-3 if you shoot progressive, the DF is a disabled choice. But the XDCam disc camera was still set to DF...so that is likely what was happening.

Funny part is that for some reason, I was under the impression that progressive was always non drop. I should probably know these things...

I thought that 24P was always non-drop, whereas 30p could be either...
The time differential should be a factor of about .001 (if I remember correctly) if that was the issue.

Well, then, if so even more curious, since we were shooting 24p. Go figure.