Canon firmware has been caught misbehaving! The Canon 5DMk II firmware has a significant bug related to bulb exposures. A fix is included with GBTimelapse version 3.3.24. If you are using a Mk II, please download and install the latest version of GBTimelapse.
GBTimelapse has stalled several times when controlling a 5D Mk II with a very short time-interval. I traced the problem down to a bug in the Mk II firmware (or maybe in the Canon driver - it's not possible to tell). Even though this bug is due to the MK II firmware, I've found a way to clear the error and continue.
The BUG: a random and intermittent failure to start a bulb
exposure. It can happen if GBTimelapse starts a bulb exposure immediately (no
delay) after having downloaded the image from a previous bulb exposure. Sometimes when GBT sends a bulb-start command to the camera, the camera returns a value
indicating success - but fails to open the shutter! This leaves GBT waiting
for an image download that never happens.
My TEST: to reproduce the bug, set a 5D Mk II to RAW images, bulb-time = 2 seconds, and
time-lapse interval = 4 seconds. With these settings it takes nearly 5
seconds to do the bulb exposure and download the image. Because the
interval is only 4 seconds, GBT tries to capture the next image immediately
after the download. The 5D Mk II will fail to capture an image about
once every forty images under these conditions.
The FIX: the new GBT version detects the error, clears the Mk II,
and continues with the time-lapse. However, the interval for that one
image will be a few seconds longer than normal. You will see a message
on the main screen when this happens "Bulb Start error
'EDS_ERR_DEVICE_BUSY'", but the time-lapse will continue.
TIP: although the error is fixed, it's best to avoid the situation - I suggest not pushing it. Make sure your time-lapse interval is longer than the total capture time (bulb time + image download time). Just take a test exposure to see what the total capture time is.