I have found that when taking a series of very short exposure images, at some point the camera reports BUSY, stops taking capture requests, and everything comes to a halt. I have to power cycle the Canon to recover.
My take is that the Camera is reporting COMPLETE after taking the image, puts the image in a buffer, and accepts another capture request. If the time to download is long enough, eventually the buffer gets full and can't take another image. That's when the BUSY is reported.
Specifically, this happens when taking a series (100) of bias images (shortest possible exposure time). I have had to insert a 4 second pause between captures to prevent this from happening. The delay time was calculated based on how long it takes to download an image.
So the real problem is that astro software developers are depending on the sdk documentation, and this little factoid was overlooked. To make things work flawlessly for software, a COMPLETE message should not be issued until the image in the buffer is gone.
The Busy sign comes up on the camera when it is still in the middle of a process. That can be things like saving an image, a long exposure, or when a flash is being used the flash heads are still power cycling. In a case like this it could be taking longer to save the files to your computer. It would be good to try using the camera by itself, load a memory card in it, and check to see if the Busy sign is coming up that way.
Normally exposures that short should be saved individually rather quickly. If this issue is only happening when you use your astrophotography software we would recommend contacting the maker of that software to trouble shoot it. When an issue like this is only happening with a specific piece of software or connection that indicates that is causing the delay.
We are limited on the trouble shooting and compatibility information we can offer on non-Canon software. We only have information on functionality and compatibility on software made by Canon. Canon does not test non-Canon software so we are unable to confirm whether non-Canon software is fully compatible. For trouble shooting and compatibility information on non-Canon software we would recommend contacting the maker of the software.
The flash is disabled for astro photos. Mirror lockup also disabled and even removed by the mod.
I use two different astro apps. Backyard EOS works fine when taking a 100 bias images with shortest possible exposure. N.I.N.A. is the one that fails. This isn't conclusive of anything, because Backyard EOS might be waiting until the image is processed before taking another image.