Dave Liquorice presented the following explanation :
The controller has crashed a couple of times over the years, failing to
work via the remote, which has meant my entering by another door to
reset it.

How would you reset it if there is no "other door" to allow access to
the control unit if the door won't open?

I suppose I could switch off the main supply to the garage to reset the
controller's processor, but all of the electric door makers seem to
have a manual cranking system built-in by default in case the whole
thing dies or their is a power cut.

Where there is no 'other door', they suggest you install an add-on to
link the manual cranking system to the outside of the door.

Mine runs on 24v DC, so I could add some back up batteries, but over
the years we have had it, not once I we had a power cut, when I have
needed to operate the door. I did once test the hand cranking system,
just to see it worked when I first installed it - its a bit of a PITA
compared to pressing a button, but it works.

Mine came with two remotes, one in the house, one in the car. I soon
added a third wired remote (up/down/stop) in the garage. I then later
added a beam sensor across the door level with my car's tow ball, to
prevent the door closing unless the ball was clear of the door. It's
red LED also serves as a means to see when parking that I have stopped
clear of the door.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk