TT21 Stuck in Configuration Mode

I installed a TT21 last fall and *thought* I had it configured and
working properly. I fired it up this morning in preparation for the
start of the soaring season, and it came up in configuration mode
instead of the normal operating mode. I ran through the configuration
sequence again, turned it OFF when prompted, but it still comes up in
config mode when I apply power.

As a possible data point, I still have all zeros in for the system
Mode S 'address' - could this be the problem? If so, how does one go
about acquiring an address?

TIA,

Frank (TA)