Stubby wrote:

The explanation I received years ago was your transponder replies with
12 bits of switch settings (4 switches with 3 bits each) and one more
that reflects the IDENT button.

The twelve data bits (which are either the transponder code or the
encoded altittude) are always sent along with a beginning and
ending framing pulse. The SPIP is sent 4.35 microseconds
after the last framing pulse only when identing.