Transponder

A squawk is four digits from 0 to 7. A code containing an 8 or a 9 is not a squawk the server can read, and because it cannot be read the whole line is refused, not just the squawk.

The second short form s<CODE> shares its letter with the speed, and the count of digits is what tells them apart: s1000 is a squawk because it has four, s250 is a speed because it has three. That is why a speed cannot be typed with a leading zero, since s0250 has four digits and is squawk 0250. Use s 250 or ,250 if you want the speed.

sq ident needs no argument, and shortens to its initials: si. The server marks the aircraft as identing for 15 seconds; what your scope does with that, and for how long it shows it, is your client's own behaviour.

The two transponder verbs share the sq word and are told apart by what follows it, the same way s is: four octal digits set a code, the word ident squawks ident.

To set a code and ask for an ident in one go, write it the way it is said on frequency, code first: sq 2000 ident, or sq2000 si in the short form. The aircraft reads it back as "Squawk 2000 and ident." A number typed after the ident, as in si 2000, is refused and the reply shows the spelling that works. Nothing on that line is flown, so a code typed after the ident can never turn into a descent or a turn.