Mode S Interrogations

[Golden Rivet]

Currently studying for more exams and have come up against conflicting explanations as to the duration of the P4 pulse.

One book states the mode S transponder will reply with a Mode S reply on receipt of a 0.8 microsecond P4 pulse, and a second book says it will remain silent and only reply with a Mode S reply on receipt of a 1.6 microsecond P4 pulse.

Any transponder Gurus out there ?

Thanks in advance

GR