[NRL, CMU; prob. fr. the movie Tron] To become inaccessible except via email or talk(1), especially when one is normally available via telephone or in person. Frequently used in the past tense, as in: “Ran seems to have tronned on us this week” or “Gee, Ran, glad you were able to un-tron yourself”. One may also speak of tron mode; compare spod.

Note that many dialects of BASIC have a TRON/TROFF command pair that enables/disables line number tracing; this has no obvious relationship to the slang usage.