There is disclosed a portable electronic instrument (1) including a case (2), at least one electronic unit (72, 74) housed inside the case, and at least a first input and/or output terminal (A; B) accessible from the exterior of the case and including an electrically conductive element (100) which is mounted so as to be mobile with respect to the case. The input and/or output terminal is adapted to be electrically connected to an input and/or output terminal (72A, 74A; 72B, 74B) of said electronic unit via a transmission line (I/OA; I/OB) and for allowing transmission of electric signals on the transmission line through the connecting element. The connecting element can occupy a first or second position in which the input and/or output terminal is respectively uncoupled from or coupled to the input and/or output terminal of the electronic unit. The portable electronic instrument further includes a transmission gate (TGA; TGB) inserted on the transmission line between the input and/or output terminal and the input and/or output terminal of the electronic unit, said transmission gate having a transmitting and non-transmitting state allowing the input and/or output terminal to be respectively coupled to or uncoupled from said input and/or output terminal of the electronic unit. A protective element (TVSA, TVSB) against electrostatic charges is also inserted on the transmission line between the input and/or output terminal and the transmission gate.