The GTX 328 transponder is powered on by pressing the STBY, ALT or ON keys, or by a remote avionics master switch (if applicable). After power on, a
Mode Selection Keys
OFF Powers off the GTX 328. Pressing STBY, ON or ALT Key powers on the transponder displaying the last active identification code.
STBY Selects the standby mode. When in standby mode, the transponder will not reply to any interrogations.
ON Selects Mode A. In this mode, the transponder replies to interrogations,
as indicated by the Reply Symbol ( 
). Replies do not include altitude information.
ALT Selects Mode A and Mode C. In ALT mode, the transponder replies to identification and altitude interrogations as indicated by the Reply Symbol ( 
). Replies to altitude interrogations include the standard pressure altitude received from an external altitude source, which is not adjusted for barometric pressure. The ALT mode may be selected in aircraft not equipped with an optional altitude encoder; however, the reply signal will not include altitude information.
Any time the function ON or ALT is selected the transponder becomes an active part of the Air Traffic Control Radar Beacon System (ATCRBS). The transponder also responds to interrogations from TCAS equipped aircraft.
Code Selection
Code selection is done with eight keys (0 – 7) providing 4,096 active identification codes. Pushing one of these keys begins the code selection sequence. Digits that are not yet entered appear as dashes. The new code is activated when the fourth digit is entered. Pressing the CLR Key moves the cursor back to the previous digit. Pressing the CLR Key when the cursor is on the first digit of the code, or pressing the CRSR Key during code entry, removes the cursor and cancels data entry, restoring the previous code. Press the CLR Key up to five seconds after code entry is complete to return the cursor to the fourth digit. The numbers 8 and 9 are not used for code entry, only for entering a Count Down time, and contrast and display brightness.