The new  plug connects the RT 279  to the  external coder unit KY95/APX which replies  with 5, 12 or 6 bits for mode 1, 2 or 3  on each challenge.


The APX6 control panel was changed as well: the destructor switch is gone, and the Identify Position (I/P) function was added which  gave an extra blib on the ground radar near his echo when a pilot used the PTT switch on his microphone. So, the traffic controller could see who was talking.

Circuit diagram C 1158/APX




The Mode 1 and Mode 3 codes  for the KY95 are set on this control panel, and  can be changed during flight.

Plug pinout is the same as on the KY95.


This KY95 gets 115Vac and 28Vdc , as well as the mode 1, 2 or 3 pulses from the RT 279/APX unit, and returns  a  string of pulses as reply.


All reply pulses are on a 1.45us grid, from which the first and last pulse at 20.3us are always  present.

In-between these, the mode 1 reply can have up to 5 pulses ( 32 combinations), the mode 2 reply can have 12 pulses ( 4096 combinations) and the mode 3 reply has up to 6 pulses ( 64 combinations)

For more info, see the second page of the  circuit diagram.


The mode-2 code is set on the KY95A front panel with 4 octal thumbwheels.

The APX-6 was introduced in WWII, supporting only single pulses as  Mk X reply for a mode 1, 2 or 3 challenge.

In 1953. an upgrade to the RT-82/APX-6 was made to include an external coder unit, the KY95 which implemented the new selective information features (SIF).  The combination was renamed to APX25.   Its components are shown below.


IFF transponders


AN/ APX-25






From top to bottom on the left  side of this picture you see :

The KY95 coder 

      circuit diagram is here

C– 1128   coder control

C– 1158  TR controlbox


And to the right the

RT 279/APX  ( = modified RT82 /APX-6)   The RF part  is identical to the RT 82, see here


The circuit  diagram of the modified  coder part  is here


Only minor modification were made :  the addition of a plug, and an internal switch serving to undo the modification, so the modified RT 279 has exactly the same functionality as the former RT82/APX-6 , and will run without external coder.