Switching of the display screens on early Airbuses were performed by the SDACs (System Data Acquisitions Concentrators) and the Display Management Computers. The PFD ND XFR pushbutton found on these aircraft would act as a hardware switch (relay operated) for these systems.
The purpose of the XFR transfer (auto or manual functionality) is to allow for operator switching if there is a display failure. Data from the E/W and S/D (upper and lower ECAM display) can also be displayed on the the Captain or FO primary or navigation display unit by using the switching panel on the pedestal.
PFD ND TRANSFER Push Button Interfacing
The PFD ND XFR push button found on the lighting / speaker control panel is not your typical push button. Switching operations requires a 28VDC supply to the relay box found on the 301VU and 500VU panels. With a powered supply the relay is now functional and the PFD ND XFR discrete output can be interfaced to an input board.
The discrete output operates as a latch (on or off position) and will also need to be configured correctly in your avionics software for correct operations. In ProSimA320 the push button can be setup as a latched-switch type in the configuration screen.
Lighting / speaker control panel pinouts
Thank you to David for pinout confirmation