Philips HEARTSTART XLT Service Manual page 192

System Functional Descriptions
6-14
Key Functions
All front panel keys connect to the Interface PCA via flex circuits.
Main Keys
The key presses from the 4 main keys (OFF, 1, 2, 3) pass through the Interface
PCA and then on to the Control PCA via a large ribbon cable. The Control
PCA then interacts with the other parts of the system as needed to respond to
the keypress.
Other Keys
Keypresses from all other keys (printer, display, manual/pacing) are detected
and processed by the Interface PCA and then passed to the Control PCA via a
large ribbon cable. The Control PCA then interacts with the other parts of the
system as needed to respond to the keypress.
Theory of Operation