The PT2009-A1 provides 2 extension connectors.
One of the extension connectors is used to connect the 'Call station extension (RS485, 8)' PTO2006-A1, the other is not used on the PT2009-A1.
Extension connectors - 'X1' and 'X5'
GND reference voltage
Do not use this pin
3.3 V is not required to connect the extension module
Serial I2C bus interface, data
Serial I2C bus interface, clock