Anyway here I will show you one of the most used parts by the pilots those days, the MCP.
The MCP sends and receive data to/from many equipements, mainly involved are the 3 FCC (Flight Control Computers) and the TMC (Thrust Management Computer) and it's only a part of the Autoflight system.
The following scheme simplifies the interaction between the various equipements operating the autoflight functions.
The MCP has 3 connectors in the back which directly correspond to the 3 channels it operates (L, C, R)
The following pictus shows internal autopilot block scheme and the connections that each channel features
As you can see the MCP operates with 28VDC and a simple power up test allows some functions to work as you can see in the following video:
The following video shows the Airspeed's Speed Bug driven from the MCP dedicated ARINC channel