Ports come in two flavours: Input and Output.
You can have any number of input or output ports on an object.
You define ports on an object using the PRT: INEW and PRT: ONEW commands, and need to specify a port number (to identify a particular port) and an X-Y offset from the agent it is positioned on. There are also areas for giving the port a name and a description. These parameters are relics from the early days of development and are not used by anything so can safely be specified as an empty string - "".
If the port is an input port then you also need to define an event number that is triggered whenever an input is received.
All agents in the game use a system whereby the value outputted is in the range of -255 to +255, this allows a wide range of devices performing completely different functions to all make utilise another devices output (whether the input actually makes sense to that device is another matter!).
Agents can only be wired up so that an input port connects to an output port.
Agents can only have one connection on their input port, but can have multiple connections from their output port. This means one device can send an output to multiple devices, but can only receive input from one device.
Even if a device has no need for an input port you should include one anyway and simply use it to pass-through any values it receives.