Principles
The HP-IL can be seen as a precursor to the Universal Serial Bus now largely in use on desktop computers. However, as its name implies, the cable formed a loop: the cable originated in the HP-IL extension module, ran through all connected devices, then came back to the module.
On the bus, devices could act as controllers (the calculators/computers) or slaves (the peripherals). Certain controllers like the HP-71 module or the HP82973A ISA interface could act as slaves as well, enabling a small network of calculators to be set up.
The IL used a form of Token passing protocol for media access control.
Every device on the bus has a ring-in and a ring-out connector, either on pigtails or built in. HP used a proprietary two-pin connector design with polarizing "D"-shaped shells. HP-IL cables can be interconnected without further adapters to extend their length.
Read more about this topic: HP-IL
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—Ralph Waldo Emerson (1803–1882)