The HP-IL ( Hewlett-Packard Interface Loop) is a short range network cable enabling several devices such as printers, floppy disk drives, tape readers, etc. to be connected to the HP-41C programmable calculator.The HP-IL had a master controller, the HP-IL extension module, that plugged at the back of the HP-41C, and any number of slave devices.
The HP-IL can be seen as a precursor to the Universal Serial Bus now largely in use on desktop computers. However, as it names 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.
Hewlett-Packard developed a wide range of devices to be connected to the HP-IL, but the main target was to enable the connection of scientific equipment though HP-IB and RS-232 interfaces to the calculator. Most HP-IL peripherals had therefore a price tag that made them out of reach for common users.
