The new Hewlett-Packard Interface Loop: The new Hewlett-Packard Interface Loop (HP-IL) is a bit-serial interface for battery-operable systems with controllers, like the HP-41

Hewlett-Packard has introduced a new bit-serial interface that makes it possible for hand-held computers, such as the HP-41, to control instruments and peripherals. The interface, called the HP-IL for Hewlett-Packard Interface Loop, uses a two-wire cable that leads from the output port of one device to the input port of the next, until all devices form a closed loop. The loop is unidirectional with commands and data moving around the loop at rates up to five kilobytes per second. The new interface enables hand-held computers to control and read data from a new HP digital multimeter and to interact with a new digital tape cassette drive and a new thermal printer/plotter.