Hewlett-Packard Interface Loop (HP-IL)

The Hewlett-Packard Interface Loop, HP-ILm is a bit-serial interface designed for low cost battery-operable systems. HP-IL allows HP-41 calculators and HP-71, The PORTABLE, HP 150 and other computers to be used as system controllers, capable of transmitting and receiving data, and performing a wide variety of information managaement functions. In addition, HP-IL allows the HP-41 and HP-71 to be used for instrument control.

In HP-IL systems, devices are connected by two-wire cables leading from the output port of one device to the input port of the next, until all devices form a closed loop. This loop structure provides a unique capability through: auto address assignment, device capability identification, power ON/OFF control, and error checking.


HP-IL is a low-cost, low-power alternative below HP-IB in price and performance.

Although HP-IB and HP-IL serve the same basic function - interfacing controllers, instruments and peripherals - they differ in many respects.

1. Because of HP-IL's low power consumption, it is usable with portable, battery powered systems. Generally HP-IB is not.

2. HP-IL system components will generally be low cost and have moderate performance; HP-IB system components are medium- to high-performance and generally cost more.

3. HP-IL systems work at relatively low data rates compared to HP-IB, and relatively high data rates compared to RS-232C. For example, the HP-71 and The PORTABLE can transmit at speeds of 5K to 6K bytes per second (50 000 to 60 000 baud on RS-232C). HP-IL maximum data rate at a 100 metre distance is 20K bytes per second; this rate is not dependent on HP-IL cable length.

4. HP-IL allows device separations of up to 100 metres with shielded, twisted pairs (10 metres with zip cord). HP-IB requires extender hardware for long distance connections.

The above text is directly from 1987 Hewlett-Packard catalog.