HP-IL

HPIL (Hewlett-Packard Interface Loop)

HP-IL connectors (on an HP-71B)
Type	Peripheral data bus
Designer	Hewlett-Packard
Designed	1980s
Manufacturer	Hewlett-Packard
Superseded by	USB (mid-1990s)
External	yes
Pins	2
Data signal	Token ring
Bitrate	160 kbit/s (limited to 16 kbit/s) [1]
Max. devices	31 (with extensions up to 960)
Protocol	Serial

The HP-IL (Hewlett-Packard Interface Loop), was a short-range interconnection bus or network introduced by Hewlett-Packard in the early 1980s. It enabled several devices such as printers, floppy disk drives, tape readers, etc. to be connected to programmable calculators such as the HP-41C, HP-71B and HP-75C/D, the 80-series and HP-110 computers, as well as generic ISA bus based PCs.

As its name implies, the HP-IL cable formed a loop: the cable originated in the HP-IL extension module, ran through all connected devices, then came back to the module. 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.

The IL used a form of token passing protocol for media access control. Each device on loop receives a sequentially assigned address automatically, from 1 up to 30. 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.

In the loop, there is one controller, one listener and one talker device at any time, others are inactive. The talker is the device that is sending information to the loop, and the listener is that one that receives information from the loop. The controller is the device that commands devices to talk and listen. In calculator-based systems (e.g. using the HP-41C), the calculator is always the active controller.

Hewlett-Packard developed a range of devices to be connected to the HP-IL, mostly peripherals such as printers and storage devices for calculators. Through the 82169A HP-IL/HP-IB Interface, HP-IL controllers could be connected to instruments with an HP-IB (aka GPIB or IEEE-488) interface, or vice versa. There were also plans to make test equipment with IL interfaces, but apart from the somewhat popular 3468A multimeter, only a few devices were introduced before HP-IL itself became obsolete.