Where multiple devices per type were present, a manual ("MANIO") assignment mode could be used. For ease of use, the calculators supported automatic I/O address assignment ("AUTOIO"), where printer or mass storage commands are directed to the first available device of the appropriate type. Popular uses for the HP-IL on the calculators included printing (using the HP 82162A thermal printer) and cassette file storage (through the HP 82161A digital cassette drive). In the HP-75C/D it was built in, in others such as the HP-71 and HP-41, plug in interface modules were available (such as the HP82160A for the HP-41). Several HP calculators were offered with HP-IL interfaces. In addition to the HP-IB interface, HP also sold RS-232 and general-purpose parallel I/O interfaces as well as a prototype kit to create custom interfaces. 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. 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. Hewlett-Packard developed a range of devices to be connected to the HP-IL, mostly peripherals such as printers and storage devices for calculators. Most devices that were designed to be used as controllers were fixed in that role, but the HP-71B was capable of assuming either controller or device mode and with the HP 82402 Dual HP-IL Adapter, the HP-71B could even be configured with multiple loops. Data transfer between loop devices is accomplished by the controller designating a talker and one or more listeners, and then sourcing a "Send Data" message. Multiple devices can be assigned the role of listener at once, but the role of talker can only be assigned to a single device at a time. The role of talker allows a device to originate data on the loop. When the controller assigns listener role to a device, that device accepts and processes data received from the loop. By addressing each device in turn, and using the "Send Accessory ID" message, the controller can determine the role and capability of each device on the loop. Once addresses are assigned, the controller can then assign "talker" or "listener" roles to any device on the loop. Up to 31 devices can be addressed using this method. When the "Auto Address n" message finally returns to the controller, it can tell how many devices are on the loop ( n-1). When the loop is initialized, the controller sends an "Auto Address 1" message to the first device that device (and each subsequent device) takes the number in the message it receives as its own address, and then forwards the message with the address incremented to the next device. HP-IL uses a token passing protocol for media access control: messages are passed from one device to the next until they return to the originator. HP-IL cables utilize a unique two-pin connector design with polarizing "D"-shaped shells, and can be connected together without further adapters to extend their length.
One device on the loop is designated the controller, and manages all other devices on the loop. it was a Ring network): each device in the loop had a pair of two-wire connections, one designated in, which received messages from the previous device in the loop and one designated out, which delivered messages to the next device in the loop. As its name implies, an HP-IL network formed a loop (i.e.