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.
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.