1822 interface

The 1822 interface (named after the number of the BBN report which specified it) is bit-serial data transfer interface used in data networking; it was originally specified for use in connecting a host to an IMP on the ARPANET. It is asynchronous, with a handshake on every bit; there is no fixed word length, the IMP was prepared to talk to machines with different word lengths.

At the very lowest layer, there are two different choices for the physical layer; 'Local Host' (LH), and 'Distant Host' (DH). (Technically, there is also a third, the 'Very Distant Host' (VDH), but it is an entirely different system from the other two; it is a synchronous serial line through a modem, exactly the same as the system which the IMPs use to talk to each other, allowing a host to be many miles from the IMP it is attached to. It is not at all similar to the interface discussed here, so is not covered here.)

The first two are operationally the same, and differ only in the lowest-level physical details: LH uses TTL signals over twisted pairs, with a ground being the other conductor in the pair; DH uses differential pairs, with ground isolation on the IMP end.

The 1822 interface also includes two pairs of 'ready' lines (one pair from the IMP, and one from the Host); one conductor of each pair is connected to the other (which is usually connected to ground) via a relay when that entity wants to signal that it is ready.

A number of different 1822 host interfaces were produced over the years. Somewhat later, the 1822 interface was adopted for the interface used to connect to the PRUs of the early Packet Radio Network‎‎; this was likely due in part because of the easy availability of a large range of host interfaces.

Host interfaces

These were some of the most popular:

• IMP11-A ARPANET interface
• LH-DH/11 Local/Distant Host Controller
• MLH-DH/LSI11 Multiple Channel Controller
• Stanford 1822 Interface

but there were many others (many being 'one-off' local creations).

See also

• BBN Report #1822, Specification for the Interconnection of a Host and an IMP - The 1976 revision of this key document