1822 interface

The 1822 interface (named after the number of the BBN report which specified it) was bit-serial data transfer interface used in data networking. It was asynchronous, with a handshake on every bit.

It was originally specified for use in connecting a host to an IMP on the ARPANET, leading to the common naming of device controllers which produced an 1822 interface as IMP interfaces; a number of different host 1822 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.

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 single-sided 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 included two pairs of 'ready' lines (one pair from the IMP, and one from the Host); one conductor of each pair was connected to the other (which was usually connected to ground) via a relay when that entity wanted to signal that it is ready.

There was no fixed word length, the IMP was prepared to talk to machines with different word lengths.

See also

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