Alto-1822 Interface

The Alto-1822 Interface was an 1822 interface for the Xerox Alto. In common with almost all high data rate Alto devices, it was connected to the Alto CPU bus, and used custom microcode to move data to and from main memory. It was full-duplex, but included no buffering other than input and output shift registers. It could operate in both models of the Alto (Alto I and II).

Physically, it was a single PCB (initial prototypes were wire-wrapped) which plugged into the main backplane. The 1822 interface was run through a flat cable from a row of contacts on the back edge of the PCB to a connector on the Alto's back panel, and thence via a cable to the entity to be connected. Both LH and DH 1822 interface variants were supported.

It was used on the Maxc2 front end to connect to the ARPANET for NCP, on PUP gateways to talk to PRUs, and apparently eventually on the Alto ARPANET IP gateway.

A great deal of information about it has survived in the Alto archive (see below): informal documentation; hardware engineering details; microcode; higher-level software which used it.

External links

• Alto 1822 interface information
  • general documentation
    • Alto-1822 Interface Request for Comments January 2, 1978
    • New microcode, other changes January 26, 1978
    • New, Up-to-date stuff February 7, 1978
    • Microcode & Emulator Interface March 30, 1978
    • Alto-1822 PC Board Internal Cable May 19, 1978
    • Alto 1822 Interface cable descriptions November 5, 1978
    • Installation and Testing March 13, 1981
  • IMP front end software - Contains BCPL source code
    • ImpFrontEnd August 22, 1983]
    • Alto Imp driver August 22, 1983
• John Shoch and Larry Stewart, Internetwork experiments with the Bay Area Packet Radio Network, IEN 78, January 1979 - has an image of the Alto-1822 Interface