Computer Interconnect

The Computer Interconnect (usually given as the acronym, CI) is a high-speed packetized data networking system from DEC; basically a fairly short-range LAN; it was the first implementation of DEC's System Communication Architecture.

It consists of uni-directional point-point links, all running between an interface on each host (called 'nodes' in the CI system) and a 'coupler'. The network's physical topology is thus a star (an approach pioneered by LCS's Version 1 ring), but electrically and logically, it is a linear bus, like the Ethernet. It too uses CSMA for access control to the shared transmission medium, but different in detail from that of the Ethernet.

In addition to processors (mostly VAXen, although there was the CI20 Computer Interconnect for the KL10), intelligent mass storage controllers (such as the HSC50) could be connected to a CI network; this was a key role in the widely-deployed VAXcluster. Acknowledgements are provided by the hardware; if one is not received, a retransmission can be performed.

Node addresses are 8 bits; so the address space could hold up to 256 nodes. The initial coupler (the SC008 Star Coupler) - a passive one - supported up to 16 links. All CI links are duplicated, for robustness. (Later versions used them separately, to double the capacity.) Thus, each host has 4 links to it; two transmit, and two receive.

The links are coaxial cables. Data speeds were 70 megabits per second; link lengths could be up to 45 meters. At the analog level, it used Manchester encoding, on baseband signaling.

See also

• SC004 Star Coupler 4 Nodes max.
• SC008 Star Coupler 8 Nodes max.
• CISCE CI Star Coupler Extension 16 Nodes max.
• CI20 KL10 CI Interface
• CI780 VAX-11/780 CI Interface
• CI750 VAX-11/750 CI Interface
• CIBCI - VAXBI CI Interface, 1st generation
• CIBCA-A - VAXBI CI Interface, 2nd generation
• CIBCA-B - VAXBI CI Interface, 3rd generation
• CIXCD - XMI CI Interface
• CITCA - TURBOchannel CI Interface
• CIQBA - QBUS CI Interface
• CIPCA - PCI CI Interface
• DEC CI Hierarchical Storage Controllers

External links

• CI - CI documentation at Bitsavers
  • Computer Interconnect Specification
• VAXclusters: A Closely-Coupled Distributed System - paper on VAXclusters which describes the CI in some detail