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Octobus (or OCTObus, as it was written in some documentation) was a high-speed serial command bus for system-internal high speed signal/command transfer. It could send short messages used for synchronization of directly coupled processors in multi-processor configurations including DOMINO I/O controllers.

OCTOBUS card in a ND-5700 crate.


Octobus was a part of transforming the original ND-500 design, where the ND-110 processor was an I/O bottleneck, to a multi-processor system with DOMINO I/O boards connected directly to the MF-Bus. The role of OCTObus was to manage and synchronize the processors.

Physical implementation

The MPM-5 (MultiPortMemory) system was extended with the physical wiring and features for Octobus support and was then called Multi-Function bus, or MF-Bus, introduced with the ND-5000 series.

Protocol overview

  • An OCTObus message is 32 bits. It includes priority, destination, source, information, etc.
  • Each device connected to the OCTObus is called a node.
  • There can be up to 62 nodes on one OCTObus.
  • An OCTObus can be bridged to another OCTObus if 62 nodes isn't sufficient.
  • One node must be set up as MASTER. This node supplies the OCTObus clock (XCLK)
  • Any node can be set up as MASTER, the MASTER node is not special.
  • All nodes can take control of the bus, by issuing a request (XREQ) intercepted by the MASTER.
  • Power failures are tolerated by all nodes.
  • Retries are handled by hardware.


  • XREQ - Transmit request
  • XCLK - Clock
  • XDAT - Data
  • XRFO - Refresh oscillator

Data rate

Data rate depends on cable length.

Cable length in meters Clock frequency in MHz Data rate in Mbits/s
6 4 1.0
60 1 0.250
120 0.5 0.125


The internal backwired OCTObus in the MF-Bus is called a local OCTObus and is TTL. Non-backwired OCTObus is called global OCTObus and uses differential cable.


  • ND-14.001.1A DOMINO Standard Hardware Description
  • "Sales information ND-5000 series" (found at www.sintran.com, original published by Norsk Data). Filesize 1.1 MB. Accessed on 03 August 2010.