I/O buses: Transactions
P.J. Drongowski


A bus transaction is a two step process.

  1. Obtain the use of the bus.
  2. Transfer data on the bus.

Arbitration.

  > Arbitration is required since data transfers may have
    more than one source.
  > There are twelve kinds of arbiters. Categorization is
    based upon three discriminating criteria.

    1. Location of arbitration logic. (Where?)
       + Centralized.
         - Signal must pass from a requesting connection to
           a central arbitration point.
         - Response must be returned before transfer may begin.
       + Distributed.

    2. Allocation rules. (How?)
       + Priority.
       + Democratic.
         - No priority rules.
         - Apparent tie is resolved arbitrarily or by "fairness."
       + Sequential.
         - Opportunity for request is given to only one
           connection at a time.
         - No ties are possible.
         - Sequence is not necessarily round robin.

    3. Timing relationship of arbitration to data
       transfer. (When?)
       + Fixed.
         - Connection must request a data transfer at a fixed
           time relative to the data transfer.
         - Examples.
           * Busses that use the same signal lines for
             data transfer and arbitration.
           * Polled busses. (Transfer always occurs immediately
             after a connection is polled.)
       + Variable.
         - Data transfer can be requested at any time.
         - Independent of the current state of the data
           transfer facilities.

Data transfer synchronization.

  > Source of synchronizing signals.
    + Centralized.
    + 
  > Periodicity.
    + Periodic.
    + Aperiodic.    

UNIBUS.

  > Centralized arbitration.
  > Priority allocation by groups.
  > Variable arbitration.
  > 56 bus lines.
    + Data bus (16 bits.)
    + Address bus (18 bits.)
    + Data transfer.
      - Parity (2.)
      - Master sync (MSYNC.)
      - Slave sync (SSYNC.)
      - Operations (2.)
        * Read word.
        * Write word.
        * Write byte.
        * Modify (data in pause.)
    + Arbitration.
      - Bus request (5.)
      - Bus grant (5.)
      - Selection acknowledge (SACK.)
      - Interrupt.
      - Bus busy (BBSY.)
      - Initialization.
    + Power failure (2 bits.)
  > Arbiter uses three key signals.
    + BR: Bus Request.
    + BG: Bus grant.
    + SACK: Selection acknowledge.
  > There are BR and BG signals for each of four priority
    groups (levels) (4, 5, 6, 7) and the Non-Processor
    Request (NPR) group. All share a common SACK line.
  > Arbitration sequence.
    + Controller asserts BR.
    + Arbiter responds with BG.
    + Controller asserts SACK, then drops BR.
    + Arbiter drops BG.
    + When bus is idle (i.e., no on-going data transfer)
      arbiter raises BBSY.
    + Controller drops SACK in response to BBSY and begins
      a data transfer.
  > Data transfer sequence (input operation.)
    + Master sends slave's address and operation to be
      performed.
    + Master asserts MSYNC and slave fetches the data.
    + Slave puts the data on the bus and asserts SSYNC.
    + Upon receiving SSYNC, master copies the data from
      the bus and drops MSYNC.
    + After 75 nsec, master clears address and control lines
      and drops BBSY.
    + After seeing MSYNC drop, slave clears the data bus
      and drops SSYNC. The UNIBUS is available again.

Ethernet.

  > Source: Evolution of the Ethernet local computer network,
    J.F. Shoch, et al., IEEE Computer, August 1982.
  > Multi-access, packet-switched communication system.
    + Shared communication medium is a passive coaxial cable.
    + No central control.
    + Address recognition at each connection receives packets
      from cable.
    + Transmission is distributed using a statistical arbitration
      scheme.
  > Distributed control policy called "Carrier sense multiple
    access with collision detection" or "CSMA/CD."
    + Sequence.
      * Check to see if network is busy (carrier sense.)
      * Wait for a quiet net.
      * When quiet, begin to transmit packet.
      * Listen for collision.
      * If a collision is detected, aborted packet transmission and
        briefly jam the channel. Schedule retransmission to take
        place after a random delay period.
      * If no collision is detected, continue transmission until
        end of packet.
    + Collision should occur early in packet transmission. This
      collision window is a function of end-to-end propagation delay.
  > Arbitration and transfer characteristics.
    + Distributed, fixed arbitration.
    + Contention arbitrated.
    + Data transfer is the request for use of the bus.
      
                                Experimental     Specification

Data rate                       2.94 Mbps        10 Mbps
Max end-to-end length           1 km             2.5 km
Max segment length              1 km             500 m
Encoding                        Manchester       Manchester
Coax cable impedance            75 ohms          50 ohms
Coax cable signal levels        0 to +3 V        0 to -2 V
Transceiver cable connectors    25 & 15-pin D    15-pin D series
Length of preamble              1 bit            64 bits
Length of CRC                   16 bits          32 bits
Length of address fields        8 bits           48 bits

Copyright (c) 1986 Paul J. Drongowski