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Note 29.0                   Q-bus Expansion Concepts                  No replies
FURILO::GIORGETTI                                   292 lines  21-AUG-1985 23:09
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      +---------------+                                    +-----------------+
      | d i g i t a l |                                    |  uNOTE # 029    |
      +---------------+                                    +-----------------+

                                                                   
      +----------------------------------------------------+-----------------+
      | Title: Q-bus Expansion Concepts                    | Date: 24-Jun-85 |
      +----------------------------------------------------+-----------------+
      | Originator: Charlie Giorgetti                      | Page 1 of 5     |
      +----------------------------------------------------+-----------------+


      This   MicroNote   discusses   the   expansion   (multiple   backplanes)
      characteristics of a Q-bus system.  Understanding this topic is critical
      when configuring a system.  The loading, impedance, and single backplane
      characteristics  of  the  Q-bus and some assumptions and definitions are
      discussed prior to defining the expansion rules.  The specific  products
      used in expansion are not discussed here.

                     Viewing the Q-bus for Electrical Analysis
                     Viewing the Q-bus for Electrical Analysis
                     -----------------------------------------

      When analyzing the Q-bus from a configuration rule standpoint the bus is
      treated as a transmission line.  The reasons for this:

       o  The Q-bus has voltage sources at both ends of a conductor.

       o  When one of these voltage sources (typically  a  processor)  changes
          state  (a  control/data signal transitioning) its effect is not seen
          instantaneously at the other end, but after some propagation  delay.
          The  propagation delay could result in signal reflections on the bus
          if it is not properly terminated or expanded.


                                Loading Definitions
                                Loading Definitions
                                -------------------

      The  Q-bus  specification  defines  two  loading  parameters  used  when
      configuring a system.  These parameters, AC and DC loading, indicate the
      load presented to the system by individual elements  on  the  Q-bus.   A
      system  element is either a Q-bus module or a backplane.  The definition
      of AC and DC loads are:

       o  AC loading is the capacitive loading added to a Q-bus  system  by  a
          Q-bus  module  or  by the backplane itself.  Capacitive loading will
          cause bus reflections and impact signal rise and fall  times.   This
          is  measured  at the time the module or backplane is being designed.
          An AC load is 9.35 pf/signal line.

       o  DC loading is the amount of leakage current presented to  the  Q-bus
          by  an  undriven signal line on a Q-bus module.  This information is
          obtained  from  the  specification  data  for  Q-bus   drivers   and

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          receivers.  A DC load is defined as 210 uA.

      The number of AC and DC loads allowed in a configuration is dictated  by
      the  number  of  backplanes  and  the  termination  used.   This will be
      discussed in later sections of this MicroNote.

      The AC and DC values for Q-bus modules and backplanes can  be  found  in
      either   the  Microcomputer  Products  Handbook  (#EB-26078-41)  or  the
      Microcomponents Configuration Guide (#EB-27318-68).

                              Backplane Configurations
                              Backplane Configurations
                              ------------------------

      The rules that govern Q-bus system  implementation  must  be  viewed  in
      light  of  the  backplane  arrangement  used.   The  two supported Q-bus
      configurations are:  single backplane or multiple backplanes.   How  the
      Q-bus   is   treated  as  a  transmission  line  varies  for  these  two
      configurations and is the foundation for the implementation rules.

                     Impedance and Termination Characteristics
                     Impedance and Termination Characteristics
                     -----------------------------------------

      The characteristic impedance of the Q-bus  is  approximately  120  Ohms.
      Therefore, when implementing a system (single or multiple backplane) the
      basic configuration is:
                            Transmission Line Impedance = 120 Ohms
                               +-----------------------------+
            Source Backplane   |                             |
               +---------+     |                        +----+----+
               |         |     |                        |    |    |
               |  +---+  |     |                        |  +-+-+  |
               |  | S |  |     |                        |  | Z |  |
               |  +-+-+  |     |                        |  +-+-+  |
               |    |    |     |                        |    |    |
               |  +-+-+  |     |                        |  +-+-+  |
               |  | Z |  |     |                        |  | S |  |
               |  +-+-+  |     |                        |  +---+  |
               |    |    |     |                        |         |
               +----+----+     |                        +---------+
                    |          |                   Destination Backplane
                    +----------+ 

            Source (usually the processor)           Far-end termination

                    Z - Bus Termination with 120 Ohms Characteristic Impedance 
                    S - Voltage Source
            
                    The transmission line in this diagram could be a single 
                    backplane or multiple backplanes connected with expansion 
                    cables.

                       Figure 1 - General Q-bus Configuration

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                                                           uNOTE # 029
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                      Q-bus Configuration -  Single Backplane
                      Q-bus Configuration -  Single Backplane
                      ---------------------------------------

      For the single backplane case the transmission line is the length of the
      etch  runs  on  the  Q-bus  connector  blocks  and the backplane printed
      circuit board.  This orientation has a signal generator at one end  (the
      processor)  and potentially a terminator at the far end of the bus.  The
      length of the etch runs cannot exceed 14 inches (35.56 cm).  In figure 1
      the transmission line is the backplane itself.

      A single backplane system does not require termination if there are less
      than  20 AC loads.  In this case the signals do not lose their integrity
      because the reflections, caused by the mismatched  impedances,  are  not
      significant  enough to disrupt bus activity.  However, in a high ambient
      electrical noise environment, system integrity may be further insured by
      proper termination.

      The single backplane configuration requires termination if the number of
      AC  loads  is  20  or greater.  The number of allowable AC loads in this
      case is dictated by  the  termination  on  the  processor.   A  120  Ohm
      processor  can  have up to 45 AC loads.  A 240 Ohm processor can have up
      to 35 AC loads.

                     Q-bus Configuration - Multiple Backplanes
                     Q-bus Configuration - Multiple Backplanes
                     -----------------------------------------

      For the multiple backplane case (where the multiples are  two  or  three
      backplanes) the transmission line is the cables used to interconnect the
      multiple backplanes.  The expansion cable set consists of:

       o  A module in the source backplane

       o  A module in the destination backplane

       o  Cables to connect the two modules

      The maximum length of the cables is 16.0 feet (4.88 meters).  The length
      of these cables is by comparison significantly longer then the length of
      the Q-bus connector blocks and the backplane etch  used  in  the  single
      backplane  case.  Therefore, only the interconnect cables are considered
      for configuration purposes.  This arrangement has a signal generator  at
      one end and requires termination at the far end.

      The far end termination  must  reside  in  the  last  backplane  of  the
      configuration.  The location of the far end termination can be any place
      in the last backplane, since the backplane etch runs do not  enter  into
      transmission  line considerations.  The lump sum termination must be 120
      Ohms.

      The termination in the source  box  must  also  be  120  Ohms.   If  the
      processor  is  240  Ohms  then  the  expansion  cable  set module or the
      backplane printed circuit board must have 240 Ohm termination to achieve

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      the 120 Ohms for the lump sum load.

      Lump sum implies that the 120 Ohms  can  be  achieved  by  one  or  more
      expansion  module or backplane printed circuit board mounted terminators
      and its location is position independent in a given backplane.  Figure 2
      shows an example of how such a lump sum load can be accomplished.



                     |  | <--------------------- Expansion Cable from 
       Backplane     |  |                          Source Backplane
         Slot #      |  |
              +------+  +---------------+
              |  +---+  +------------+  |
            1 |  | --+--+--  <-------|--|------- Expansion Module
              |  +-------------------+  |        with Termination (Z1)
            2 |  |                   |  |
              |  +-------------------+  |                               
            3 |  |                   |  |<------ Printed Circuit Board      
            . |  +-------------------+  |                               
            . |  |                   |  |
            . |  |                   |<-|------- Backplane 
            . |  +-------------------+  |
            n |  |                   |  |
              |  +-------------------+  |                               Z1 * Z2
              |    +-+           +-+    |        Lump Sum Termination = -------
              |    | |   <--->   | |    |                               Z1 + Z2
              |    +-+     |     +-+    |
              +------------+------------+
                           |
                           +-------------------- Printed Circuit Board
                                                 Mounted Termination (Z2)


        Figure 2 - Example of Lump Sum Termination in an Expanded Backplane

      Figure 1 shows the double backplane configuration  where  the  expansion
      cable  set is considered the transmission line.  The far end termination
      is  required.   Figure  3  shows  the  three  backplane   configuration.
      Backplane  #2  in  figure  3  for  all practical purposes is part of the
      expansion cable set when looking at it from an expansion point of view.

      The lengths of the expansion cables in multiple box  configurations  are
      strictly  specified.   As  mentioned  the  maximum length of the overall
      cable is 16.0 feet.  The minimum  length  is  2.0  feet  (0.61  meters).
      Therefore,  in a two backplane configuration the expansion cable must be
      between 2.0 and 16.0 feet.

      In the three backplane configuration the maximum cable length  is  still
      16.0  feet.  One of the two interconnect cables must be between 2.0 feet
      and 6.0 feet (1.83 meters) in length.  The other interconnect cable must
      be  at  least  4.0  feet (1.22 meters) but not longer than 10 feet (3.05

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                                                           uNOTE # 029
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      meters).  The difference in the two cable lengths must be 4.0 feet.

      The cable lengths are specified to insure that any reflections occur  in
      the expansion cables and not in the backplane (if they happen).

      The etch runs on the backplane printed circuit board used in a  multiple
      backplane configuration must be no longer than 10 inches (25.4 cm).  Not
      all backplanes used in single configurations can  be  used  in  multiple
      backplane configurations.


                             Expansion Cable     Expansion Cable
                                #1 to #2            #2 to #3

                              +---------+         +---------+
                              |         |         |         |
               +---------+    |    +----+----+    |    +----+----+    
               |         |    |    |         |    |    |    |    |
               |  +---+  |    |    |         |    |    |  +-+-+  |
               |  | S |  |    |    |         |    |    |  | Z |  |
               |  +-+-+  |    |    |         |    |    |  +-+-+  |
               |    |    |    |    |         |    |    |    |    |
               |  +-+-+  |    |    |         |    |    |  +-+-+  |
               |  | Z |  |    |    |         |    |    |  | S |  |
               |  +-+-+  |    |    |         |    |    |  +---+  |
               |    |    |    |    |         |    |    |         |
               +----+----+    |    +----+----+    |    +---------+    
                    |         |         |         |
                    +---------+         +---------+        
               Backplane #1        Backplane #2        Backplane #3


                    Z - Bus Termination with a Characteristic Impedance
                    S - Voltage Source     


                   Figure 3 - Three Backplane Q-bus Configuration

      Multiple  backplane  configurations   allow   22   AC   loads/backplane.
      Therefore,  it  is  44  AC  loads  in  a  two  or 66 AC loads in a three
      backplane configuration.  To avoid lumping too many  AC  loads  together
      the total number of AC loads should be distributed as evenly as possible
      over the two or  three  backplanes.   The  entire  configuration  cannot
      exceed 20 DC loads.

      In  summary,  following  the  expansion  rules  insures  proper   system
      operation.   The  set of rules to be followed are dictated by the single
      or multiple configuration chosen and the arrangement of the  termination
      in the system.




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