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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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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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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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