================================================================================ Note 29.0 Q-bus Expansion Concepts No replies FURILO::GIORGETTI 292 lines 21-AUG-1985 23:09 -------------------------------------------------------------------------------- +---------------+ +-----------------+ | 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 221 uNOTE # 029 Page 2 of 5 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 222 uNOTE # 029 Page 3 of 5 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 223 uNOTE # 029 Page 4 of 5 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 (3.05 224 uNOTE # 029 Page 5 of 5 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. 225