Computer bus structure permitting replacement of modules during operation

Abstract: A computer bus structure (20) is provided which permits replacement of removable modules (12, 14) during operation of a computer (10) wherein means are provided to precharge signal output lines to within a predetermined range prior to the usage of the signal output lines (42) to carry signals, and further, wherein means are provided to minimize arcing to pins designed to carry the power and signals of a connector. In a specific embodiment, pin length, i.e., separation between male and female components of the connector, are subdivided into long pin (40) length L and short pin (42) length 5. Ground connections (50) and power connections (46) for each voltage level are assigned to the long pin lengths. Signal connections (84) and a second power connection (52) for each voltage level is assigned to the short pin lengths. The precharge/prebias circuit comprises a resistor divider coupled (92, 94) between a power source (49) and ground (98) with a high impedance tap (95, 82) coupled to a designated signal pin (84), across which is coupled a charging capacitor (90) or equivalent representing the capacitance of the signal line (82). Bias is applied to the precharge/prebias circuit for a sufficient length of time to precharge the signal line (82) to a desired neutral signal level between expected high and low signal values prior to connection of the short pin (84) to its mate.

Description: BACKGROUND OF THE INVENTION This invention relates to structures of computer buses and the like which permit insertion and withdrawal of connectors for modules such as printed circuit boards during powered operation. It is frequently desirable for electronic systems, and in particular computer systems to operate continuously. Computer systems typically comprise modules interconnected through a bus structure in a backplane. The modules or subsystems are generally separately addressable under control of operating systems software. Insertion and withdrawal of a module from a backplane through a module connector and the backplane connector during operation of the backplane where there is application of power through the backplane presents numerous problems. Undesired transient signals can be created during the insertion or removal process which disturb backplane logic signals. Arcing between contacts occurs whenever power is instantaneously applied or removed from a contact if there is a significant capacitive or inductive component in the power source or the load. What is needed is a mechanism for addressing the problem of undesired transients in signals and power which would permit online withdrawal and insertion of modules, as part of a scheme for online replacement or repair by module exchange in an operating computer system. DESCRIPTION OF THE PRIOR ART The following patents were uncovered in a search of U.S. Patent Office records relating to computer bus structures suited for making and breaking signal and power connections of modules in an operating electrical circuit by withdrawal and insertion of a connector. The Glover et al. patent relates to a high voltage connector having a male connector member with a plurality of pins, some of which are longer than others, whereby a plurality of electrical signal lines may be connected simultaneously or sequentially. The Ohnuma et al. patent discloses a printed circuit board wherein connector plugs are connected to ground into an interim power connection through assent of extended pins prior to a connection to a shorter pin carrying continuous power. The interim power connector pin is connected to the permanent power connector pin through an impedance element which results is a disconnection when steady d.c. power is applied. The impedance element reduces electric current to minimize the effects arcing. The Yamamoto patent shows a connector system with two lengths of connector members and contains a suggestion that signal contacts do not become electrically coupled until power contacts are engaged. The Goff et al. patent shows two-connector system in which a.c./d.c. power is applied through a connector prior to completion of circuit for signal connectors. The Reimer et al. patent shows a circuit board positioning arrangement in which two circuit boards are offset mounted relative to one another on a movable support such that one circuit board engages a socket prior to the second circuit board. The Faultersack patent shows a backplane mounting assembly using guide pin of different lengths. Three pin lengths are defined; a ground pin length, a power pin length and input/output signal pin lengths. Three different pin lengths are required to satisfy the circuit requirements and safeguard the circuit components. The Reichback patent describes an integrated circuit package in which pins are mounted in concentric circles with pins close to the center of the circuit package being longer than the pins at the periphery. The Laursen et al. patent discloses a printed circuit board connector system which a first power source isolated from a main power source is connected to a circuit board before other connections are made after which the first power source is disconnected upon further insertion of the circuit card. The Jayne patent discloses a connector having pins of varying lengths. The Phillips et al. patent describes a printed circuit board connector for a computer bus having a plurality of pin lengths wherein each of the pin lengths is selected for specific signal types. There are at least four different pin lengths where ground, power, signal and reset are used. The Kawasaki et al. patent describes a connector for an automotive application which includes mechanical interlock in connection with an electrical connection system. Pins of varying lengths are disclosed in connection with the interconnect system. The Busby patent discloses a circuit card of the type for connection to a system by means of connector pins. The card includes a soft power switch for reducing the power transient effects of inserting the card by gradually coupling the circuit load on the card to the system power supply voltage. Connector pins of various extensions are used to initiate events in the coupling of the circuit load to the power supply voltage. The card further includes logic which causes the circuit load to be gradually decoupled from the power supply upon removal of the card from the system. The features of the invention are defined by Claims 1 and 5. At least the preferred embodiment of the invention permits replacement of removable modules during operation of a computer by precharging signal output lines to within a predetermined range prior to the usage of the signal output lines to carry signals. Also arcing to pins designed to carry the power and signals of a connector is minimised. In a specific embodiment, pin length, i.e. separation between male and female components of the connector, are subdivided into long pin length and short pin length. Ground connections and power connections for each voltage level are assigned to the long pin lengths. Signal connections and a second power connection for each voltage level is assigned to the short pin lengths. The precharge/prebias circuit comprises a resistor divider coupled between a power source and ground with a high-impedance tap coupled to a designated signal pin, across which is coupled a charging capacitor or equivalent representing the capacitance of the signal line. Bias is applied to the precharge/prebias circuit for a sufficient length of time to precharge the signal line to a desired neutral signal level between expected high and low signal values prior to connection of the short pin to its mate. The precharge/prebias may have a power source that consists of either an onboard battery, a monkey cable or a source of power through the long pins. The invention is better understood by reference to the following detailed description in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of a computer system with a bus having sockets connectable to modules which can be inserted and withdrawn while power is applied to the computer system. Figure 2 is a schematic diagram of features includable in a removable module. Figure 3 is a schematic diagram of a prebias circuit coupled to a computer bus in accordance with the invention. DESCRIPTION OF SPECIFIC EMBODIMENTS Referring to Figure 1, there is shown a computer system 10 suited to on line replacement of modules 12, 14, 16, 18 coupled to a bus 20. The computer bus 20 is provided with connectors, which can be either sockets or plugs, and herein depicted as plugs 22, 24, 26 and 28, which mate with corresponding sockets 32, 34, 36, 38 of the circuit modules 12, 14, 16 and 18, respectively. The plugs, for example, representative plug 22, are provided with pins of different lengths or intended separation distance from a mating pin receiver. For example, there is a category of pin designated long pin and a category of pin designated a short pin 42. By long and short it is meant that the separation between the long pin and its mating pin receiver in a socket is less than the separation between a short pin and it mating socket. (All pins may be of the same physical length if the pin receivers of the sockets are displaced such that the separation between the long pins and the socket is less than the separation between short pins and the socket.) Referring to Figure 2, the longs pins may include according to the invention, sacrificial pin 44, 48 and a ground pin 46, 50. The ground pin is used to establish connection with a ground potential when the modules 12 is first inserted and to maintain a ground potential until the module 12 is fully withdrawn. For each level of voltage, particularly for d.c. voltage above 5 volts, a sacrificial pin is provided, which is a long pin designed to encounter a corresponding socket 44 before a signal is established through the module 12. The sacrificial pin is a pin designated to withstand damage caused by arcing during the mating process when a module is inserted and during the demating process when a module is removed. During the mating process, the long pin is juxtaposed to its mating socket to a point where arcing occurs. Arcing and connection upon insertion begins to charge capacitance that is usually required on the voltage lines in electronic circuits. As the insertion progresses, the capacitance of the circuit becomes fully charged. Thereupon, the short pin or pins mate to complete the insertion process. Selected short pins carry the circuit operating power under normal operating conditions. The short pins carrying power are permanently electrically coupled to long pins intended to carry power of the corresponding voltage. Because the short power pins are not subject to arcing, short power pins provide a more reliable connection to voltage. Examples of short power pins are pins mating Co sockets 52 and 54. Pins may be redundant especially symmetrically redundant, in a connector such that slight misalignment of the mating portions does not change the mating characteristics and so that the current-carrying capacity of the connector is increased as current is distributed. As a further protection against stray signals, output drivers, such as output driver 56, are subject to enablement through three-state control as for example, through an AND gate 58 having one input 60 biased through a biasing resistor 62 to ground 64 through a long pin 46. The initial bias is overcome by a "power-on" signal applied through a short pin through socket 66 to the AND gate 58 through input 60. Control and other operation of the signals can be enabled by means of a three-state Enable input 68 in response to the signal through the AND gate 58 which, in turn, is responsive to a fixed state machine or random logic 70 driven through driver 72 in response to a signal received through a socket 74 coupled to a corresponding short pin. Referring now to Figure 3, there is shown a precharge/prebiasing circuit 80 in accordance with the invention. To prevent the insertion of a module 12 from disturbing logic signals on the backplane 20, means are provided to assure that the inserted module 12 biases its output to the signal line 82 to a logic level which is intermediate of its expected high and low logic levels. When so biased and precharged, transient noise created upon contact between the signal line 82 through its socket 84 and the corresponding pin 86 does not cause logic faults. Associated with each backplane 20 is an effective capacitance C B represented here by capacitor 88. Associated with each signal-carrying circuit is an effective circuit capacitance C C represented here by capacitor 90. The capacitor 90 is generally of a value substantially smaller than that of capacitor 88, although it is significant. Unless the effective capacitance 90 is precharged and prebiased, there will occur a switching transient which has an amplitude proportional to the required change in voltage across the effective capacitance 90 to match the voltage on the circuit pin 86 of the backplane 20. A biasing circuit, comprising resistor 92 resistor 94, and resistor 95 forming a voltage divider, is provided as one example of a biasing circuit. Resistor 92 is coupled between a node with resistor 95 and a power line 96, wherein the power line 96 has power applied to it prior to the making of a connection on socket 84. The resistor 94 is coupled between the node with resistor 95 and a zero or ground reference 98, for example, coupled to a socket 50 to a long pin 51. Resistor 95 is coupled to signal line 82 from the node with resistors 92 and 94. The biasing circuit requires a zero voltage reference prior to signal application to the output. The power bias relative to the zero voltage reference can be provided by a variety of options. In accordance with the invention, the power for the bias circuit can be provided through a socket 46 to a long pin 47, or optionally through a battery 49 in the module coupled between a ground reference and the power line for the bias circuit. The bias circuit is preferably of high impedance so as not to interfere with the signal of any output driver. The RC time constant formed by capacitor 90 and the parallel-series combination of resistors 92, 94 and 95 should be sufficiently short to permit the line 82 to charge to its desired intermediate level between the time of insertion, connection of the power carrying-line at socket 46 and the signal carrying line at socket 84. To assure that the impedance is sufficiently high, the feeder resistor 95 should be of sufficiently high resistance. The invention has now been explained with reference to specific embodiments. Other embodiments will be apparent to those of ordinary skill in the art. It is therefore not intended that this invention be limited, except as indicated in the appended claims.

Claims: A computer bus structure to permit replacement of removable modules (12, 14, 16, 18) connectable to a bus (20) during operation of a computer (10) comprising: a plurality of connectors (22, 24, 26, 28; 32, 34, 36, 38) coupled to bus lines of said bus (20) for connecting said modules (12, 14, 16, 18) to said bus lines; drivers (56) within each said module (12, 14, 16, 18) for coupling signals on output signal lines (84) to said bus (20) through said connectors (22, 24, 26, 28; 32, 34, 36, 38); and means for setting output level of output signal lines (82) of said drivers (56) prior to connection to said connectors, said output setting means comprising a high-impedance voltage divider (92, 94, 95) coupled to said output signal lines (82) and having a voltage terminal coupled to a voltage reference connection (96) and a reference terminal coupled to a zero-voltage reference connection (98), said voltage reference connection being activated to a predetermined voltage level prior to connection of said output signal lines (82) to said bus. The bus structure according to claim 1 wherein said connectors comprise first pins (47, 51) and sockets (46, 50) and second pins (86) and sockets (84), said first pins and sockets being disposed to mate with one another prior to the mating of said second pins and sockets upon insertion of said first and second pins into said sockets. The bus structure according to claim 2 wherein said first pins include first power connections (47) for each one of a selected number of voltage levels, and said second pins include second power connections (46) coupled directly to a corresponding first pin for each one of said selected number of voltage levels, said first power connection being disposed to encounter initial making of a power connection and final breaking of a power connection at each said power level in order to absorb and dissipate arcing due to voltage differentials and current differentials between said modules (12) and said bus (20), thereby to provide hot safe mating and reliable power-carrying connection through said second pins (86). The bus structure according to claim 1 wherein said drivers (56) comprise three-state drivers, and wherein said modules (12) include means (58) for establishing disassertion of a power-on signal received from a power-on signal pin as a default state whenever said module is at least partially disconnected from said bus, and wherein each said three-state driver (56) is constrained by said disassertion means to default to a passive state during disassertion of said power-on signal. A computer bus structure to permit replacement of removable modules (12) during operation of a computer (10) comprising: a plurality of connectors (22, 32) coupled to bus lines for connecting input/output modules (12) to said bus lines, each of said connectors having first (L) and second (S) interface pins, said first interface pins (L) being of a first defined length, and said second interface pins (S) being of a second defined length shorter than said first defined length; selected ones of said first pins (47,51) being coupled to zero voltage reference connections, and to power supply connections for driver elements of said input/output modules (12); selected ones of said second pins (86) being coupled to power supply connections and corresponding first pins; means (58) within each said-module (12) for establishing disassertion of a power-on signal received from a power-on signal pin as a default state whenever said module (12) is at least partially disconnected from said connectors; and three-state drivers (56) within each said module (58), each one of said three-state drivers defaulting to a passive state during disassertion of said power-on signal. The bus structure according to claim 5 wherein selected ones of said first pins (47) are redundant in order to distribute current loads during plug-in to extend voltage range for hot safe mating. The bus structure according to claim 5 wherein said three-state drivers (56) include means (92, 94, 95) for setting outputs thereof to a default voltage in a midrange between asserted and disasserted in order to minimize voltage transients on said bus lines. The bus structure according to claim 7 wherein said output setting means comprises a high-impedance tap (95) on a voltage divider (92, 94) coupled between a voltage reference connection coupled to one of said first pins (47) and a zero-voltage reference connection (50) coupled to one of said first pins (51). The bus structure according to claim 5 wherein power supply connections (44, 48) are distributed among selected ones of said second pins in order to distribute power loads of said module (12).

Inventors: O'BRIEN TERRY P, TELIAN DONALD A, WATANABE TAK, YOUDEN JOHN J, O'BRIEN, TERRY P., TELIAN, DONALD A., WATANABE, TAK, YOUDEN, JOHN J.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1990
Main classification: H01R23
IPC codes: H02H9, G06F3, G06F1