Abstract:
An electronic circuit board electrically connected to other circuits of a data processing system by means of a bus, may be removed and re-inserted in the system without the necessity of disabling other circuits connected to the bus. A latch actuated switch provides a control signal in anticipation of circuit board removal. The control signal activates a finite state machine which seizes control of the bus after completion of any current bus communications and stops the generation of clock pulses normally required in bus communications. When contact is physically broken between the board and its corresponding connector, the finite state machine restores the bus clock pulses and relinquishes control of the bus. When a board is to be inserted in an open connector, contact between the board and the connector is sensed by the finite state machine which causes the bus to be seized and the bus clock pulses to be temporarily inhibited. When the board is fully inserted, the finite state machine restores the bus clock pulses and relinquishes control of the bus.

Description:
TECHNICAL FIELD 
     The invention relates to electronic circuit modules and particularly to a method and apparatus for removal and insertion of circuit modules from and into connectors forming part of an electronic circuit arrangement. 
     DESCRIPTION OF THE PRIOR ART 
     Electronic circuit modules such as circuit boards and other component carriers are commonly connected to other circuits by means of a bus and connectors in which the circuit modules may be inserted. It is well known that removal of a circuit module from a connector in an active system may cause arcing at the connector and in the prior art, switches are mounted on circuit boards to remove electrical power from the board during removal and insertion of the board from and into an associated connector. Circuit boards usually have communication and interaction with other circuits by means of a backplane bus arrangement to which connectors are connected. The abrupt disconnection or connection of power to a board so connected to the bus tends to cause electrical transients on the bus. Such a disturbance on the bus is likely to cause error conditions to occur throughout the circuits connected to the bus. It is therefore not uncommon to shut down operations of a whole system when one board has to be removed or inserted. A problem in prior art systems is that such a procedure requires a reinitialization of the complete interconnected system or subsystem and a restoration of interrupted bus communications when the power is restored. Clearly, such shutting down and reinitializing is interruptive and time consuming. It is particularly disadvantageous in multiprocessor systems wherein the several processors function independently and the system operates normally with certain boards removed. 
     SUMMARY OF THE INVENTION 
     These and other problems of the prior art are solved and an advance is made in accordance with this invention in a system wherein circuit modules are interconnected by a bus, by inhibiting the operation of the bus during the period that a module is being inserted or removed from a connector connected to the bus and reactivating the bus after the module has been inserted or removed. In the removal of a module from its associated connector, a switch on the module is operated to provide an inhibit signal via the associated connector to a control circuit which inhibits operation of the bus. As the module is removed from its associated connector, the inhibit signal is deactivated causing the control circuit to again enable the bus for the performance of normal bus functions for the remaining modules of the system. Similarly, when a module is to be inserted in an associated connector, the switch on the module will be positioned such that the inhibit signal is transmitted via the associated connector to the control circuit. In response, the control circuit inhibits operation of the bus. Upon full insertion of the module in the associated connector, the switch is operated to a second state in which the inhibit signal to the control circuit is deactivated. As a consequence, the control circuit again enables the bus to perform normal operations. Advantageously, this procedure causes at most a transmission delay for other circuits attempting to transmit over the bus during the idled periods but does not in any way interfere with any other operation of any of the circuitry connected to the bus. In one particular embodiment of the invention, the control circuit in response to the inhibit signal seizes control of the bus and halts the clock signals which control the operation to the bus, thus preventing any other circuits from seizing or transmitting data on the bus. When the inhibit signal is deactivated, the clock pulses will be restarted and the bus will again be available to all circuits connected to it. Advantageously, this arrangement does not interfere with the operation of any circuits other than those on the affected module except for possible bus access delays and avoids the necessity of a lengthy power down and reinitialization procedure. 
     In one particular embodiment the connectors in which circuit boards are inserted are each equipped with a pair of extra length pins at the top and at the bottom of the connector. The longer pins are connected to the actuator operated switch and this arrangement assures that contact is made with these pins before other pins. Furthermore, in one embodiment the switch is a double throw switch having one connection for sending a power unit inhibiting signal to the power supply associated with the board and an opto-isolator connected across the switch. An initial opening of the switch on a fully inserted board does not affect operation of the power supply unit since this switch is short circuited by the opto-isolator. However, after the bus control clock pulses have been appropriately inhibited, a signal is transmitted by the control circuit to the opto-isolator, thereby opening the path and causing the power supply unit to be shut down. When a board is inserted in a connector, the opto-isolator control signal is set to such a state that the opto-isolator will be in the open state when the board is first inserted. Upon complete insertion of the board and operation of the switch, the power control path will be closed causing the power supply to provide power to the newly inserted board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention may be better understood from the following detailed description of an illustrative embodiment of the invention, taken together with the drawing in which: 
     FIG. 1 is a block diagram representation of a lever actuated switch on a circuit board connected to a bus and control circuitry for controlling the bus during insertion and removal of the circuit board; 
     FIG. 2 is a schematic representation of a connector and a circuit board with a lever actuated switch; 
     FIG. 3 is a state diagram of the control circuitry of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a representation of a multiple conductor bus 150 and a plurality of circuit boards 101 and 102, as well as control circuitry on circuit board 103. The boards shown are modules representative of a number of boards plugged into a backplane and connected to the bus 150 on the backplane by means of circuit board connectors as depicted in FIG. 2. The circuit boards connected to the bus 150, will normally include the circuitry shown on board 101 as well as numerous other circuit elements and other connections to the bus. The boards may, for example, be bus interface boards providing an interface from processors or controllers to the bus. The additional connections and circuits on the boards are not shown in the drawing since they are not required for an understanding of the invention. The bus 150 is viewed as a multiple conductor bus which may be a standard bus such as the well-known Multibus II or the S-100 bus defined as an IEEE standard. Communication between the circuit boards connected to the bus takes place via these conductors which will include address leads in addition to data and other control leads. The address leads in this particular illustrative embodiment are identified as destination ID (DID) leads 0 through 6. Since a number of units communicate via this bus, an arbitration scheme is provided. This may be a well known arbitration scheme such as, commonly used for example, with the Multibus II or the S-100 bus. In this illustrative system, each circuit board is provided with an arbiter circuit 107 which, by monitoring several bus leads, determines when it is entitled to bus access. When a circuit has gained access to the bus it will assert the HOLD lead, which is one of the control leads of bus 150, to notify other circuits that the bus is occupied. 
     The control board 103 is shown as a convenient arrangement for housing bus control circuitry activated in connection with the removal and insertion of one of the circuit boards 101. This control circuitry will normally be only a part of the circuitry on the board. A DC to DC converter 105 supplies electrical power to circuit board 101 in a standard fashion. A similar power supply connection not shown in the drawing exists for board 103 and other circuit boards connected to the bus. 
     Shown as part of board 101 in FIG. 1 is a double-pole, double-throw switch 113, having contacts 114 and 116. This switch is actuated to its open and closed position by means of a latch 210 shown in FIG. 2. When the board is fully inserted in its associated connector, the latch 210 is in the closed position shown in broken lines in FIG. 2. In this position switch contact 114 is open and contact 116 is closed. In preparation for removal of a board from its connector and prior to insertion of the board into the connector, latch 210 is operated to the open position, shown in solid lines in FIG. 2. In the open state of the latch, contact 114 is closed and contact 116 is open. The closing of the contact 114 causes a connection to be established between the ground lead 151 and the MTCHLD lead 153 of the bus 150. This is equivalent to placing a logical 0 on the MTCHLD lead. As will be discussed further in subsequent paragraphs, the assertion of this lead is used to initiate action of the circuitry on the control board 103. 
     In the closed state of the latch 210, contact 116 provides a connection from ground lead 151 to a control input of the DC to DC converter 105 via control lead 115. The DC to DC converter is a standard power supply circuit which is responsive to the state of control lead 115 and ceases to provide power to the board 101 when this ground connection via lead 115 is opened. A single power supply may be provided for each of the boards 101 connected to the bus 150. Alternatively, a shared power supply may be used which may be disconnected individually from each of the boards. An opto-isolator 118 is connected across switch contact 116. The opto-isolator is a commercially available optical device which may be electrically controlled to present either an open electrical circuit or a closed electrical circuit. The opto-isolator, as will be discussed in subsequent paragraphs is controlled from the control board 103 and provides a current path between the terminals of switch contact 116 when it is opened by operation of the latch 210 to the open position in anticipation of removal of the board. A control signal from control board 103 opens the path through the opto-isolator at the appropriate time. 
     FIG. 2 shows a representation of the circuit board 101 which may be pluggably engaged with a circuit board connector 203 shown in a partial cutaway view. A plurality of contacts 205 on board 101 are arranged to be engaged with a number of connector pins 207 of the connector 203. The connector 203 is fastened to a backplane 209 and pins 207 extend through the backplane and connections may be made to the pins in a standard fashion by wire wrap or printed circuitry interconnections on the backplane. The bus 150 (not shown in FIG. 2) physically resides on the backplane 209. Circuit board 101 is equipped with a latch 210 which is shown in the open or actuated position in solid lines and in the closed position in dotted lines. The latch operates toggle switch 113, depicted in circuit diagram form in FIG. 1, to its normal on position when the latch is closed and to its normal off position when the latch is opened. The switch 113 is connected by so-called printed wiring conductors to certain of the contacts 205 and via pins 207 to the backplane 209 when the board is inserted in the connector 203. The switch 113 further includes the opto-isolator 118 shown in FIG. 1. FIG. 2 shows that the uppermost pair of pins 202 and the lowermost pair of pins 206 of the set of pins 207 is longer, for example one eighth of an inch longer, than other pins of the connector. One pin of the upper pair 202 and one pin of the lower pair 206 are connected to the ground lead 151 (FIG. 1) and the other pins of the two pairs are connected to the MTCHLD lead 153 (FIG. 1). The corresponding contacts on board 101 are connected in parallel, as shown in FIG. 2, and on the backplane 209 as well. The purpose of the longer pins is to assure that electrical contact is made between these pins and the corresponding ones of the contacts 105 before other pins. That assures a proper sequence of events, as the board is removed or inserted, as will be discussed later herein. 
     FIG. 3 is a state diagram representing the functions of the state machine 120 of FIG. 1 which is part of the control circuitry on board 103. The sequence of functions depicted in FIG. 3 are carried out in connection with the insertion and removal of a circuit board 101. State machine 120 consists of sequential logic circuitry for performing the sequencing function defined by the state diagram of FIG. 3. It may be readily implemented using standard logic circuit building blocks. The state machine 120 receives input signals from the MTCHLD lead 153, timing circuits 125, 126, and 127, from AND gate 121 and AND gate 122. It generates control signals to clock circuit 112, to HOLD lead 157 and to the power down enable (PRDNEN) lead 155. A control register 123 contains a bit for selectively enabling the AND gate 122. The clock circuit 112 may be any of a number of well-known clock circuits capable of generating bus clock pulses. In this illustrative system, two pairs of clock pulses, CLOCK1 and CLOCK2, 90 degrees out of phase are provided on bus 150 to enable various circuits connected to the bus to communicate on the bus at the bus clock rate. Specifically, these clock leads are used in access circuitry (not shown) in each of the circuits connected to the bus. Communications on the bus are inhibited under control of the state machine 120 by inhibiting the CLOCK1 and CLOCK2 leads, thereby preventing access to the bus and thus isolating the connected circuits from any transients which may occur on the bus during insertion and removal of a bus connected circuit board. This inhibiting of the CLOCK1 and CLOCK2 leads will not affect the operations of other clock circuits (not shown) on board 103, such as the clock circuits which control operation of the state machine 120. Neither does this inhibiting interfere with other clocked operations on any other bus connected circuit boards. This feature allows other circuits to continue to perform functions which do not require bus access and allows the entire system to resume operation without reinitialization when the signals on the CLOCK1 and CLOCK2 leads are restored. 
     As mentioned above, operation of the latch 210 to the open position, which is indicated in solid lines in FIG. 2, causes switch contact 114 to close and switch contact 116 to open. If the circuit is fully inserted in its associated connector, a logical 0 is asserted on the MTCHLD lead 153 of the bus 150 via switch contact 114 when the latch is opened. If, on the other hand, the board is being inserted with the latch in the open position, the signal is applied to the MTCHLD lead when the appropriate contacts 205 make electrical contacts with either of the two pairs of long pins 202 and 206 during the insertion process. 
     The signal on the MTCHLD lead is propagated through AND 122 if an appropriate enable bit is set in control register 123. This enable bit is provided to control the response of the state machine to the MTCHLD signal. The state machine 120 which is initially in an idle state represented by block 301 in FIG. 3, responds to the signal from AND gate 122 by making a transition to state 1 in block 303 as shown in FIG. 3. Upon entry into this state, T1 timer 125, which is a 20 microsecond timer, is initiated and the state machine remains in state 1 until the 20 microsecond period has lapsed. This is done to avoid starting the process due to transient signals which may appear on the MTCHLD lead. If the MTCHLD signal is negated during this period, a return is made to the idle state. Upon expiration of the 20 microseconds in state 1, a transition is made to state 2 in block 305. In this state, the HOLD lead 157 is asserted by the state machine 120. This is done even though the HOLD lead may already be asserted by another circuit on the bus, since the assertion of the HOLD lead by the control circuitry prevents other circuits from attempting to seize the bus upon completion by any current user of the bus. The state machine, however, will not take any action to inhibit the bus until it has been relinquished by any current user. To that end, the bus address leads DID0 through DID6 are monitored by means of AND gate 121. In this illustrative system, activity on the bus is indicated by a logical `0` on one of the DID leads. Thus, AND gate 121 provides a logical `1` output to the state machine on the lead labeled FREE when the bus is free. Upon the concurrence of the logical `1` on the FREE lead and an indication that the MTCHLD lead is still activated, a transfer is made to state 3, block 306 of FIG. 3. If the MTCHLD lead becomes deactivated at any time while in state 1, 2 or 3 a return is made to the idle state. A return is made from state 3 to state 2 if for any reason the FREE lead is negated. Upon entry in state 3, the T2 timer 126, which is a 1 microsecond timer, is activated. After 1 microsecond a transition is made to state 4, block 307 of FIG. 3. The 1-microsecond delay is used to assure that the bus is indeed free before inhibiting the bus. 
     In block 307 the bus is inhibited by means of a stop-clock signal from the state machine 120 to the clock circuit 112 on conductor 128. This signal results in a deactivation of the clock pulses on the bus clock leads CLOCK1 and CLOCK2. This deactivation will prevent any other circuits from responding to transients which may occur on the bus since the bus access circuitry of any circuits connected to the bus is dependent on these clock signals. Subsequent to the generation of the stop-clock signal, in a subsequent cycle of the state machine, a transition is made in the state machine to state 5, block 309, in which a power-down enable signal will be applied to the PRDNEN lead 155. This signal is applied to the opto-isolator 118 of board 101, and like opto-isolators of all circuit boards connected to the bus, causing each opto-isolator to present an open circuit. During removal of a board, switch contact 116 will have been opened by the opening of the latch 210 and thus, application of the control signal to the associated opto-isolator causes an open circuit at the switch. Other boards for which the latches remain closed, will not be affected by the opening of the opto-isolator devices. For the board being removed, there is an interruption in the path from the ground lead 151 to the associated DC to DC converter power supply 105. This in turn shuts down the power supply. At this point the board may be removed without causing arcing or causing unforeseen electrical transients in the circuitry. During insertion of a board, the latch 210 and switch contact 116 are in the open position. Likewise, the opto-isolator is in the open position and thus, the signal on the PRDNEN lead will have no effect on the opto-isolator of the board being inserted. 
     The progress from the idle state in block 301 through state block 309, takes less than 200 milliseconds. Thus, the shutdown of power to a board to be removed takes place after initial operation of the latch but before the human operator will have had an opportunity to disengage the circuit board from its connector. The state machine remains in the power-down state 5 as long as the MTCHLD lead remains asserted, that is, as long as there is a path through the circuit board and switch contact 114. Both the MTCHLD lead 153 and the GRD lead 151 are connected to the board via the two pairs of extra long pins 202 and 206 of connector 203. Thus, during board removal, the signal on the MTCHLD lead is negated only after all other connector pins have been disconnected from the board. When a board is being inserted, the latch 210 is closed as part of the manual board insertion operation. 
     When that happens, switch contact 114 opens and switch contact 116 closes. Consequently, the signal on the MTCHLD lead is negated, and the power supply control signal is asserted on conductor 115, causing the DC to DC converter 105 to apply power to the newly inserted board in a standard fashion. Negation of the MTCHLD signal causes a transition to be made from state 5 to state 6, block 311 in FIG. 3. A transition may be made back to state 5 in case of spurious signals on the MTCHLD lead resulting from the removal of the board. Upon entry into state 6, a T3 timer 127, which is a 200 millisecond timer, is initiated. This timer is used to assure that a clean disconnect has been made in the case of removal of a board, and to allow sufficient time for power to be properly applied to the board in the case of board insertion. Upon the elapse of this time period a transition is made to state 7, block 313 in FIG. 3, in which the HOLD lead 157 of bus 150 is released. Furthermore, the PRDNEN lead is negated in state 7 thereby enabling the opto-isolators of all the boards. In a subsequent cycle of the state machine, a transition is made to the idle state 301, in which the stop-clock signal on conductor 128 is released allowing the clock circuit 112 to again produce the bus clock pulses on leads CLOCK1 and CLOCK2. At this point the bus is available for use by all of the circuits connected thereto. In the meantime, however, none of the operations of the other circuits has been interfered with other than those resulting from a denial of access to the bus. 
     It is to be understood that the above-described arrangement is merely an illustrative application of the principles of the invention. Various changes and modifications may be devised by those skilled in the art without departing from the spirit and scope of the invention.