Abstract:
A digital control circuit is constructed to use a single signal line for the dual functions of control and sensing the presence of the controlled device. The control circuit includes a driver element having an output capable of being placed in a high impedance allowing the signal line to be pulled to a first predetermined voltage potential when the controlled device is connected thereto, and a second voltage potential when the controlled device is absent therefrom. A receiver circuit monitors the signal line to sense presence of the one or the other of the voltage potentials.

Description:
This is a continuation of application Ser. No. 08/269,448, filed Jun. 30, 1994, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention is directed generally to digital systems, and in particular to a technique that allows a single signal line to be used for multiple functions, including the function of detecting the presence of a unit at the receiving end of the signal line. 
     Today&#39;s very large scale integration techniques have achieved amazing success in implementing large amounts of function in extremely small domains. The packaging industry has followed, using such large scale integrated circuits to greatly increase the amount of function carried by or found on a circuit board, or contained in or forming a part of a sub-system, and/or a unit. 
     While there are a number of obvious advantages flowing from such minimization and packaging techniques, they also give rise to certain problems. One such problem the question of how to communicate with the function of such large scale integrated circuitry, boards, etc. in the sense of the signal lines needed for carrying information. For example, reducing the chip area used for the circuitry that implements the particular function or functions similarly will also reduce the periphery available for signal pins to carry signals to and from the circuitry. One solution has been to use large planar surfaces of the carrier or package containing the actual integrated circuit, but even this surface has a limited amount of room. 
     This problem is not limited to integrated circuit chip packages; to the contrary, the use of large scale integration can also crowd a large amount of function onto a circuit board or other sub-system package, and the problem of pin limiting is again presented. 
     SUMMARY OF THE INVENTION 
     The present invention provides a circuit capable of using a signal line for multiple purposes. Implementation of the invention is simple and inexpensive, making it an ideal solution to pin limitation problems where applicable. 
     Broadly, the invention is directed to system components that operate to use a communicating signal line to deliver a control signal from a driver circuit to a receiving circuit. Removably connected to a receiving end of the signal line is a system component that includes the receiving circuit. The output of the driver circuit is capable of being placed in a high impedance state, allowing the signal line to assume one of two voltage states that depend upon the presence or absence of the system component at the receiving end of the signal line. Thus, when the output of the driver circuit is not in its high impedance state, the signal line communicates a control signal; when in the high impedance state, the signal line communicates information of whether or not a system component is at the receiving end of the signal line. 
     In one embodiment of the invention first and second system components are coupled to one another by wiring interconnect that incorporates a plurality of signal lines, including at least one control signal line. The first system component includes the driver circuit, coupled to drive the signal line with a control signal from a driver output that is capable of being placed in a high impedance state. An enable signal, when asserted, permits the driver element to drive the control line, and when not asserted, causes the driver output to assume its high impedance state. The first system component also includes a pull-up resistance coupling the control signal line to a first voltage potential. The receiving end of the signal line has the second system component connected thereto, carrying the receiving circuit and a second resistance that couples the signal line to a second voltage potential. When the driver output is placed in the high impedance state, the presence or absence of the second resistance will cause the control signal line to assume one of two voltage states, thereby indicating the presence or absence of the second system component. The voltage state of the signal line may be sampled at the first system component to thereby determine the presence or absence of the receiver element. The driver may then be returned to an operating state whereby control signals are communicated to the receiver element. 
     Thus, by the present invention the control signal line is utilized in a manner that allows it to perform two separate functions: (1) to communicate a control signal, and (2) to sense the presence of the second system component carrying the receiving element. 
     It will be evident that there are a number of advantages obtained by the present invention. First is that what would previously require two signal lines to perform the aforementioned control and sensing functions, now only requires one. Thus, the freed signal line can now be used for other purposes. 
    
    
     These and other advantages will become apparent to those skilled in this art upon a reading of the following detailed description of the invention, which should be taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified block diagram of the invention, illustrating its operation to (1) drive a controlled unit, and (2) sense that the controlled unit is present at the other end of the driven signal line. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to FIG. 1, a first system component, designated generally with the reference numeral 10, is shown coupled to a second system component 12, carrying circuits constructed to respond to a signal communicated on a signal line 14 that intercouples the system components 10 and 12. As will be seen, the signal line 14 performs the dual function of communicating control signals from the system component 10 to the system component 12, and of communicating an indication of the presence or absence of the system component 12. 
     It will be evident to those skilled in this art that the two system components 10 and 12 may take a variety of forms. For example, they may each be integrated circuits mounted on a circuit board and interconnected by printed circuit lines or leads that would at least include the signal line 14 to communicate signals between them. Alternatively, the system components 10, 12, regardless of their particular form, may be located on different circuit boards (not shown), or other mounting apparatus, in which case the signal line 14 represents some form of interconnect, such as a connective back plane assembly or a cable connecting different enclosures, that communicates signals between the circuit boards (and the components 10, 14). As will be seen, the invention does not depend upon the particular environment in which it is used; it is still able to operate to use a single signal line (14) for two functions. 
     The first system component 10 is shown as carrying a driver device or circuit 20, whose output in coupled to drive the signal line 14, and a buffer circuit 24 connected to receive the signal line 14. A pull-up resistor R1 couples the signal line to a positive (+) voltage or potential. 
     The second system component 12 is shown as including a receiver circuit 26 coupled to the signal line 14, an opto-coupler 28 that receives the signal line 14 via a resistance R3, and a resistance R2 that couples the signal line 14 to a ground potential G. The opto-coupler 28 operates in response to a control signal (e.g., a logic HIGH or ONE) to develop a &#34;Power On&#34; signal. The Power on signal is, in turn, delivered to power control circuits (not shown) that operate to turn on supply power to the second system component 12 and the circuitry it carries, including the receiver circuit 26. 
     The resistor values for R1, R2, and R3 are selected so that if the second system component 12 is connected, and the output of the driver element 20 placed in its high impedance state by asserting the ENABLE signal that is applied to its control input (C), a SENSE signal, developed by the buffer circuit 24, will assume a particular logic state, here a logic LOW. Conversely, with the output of the driver element 20 still in its high impedance state, but with the second system component 12 removed from the receiving end of the signal line 14, the SENSE signal will assume another state (e.g., a logic HIGH), indicating the absence of the receiver component 12. A detector circuit (not shown) may be connected to receive the SENSE signal to determine therefrom the presence or absence of the receiving end of the signal line 14, the second system component. Thus, the values of R1 and R2 are selected so that, with the driver element in its high impedance state, the SENSE signal will assume one digital state when the receiver component 12 is present (and operatively connected to the system component 10), and another, different state when the receiver component 12 is absent. 
     The opto-coupler 28 is not necessarily part of the invention; it is shown here to illustrate that the load exhibited by the receiver component may include resistances that may have to be taken into account when determining the values of the resistances R1 and R2. 
     With the first system component 10 using a supply voltage (Vcc) of approximately 5 volts DC, the values of the resistances R1, R2, and R3 may be approximately 1 Megohm, 10 Kohms, and 470 ohms, respectively. Thus, with the second system component 12 not connected to the receiving end of the signal line 14, the input of the buffer circuit 24 will be pulled up toward the supply voltage Vcc of the first system component 10 to cause the SENSE signal assume one voltage state that indicates the absence of the second system component. Alternately, if the second system component is connected, the resistance R2 will pull the signal line 12 toward ground potential so the parallel resistance of resistances R2 and R3 cause the SENSE signal to assume another voltage state to indicate that the second system component 12 is connected. In addition, the resistances R1, and R2, and receiver circuit 26 permit control signals to be communicated on the signal line 14 and interpreted as such at the second system component 12. 
     In use, the detector circuits (not shown) carried by the first system component may first determine the presence or absence of the second system component at the receiving end of signal line 14. Thus, ENABLE signal is asserted to place the output of the driver circuit 20 in its high impedance state. The SENSE signal is viewed, and a determination made, such as by the detector circuits (not component 10, whether or not the second system component 12 is connected. If so, the second system component may need to be powered up, in which case the output of the driver circuit 20 is removed from its high impedance state, and the driver circuit 20 used to drive the signal line 14 with a signal (i.e., a HIGH) that turns on the opto-coupler 28, causing it to develop the Power On signal. In turn, the Power On signal will cause power control circuits (not shown) to apply supply power to the second system component 12, including the receiver circuit 26. Once supply power is applied to the system component 12, the driver element can now be used to drive conventional (depending upon the particular technology used) control signals for receipt by the receiver circuit 26 (that may be coupled onto the second system component 12 for use elsewhere therein). 
     Anytime the signal line 14 is not being used to communicate control signals from the driver circuit 20 to the second system component 12 and receiver circuit 26, the ENABLE signal can asserted to place the output of the driver circuit 20 in its high impedance state, and allow the SENSE signal to be sampled to determine the presence or absence of the second system component 12. Also, it should be noted that the driver element 20 could be operated to drive the signal line 14 to a high level state before having its output placed in the high impedance state to relieve the system of concerns about the amount of time required for the large resistance (R1) to bring the signal line to a high state. 
     In summary, therefore, there has been disclosed apparatus that employs a signal line to both deliver various control signalling to a receiving component, and to develop a signal indicative of the presence or absence of the receiving component at the receiving end of the signal line. While a full and complete disclosure has been made of the present invention, those skilled in this art that various modifications and alterations can be made to the disclosed embodiment. For example, although the invention has been disclosed using positive voltages and/or positive logic, it should be readily recognized that other logic conventions, and other voltage levels can also be used. Further, also representative values for the resistors R1, R2, and R3 have been provided, it will be evident to those skilled in this art that a variety of other values may be used to achieve the desired results.