Abstract:
An electronic switching circuit system designed to be compatible with coaxial barrel style plug and socket connectors. Specific plug and socket contacts utilize a simple electronic device that causes the switching circuit associated with either or both the plug and socket to be activated only when the plug and socket pair is fully engaged.

Description:
FIELD OF THE INVENTION 
     This invention relates to electrical connections, and more particularly electrical connections that are electrically activated only when such connections are fully and appropriately engaged. The invention is specifically useful when the electrical connections are in the form of coaxial, fixed diameter, multi-contact mating plug and socket means. 
     BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
     In certain applications it is necessary to connect two active (powered) electrical circuits together, typically by using a coaxial plug and socket connectors, each having mating diameters and with multiple circumferential mating spaced apart electrical contacts. 
     The procedure of engaging a multiple coaxial plug within a coaxial socket aperture so as to form the electrical connection with the multiple electrical contacts thereon will cause many of the contacts in the plug to “wipe” past those of the socket during insertion, generally in an electrically inappropriate manner, and may damage the electronic circuits associated with such contacts before the contacts are each fully and appropriately engaged with the corresponding electrical contact. In addition, a further problem arises in that the preferred method of making such electrical connections is typically to insert by rotationally screwing one tubular housing containing the plug into a similar tubular housing containing the socket. The environment in which this occurs could also be hazardous—for instance, on the floor of an oil-drilling rig where flammable gases may be present. In such circumstances it is advisable to make certain that no potentially live electrical contacts are capable of causing a spark or thermal effect that could ignite flammable gas, dust or vapor during rotatable insertion of the plug into the socket. 
     Still further problems exist with the hazardous environment in which such plus-socket connectors may be exposed. For example, drilling strings used in the oil industry require insertion therein of electronic monitoring and transmitting devices to allow drill operators to monitor various drilling parameters at the base of the well bore during drilling operations. Electronic devices which fit within the inner diameter of drill pipe of a drill string are typically cylindrical devices consisting of sensors, telemetry apparatus and batteries or power supplies, that together on average are less than 2 inches in diameter, and when connected can be 30 feet long. Such devices typically comprise specific pressure housings that require mechanical and electrical interconnections (contacts). These connection points are particularly vulnerable to severe shock and vibration, bending, compression and tension, high pressures and high fluid flow rates in the very harsh downhole-drilling environment. 
     Various methods have been developed and used in the industry to join, both mechanically and electrically, components of such electronic devices together in order to cope with the conditions of a drilling environment. The majority of these devices comprise a multi-contact plug and socket that cannot be rotated one into the other. Such connectors are firstly joined in a specified fashion and are then typically protected by a mechanical housing able to resist the downhole pressure. Many problems arise from the relatively complicated connection procedures necessary to connect such tool modules together, since not only must the longitudinal positioning of the two components be aligned but also they must be aligned properly in the angular sense relative to each other. 
     A simplification in the connection process that allows a more robust and reliable connection to exist utilizes a coaxial barrel style plug and socket design. Such design enables the plug to be attached to a housing, the socket attached to a similar housing and the pair are then simply pushed or screwed one into the other. Significant advantages that follow from the use of such a system are that smooth barrel joints are easily implemented, thereby minimizing flow erosion; mechanical complexity is reduced leading to more reliable systems and cost-effective implementations; and tool modules themselves can be housed in larger drill collars, enabling a simplification of the process whereby ‘collar plus tool’ is attached to another ‘collar plus tool’. 
     One prior art design that is economical, basic and reliable involves a coaxial barrel style plug and socket having a single diameter. Such design would otherwise be the connector system of choice were it not for the following problems, namely, that when fully engaging a single diameter plug and socket many of the contact rings slide past each other. In a connection system of more than two contact rings (and hence more than two electrical lines) that may be electrically active, there is a danger that misappropriate or unsafe connections may be made thereby damaging associated electronic circuits. 
     A known prior-art method and configuration to avoid the above problem of “wiping” causing inappropriate electrical connection is to modify the spacing of the contacts on the plug and socket pair such that no more than a single contact is able to make contact with another before engagement. This method, and a plug and socket combination employing such a configuration, is taught in U.S. Pat. No. 6,439,932. The aforesaid method and configuration has the serious disadvantage that in order the ensure no more than one contact connection is allowed at any time prior to full engagement, the inter-contact spacings have to be implemented at increasingly large distances from each other. This leads to a costly, long and unwieldy plug and socket pair, particularly when more than six independent connections have to be made. For instance, a mathematical analysis will show that such a connector is more than twice the length of a normal coaxial connector implemented with uniform spacing. 
     A plethora of alternative schemes that use switching means that electrically isolate connections until the appropriate electrical connections are fully made and thus avoid the wiping problem are discussed below. 
     U.S. Pat. No. 6,528,746 shows a non-coaxial connector means that uses a magnet to activate a magnetic flux responsive device (typically a reed switch) that then enables connections to be made. 
     U.S. Pat. No. 5,048,914 shows a non-coaxial connector that uses an optical transmitter/receiver pair to activate its switches. 
     U.S. Pat. No. 5,580,261 teaches a means for connecting a single pair of coaxial contacts which relies on the mechanical motion of an internal switch, the switch means ultimately causing a mechanical connection of the contacts. This invention is typical of the class of mechanical movement initiating further connections. 
     Another class of mechanical switches is the subject of many inventions that rely on solid-state switches (electronic switches) to control further switched connections. U.S. Pat. No. 4,346,419 is an example of this area of prior art. It specifically teaches the use of non-coaxial contacts of differing lengths, a short pair (last to connect) that when connected enables a solid-state switch to pass relatively high current through other longer pairs of longer contacts. Disadvantageously, this design requires the last contact to be continuously supplied with a voltage. Accordingly, despite low “trigger” voltages being used, such configuration is nonetheless unsatisfactory in explosive environments due to the possibility of initiating an explosion. 
     Typical of modern coaxial connectors is the invention as shown in U.S. Pat. No. 6,435,917. This teaches an improved manner of maintaining a reliable connection specifically related to socket contacts. However, such design provides no protection against inappropriate connections being made when engaging plug into socket. 
     U.S. Pat. No. 5,984,687 and U.S. Pat. No. 5,409,403 are typical of rotatable coaxial connector patents. These examples teach the use in specific circumstances of placing each successive contact on a successively increasing diameter. The essential advantages of this class of design are that all contacts are made only when plug and socket are essentially fully engaged, and that plug and socket can rotate about a common axis. The disadvantages are that such devices are relatively expensive and usually require a significantly larger diameter implementation than a simple fixed diameter coaxial multi-contact plug and socket, such as is specified in the present invention. Furthermore, there is no means by which such devices alone could safely operate in an explosive or hazardous environment. 
     In conclusion, the prior art teaches the use of plugs and sockets in rotatable (coaxial) and non-rotatable forms that enable contacts to connect when a fully engaged position between plug and socket is achieved. The determination of this position is implemented via one or more of the following, namely:
         contact axial spacing differences;   contact diameter spacing differences;   mechanical movement of a probe enabling contacts to be connected;   optical switch; and,   a magnetic switch.       

     While these above prior art designs exist, there is a real need, however, for a plug and socket design which combines a number of features, namely:
         comparatively small in footprint;   avoids the “wiping” problem;   simple mechanical housing;   can operate in hazardous environments; and   relatively inexpensive relative to some of the prior art designs.       

     SUMMARY OF THE INVENTION 
     Our invention enables a multi-contact coaxial plug to be axially inserted into its partner socket while electronic circuits attached to either side of the plug and/or socket are isolated from any harmful electronic misalignment during the engagement procedure. The plug and socket do not require any particular contact spacing and so can be realized in the smallest appropriate volume i.e. small fixed diameter and short fixed contact spacings. 
     Accordingly, the invention, in one of its broad aspects, contemplates a very simple basic electrical diode attached to the plug, enabling a sensor circuit attached to the socket to activate various solid state switches to protect the socket&#39;s attached electronic circuitry and permit electrical supply of power only when the plug and socket combination are fully engaged, and a similar standard electrical diode attached to the socket enabling a similar sensor circuit attached to the plug to activate various solid state switches to protect the plug&#39;s attached electronic circuitry also only when the plug and socket combination are fully engaged. The sensor circuits are symmetric and allow the protection means to activate when either the plug&#39;s circuit only is implemented, when the socket&#39;s circuit only is implemented, or when both are implemented. A specific embodiment facilitates this activation for both circuits when either or both are electrically powered. 
     A specific advantage of our invention is that such electrical connections and disconnections can be safely undertaken in hazardous environments. 
     Specifically, the present invention in one of its broad embodiments comprises a multiconductor plug and socket means;
         said plug means having at least three electrically conducting plug contacts thereon, adapted for insertion in socket means;   said socket means having a corresponding number of electrically conductive socket contacts thereon;   a first of said plug contacts electrically coupled to a second of said plug contacts via a plug-side current direction-limiting means;   a first of said socket contacts electrically coupled to a second of said socket contacts via a socket-side current direction-limiting means;   said first and second plug contacts adapted for electrical communication with said first and second socket contacts only upon proper engagement of said socket means with said plug means; and   circuit isolation means, said circuit isolation means only permitting flow of electrical current through one or more remaining plug-socket contact pairs when current flow through at least one of said plug-side and socket-side current direction-limiting means is detected.       

     The current direction-limiting device referred to above is typically a diode, but may be any combination of electrical or electronic circuits capable of providing this functionality. 
     In one refinement of the present invention, the circuit isolation means comprises plug-side circuit isolation means, said plug-side circuit isolation means only permitting flow of electrical current to at least one remaining plug contact when current flow through said socket-side current direction-limiting means is detected. 
     In an alternative refinement of the present invention, the circuit isolation means comprises socket-side circuit isolation means, said socket-side circuit isolation means only permitting flow of electrical current to at least one remaining plug contact when current flow through said plug-side current direction-limiting means is detected. 
     In a further refinement of the invention, where circuit isolation means is desired to prevent unintended shorting to electronic circuits on both the plug side and socket side of the electrical connection, the circuit isolation means comprises both plug side circuit isolation means and socket side circuit isolation means, both functioning as described above. 
     A timing circuit preferentially forms part of the circuit isolation circuit, and includes a delay from the time of connection between the plug means and socket means during which time electrical connection between the contacts must be fully established. One advantage of a timing circuit is that such a time delay prevents premature or intermittent contact associated with the current direction limiting means (typically a diode) from consequently triggering the establishment of electrical power to one or both of the plug contacts or socket contacts before full engagement of the plug means within socket means has been obtained. 
     In yet a further broad aspect of the present invention, the present invention comprises an apparatus for establishing electrical connection between a pair of electrical contacts, comprising:
         plug means;   socket means;   said plug means having one of said pair of electrical contacts thereon and a further first and second electrical plug contact thereon, said plug means adapted for insertion in said socket means;   said socket means having the other of said pair of electrical contacts thereon, and a further first and second socket contact thereon;   said first of said plug contacts electrically coupled to said second of said plug contacts via a plug-side current direction-limiting means;   said first socket contact electrically coupled to said second of said socket contacts via a socket-side current direction-limiting means;   said first and second plug contacts adapted for electrical communication with said first and second socket contacts only upon proper engagement of said socket means with said plug means; and   circuit isolation means, said circuit isolation means only permitting flow of electrical current through said pair of electrical contacts when current flow is detected through at least one of said plug-side and socket-side current direction-limiting means.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings, showing preferred embodiments of the invention, are illustrative only, and for a complete definition of the scope of the invention, reference is to be had to the summary of the invention and the claims. 
         FIG. 1  is a schematic showing a generalized form of the coaxial plug and socket connection of the present invention; 
         FIG. 2  is a more detailed schematic diagram of the isolation circuit for the plug isolating electronic switch circuit shown in  FIG. 1 ; 
         FIG. 3  is a more detailed schematic diagram of the isolation circuit for the socket side isolating electronic switch circuit shown in  FIG. 1 ; 
         FIG. 4  shows plug Sensor Circuit Detection means  263  and socket Sensor Circuit Detection means  264  shown generally in  FIGS. 2 and 3  respectively and how the Sensor Lines  292 ,  294  and  280 ,  282  are activated only by the full engagement of the plug  212  and socket  226 . A positive potential +V on the plug sensor circuit side  235  is connected to a resistor R 1  ( 272 ), then to a forward-biased diode  274 , then to diode  276  that acts to block this current, and finally to another resistor R 2  ( 278 ) that is grounded  250 . Sensor Line  1  ( 280 ) from the junction of  274  and  276  is connected to plug contact  284 . Sensor Line  2  ( 282 ) from the junction of  276  and  278  is connected to plug contact  286  and also to the plug Sensor Circuit input  256 . Similarly, a positive potential +V on the socket circuit side  236  is connected to a resistor R 1  ( 298 ), then to a forward biased diode  300 , then to a [cliodel] diode  302  that acts to block this current, and finally to another resistor R 2  ( 304 ) which is grounded  250 . Sensor Line  1  ( 292 ) from the junction of  300  and  302  is connected to socket contact  290 . Sensor Line  2  ( 294 ) from the junction  302  and  304  is connected to socket contact  288  and also to the socket Sensor Circuit input  257 . 
         FIG. 5  is a sensor circuit similar to that shown in  FIG. 4 , but modified slightly to form an alternate embodiment; 
         FIG. 6  shows schematically a sensor circuit, where only the plug side has associated isolation circuits and is electrically powered; 
         FIG. 7  shows schematically a sensor circuit, where only the socket side has associated circuits and is electrically powered; and, 
         FIG. 8  is a schematic drawing showing a typical plug and socket connector which may be used in the present invention, indicating wiring connections that corresponds to the associated wiring of the respective plug and socket electrical isolation circuits. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     While there are many methods of connecting two electronic circuits together, in one aspect the invention contemplates use of a coaxial plug and socket pair  212  and  226  respectively, as indicated in  FIG. 1 , each having a plurality of coaxially situate, concentric electrical contacts  211 ,  213  respectively thereon. The advantage of using such a coaxial multi-contact system is that the plug  212  and socket  226  can be housed in tubular containers (not shown) and the containers may be screwed together, thereby engaging the coaxial plug  212  into socket  226 . The mechanical advantage of this method of engagement brings a disadvantage—the majority of the contacts  211 ,  213  wipe past each other during insertion of plug  212  into socket  226  before the plug  212  and socket  226  become fully engaged. This may cause damage to attached electronic components if they are activated by some power source. Accordingly, the invention provides for interposing specific isolation circuits  202  and/or  216  to isolate and protect such components during the engagement process. We accomplish this by connecting plug  212  via wire harness  210  to switching circuit  202 . This circuit  202  isolates a variety of input/output lines (I/O)  200  from I/O lines  208 . A pair of lines is dedicated to use as sensor lines (Sensor Line  1   280  and Sensor Line  2   282 ) and are attached to contacts  284  and  286  which are preferably but not necessarily at the distal end  207  of plug  212 . Similarly we connect socket  226  via wire harness  224  to an isolation circuit  216 . Circuit  216  isolates a variety of input/output lines (I/O)  214  from I/O lines  222 . A pair of lines is dedicated for use as sensor lines (Sensor Line  1   292  and Sensor Line  2   294 ) and are attached to contacts  288  and  290 , which are preferably, but not necessarily at the distal end  221  of socket  226 . 
     For simplicity of deployment we have designed circuit  202  to be identical to circuit  216  (ref.  FIGS. 2 and 3 ) though this feature is not a required aspect of this invention. Although we indicate seven sets of corresponding electrical contacts associated respectively with plug  212  and socket  226 , it is obvious that the number of sets of contacts applicable to this application can be any reasonable number greater than two, and the depiction of seven contacts is merely arbitrary and illustrative of the principles to be employed. 
       FIG. 2  is a more detailed schematic diagram of the isolation circuit  202  in respect of the plug contacts  211 , as shown in  FIG. 1 . The I/O lines comprise a Power Line  235  monitored by Current Sensor  242  that controls Power Switch  244 , Digital Lines  233 ,  234  controlled by Digital Switches  246 , an Unswitched Line  248 , a Ground Line  250 , two Sensor Lines  280  and  282  controlled by Sensor Circuit  256 , and Timer Circuit  258 , the Timer  258  providing an Interrupt Line  260  to control Power Switch  244  and Digital Switches  246 . A diode  276  is carried by Sensor Lines  280  and  282 . 
       FIG. 3  is a more detailed schematic diagram of the isolation circuit  216  in respect of the socket contacts  213 , as shown in  FIG. 1 . The I/O lines comprise a Power Line  236  monitored by Current Sensor  241  that controls Power Switch  243 , Digital Lines  231 ,  232  controlled by Digital Switches  245 , an Unswitched Line  247 , a Ground Line  249 , two Sensor Lines  292  and  294  controlled by a Sensor Circuit  257  and Timer Circuit  259 , the Timer  259  providing an Interrupt Line  261  to control Power Switch  243  and Digital Switches  245 . A diode  302  is carried by Sensor Lines  292  and  294 . 
       FIG. 4  shows plug Sensor Circuit Detection means  263  and socket Sensor Circuit Detection means  264  shown generally in  FIGS. 2 and 3  respectively and how the Sensor Lines  292 ,  294  and  280 ,  282  are activated only by the full engagement of the plug  212  and socket  226 . A positive potential +V on the plug sensor circuit side  235  is connected to a resistor R 1  ( 272 ), then to a forward-biased diode  274 , then to diode  276  that acts to block this current, and finally to another resistor R 2  ( 278 ) that is grounded  250 . Sensor Line  1  ( 280 ) from the junction of  274  and  276  is connected to plug contact  284 . Sensor Line  2  ( 282 ) from the junction of  276  and  278  is connected to plug contact  286  and also to the plug Sensor Circuit input  256 . Similarly, a positive potential +V on the socket circuit side  236  is connected to a resistor R 1  ( 298 ), then to a forward biased diode  300 , then to a diode  302  that acts to block this current, and finally to another resistor R 2  ( 304 ) which is grounded  250 . Sensor Line  1  ( 292 ) from the junction of  300  and  302  is connected to socket contact  290 . Sensor Line  2  ( 294 ) from the junction  302  and  304  is connected to socket contact  288  and also to the socket Sensor Circuit input  257 . 
     It will be noted that the sensor lines  292 ,  294  on the socket Sensor Circuit Detection means  264  are crossed with respect to socket connections  288  and  290 . Apart from this detail the full circuits and wiring for both plug and socket Sensor Circuits  256 ,  257  are identical. The plug-side and socket-side Sensor Circuit Detection means  263 ,  264  may alternatively be arranged as shown in  FIG. 5 , wherein Sensor Lines  280 ,  282  are crossed with respect to plug connections  284  and  286 . 
     We proceed by explaining various embodiments in order to clarify how the system determines when the plug/socket combination has achieved full engagement. 
     Embodiment 1 
       FIG. 6  denotes an arrangement where active powered electronic circuits are incorporated only on the plug side, and furthermore that electronic access to the plug side circuits does not require socket side isolation circuitry because the socket side is essentially passive. For illustrative purposes we set the power line +V at 15 volts, resistor R 1  ( 272 ) is 50,000 ohms and resistor R 2  ( 278 ) is 100,000 ohms. 
     As may be seen with reference to  FIG. 6 , the determination of the full engagement of plug  212  and socket  216  (whereby electronic circuitry which requires isolation occurs on the plug side) is achieved as follows. Current from supply line  235  flows through resistor R 1  ( 272 ), through forward-biased diode  274  and is blocked from the plug sensor circuit output by diode  276 . A current pathway is available across the plug/socket junctions  284  and  288 , through diode  302  that now acts as a sensor activation element by passing current back through plug/socket junctions  290  and  286 , and finally through resistor R 2  ( 278 ) to Ground  250 . The potential across resistor R 2  ( 278 ) with respect to Ground  250  is sensed by the plug Sensor Circuit  256  to be approximately ⅔ times 15V (set by the potential divider R 1 /R 2  i.e. ˜10V). The threshold voltage necessary to activate the plug Sensor Circuit ( 256 ) could be set at 6 or 7 volts, greater than typical logic levels of 5V. Thus the activation voltage of ˜10V is comfortably greater than the threshold, and false activations are minimized. Diode  302  is forward biased because of the crossed sensor lines  292  and  294  on the socket side. Were this not the case the required voltage potential at the plug Sensor Circuit  256  would not be available because no current could flow through resistor R 2  ( 278 ), causing the appropriate activating voltage to be absent. Thus only when plug  212  and socket  216  are fully engaged is the plug Sensor Circuit  256  activated, and the switched lines forming part of the I/O bus  200  are then electrically connected to the I/O bus  208 . Hence the switched (and also the unswitched) lines are correctly available at the socket via the fully engaged plug. 
     It will be obvious to one reasonably skilled in the art that there should be no electrical circuits associated with socket  226  such as Digital Switches  245  that are in electrical communication with any of the non-sensor contacts  213  that would be electrically mistaken for the action of diode  302 , so as to otherwise initiate a “triggering” of the Power Switch  244 . To further guard against such a possibility, in a preferred embodiment of this aspect of the invention the output of Sensor Circuit  256  in respect of the plug sensor circuitry is passed through timer  258  (ref.  FIG. 2 ). The function of Timer Circuit  258  is to delay activation of Interrupt Line  260  controlling Power Switch  244  and Digital Switches  246  until the full engagement of plug  212  and socket  226  can be reasonably expected (typically one to two minutes). 
     The only significant requirements on the passive socket side is a diode  302  that is forward biased by crossed sensor lines  292 ,  294  in order that the Sensor Circuit  256  is correctly activated. 
     Embodiment 2 
     The complementary circuit to Embodiment 1 is depicted in  FIG. 7  and denotes an arrangement where active powered electronic circuits are incorporated only on the socket side, and furthermore that electronic access to the socket side circuits does not require plug side isolation circuitry because the plug side is essentially passive. For illustrative purposes we set the power line +V at 15 volts, resistor R 1  ( 298 ) is 50,000 ohms and resistor R 2  ( 304 ) is 100,000 ohms. 
     As may be seen with reference to  FIG. 7 , the determination of the full engagement of plug  212  and socket  216  (whereby electronic circuitry which requires isolation occurs on the plug side) is achieved as follows. Current from supply line  236  flows through resistor R 1  ( 298 ), through forward-biased diode  300  and is blocked from the plug sensor circuit output by diode  302 . A current pathway is available across the plug/socket junctions  290  and  286 , through diode  276  that now acts as a sensor activation element by passing current back through plug/socket junctions  284  and  288 , and finally through resistor R 2  ( 304 ) to Ground  249 . The potential across resistor R 2  ( 304 ) with respect to Ground  249  is sensed by the socket Sensor Circuit  257  to be approximately ⅔ times  15V  (set by the potential divider R 1 /R 2  i.e. ˜10V). The threshold voltage necessary to activate the socket Sensor Circuit ( 257 ) could be set at 6 or 7 volts, greater than typical logic levels of 5V. Thus the activation voltage of ˜10V is comfortably greater than the threshold, and false activations are minimized. Diode  276  is forward biased because of the crossed Sensor Lines  292  and  294  on the socket side. Were this not the case the required voltage potential at the socket Sensor Circuit  257  would not be available because no current could flow through resistor R 2  ( 304 ), causing the appropriate activating voltage to be absent. Thus only when plug  212  and socket  216  are fully engaged is the socket Sensor Circuit  257  activated, and the switched lines forming part of the I/O bus  214  are then electrically connected to the I/O bus  222 . Hence the switched (and also the unswitched) lines are correctly available at the socket via the fully engaged plug. 
     The significant requirements on the passive plug side is a diode  276  that is forward biased by crossed sensor lines  292 ,  294  in order that the Sensor circuit  257  is correctly activated. 
     Embodiment 3 
     The discussion of Embodiment 1 and Embodiment 2 above now makes the complete understanding of Embodiment 3 as exemplified by either  FIG. 4  or  FIG. 5  straightforward. Both plug sensor circuit  236  and socket sensor circuits  264  are powered independently by +V (plug)  235  and +V (socket)  236  lines. Taking  FIG. 4  for example, the voltage level output to Sensor Circuit  256  (plug) is available via either of two routes:
         a) current from line  235  via resistor R 1  ( 272 ) and diode  274  passes along Sensor Line  1  ( 280 ) to contacts  284  and  288 , then via Sensor Line  2  ( 294 ) through diode  302 , Sensor Line  1  ( 292 ), contacts  290  and  286 , Sensor Line  2  ( 282 ) and through resistor R 2  ( 278 ) to Ground  250 . The potential at the junction of R 2  ( 278 ) and Sensor Line  2  ( 282 ) with respect to Ground  250  is now available to activate the plug Sensor Circuit  256 ; or   b) current from line  236  through resistor R 1  ( 298 ) and diode  300  passes along Sensor Line  1  ( 292 ), through contacts  290  and  286 , then via Sensor Line  2  ( 282 ) through resistor R 2  ( 278 ) to Ground  250 . The potential at the junction of R 2  ( 278 ) and Sensor Line  2  ( 282 ) with respect to Ground  250  is now available to activate the plug Sensor Circuit  256 .       

     The choice of routes a) or b) is determined solely by whether +V (plug)  235  is greater than +V (socket)  236  by more than one diode drop (typically 0.6V). In either case the significant issue is that the plug Sensor Circuit  256  is activated by an adequate +V (socket)  236  potential or by the presence of diode  302 —both are associated only with the full engagement of the plug and socket, and either will suffice. 
     Likewise, the voltage level output to Sensor Circuit  257  (socket) is similarly available via either of two routes:
         c) current from line  236  via resistor R 1  ( 298 ) and diode  300  passes along Sensor Line  1  ( 292 ) to contacts  290  and  286 , then via Sensor Line  2  ( 282 ) through diode  276 , Sensor Line  1  ( 280 ), contacts  284  and  288 , Sensor Line  2  ( 294 ) and through resistor R 2  ( 304 ) to Ground  249  . The potential at the junction of R 2  ( 304 ) and Sensor Line  2  ( 294 ) with respect to Ground  249  is now available to activate the plug Sensor Circuit  257 ; or   d) current from line  235  through resistor R 1  ( 272 ) and diode  274  passes along Sensor Line  1  ( 280 ), through contacts  284  and  288 , then via Sensor Line  2  ( 294 ) through resistor R 2  ( 304 ) to Ground  249  . The potential at the junction of R 2  ( 304 ) and Sensor Line  2  ( 294 ) with respect to Ground  249  is now available to activate the plug Sensor Circuit  257 .       

     Again, the choice of routes c) or d) is determined solely by whether +V (socket)  236  is greater than +V (plug)  235  by more than one diode drop (typically 0.6V). In either case the significant issue is that the socket Sensor Circuit  257  is activated by an adequate +V (plug)  235  potential or by the presence of diode  276 —both are associated with the full engagement of the plug and socket, and either will suffice. 
     Diodes  274  and  300  ensure that there can be no unintended reverse current flow into their associated power supply from the power supply at higher potential on the other side of the plug/socket. 
     This embodiment illustrates usefulness of the symmetry of the circuit operations attached to either plug or socket—fabrication of the switching circuits is simplified in that both assemblies can be identical. The only necessary modification is that the lines must be crossed between contacts  288 ,  290  and Sensor Lines  292  and  294  (as shown in  FIG. 4 ), or equally between contacts  284 ,  286  and Sensor Lines  280  and  282  (as shown in  FIG. 5 ). In these embodiments, when plug and socket are fully engaged,  FIGS. 2 and 3  indicate that the Power Switch lines ( 235 ,  236 ), Digital Switch lines ( 233 ,  234 ,  231 ,  232 ), the Unswitched Lines, Ground Lines and Sensor Lines are all connected appropriately. This enables power to flow as required from plug to socket or vice versa, digital information to flow as required from plug to socket or vice versa, etc. 
     Our invention does not limit us to a ‘one-to-one’ line connection correspondence, however. The obvious inclusion of more contacts in plug  212  and socket  226  would enable the independence of the information or power carrying lines. The necessary and sufficient feature for determining full engagement is that plug Sensor Line  1  ( 280 ) connects to socket Sensor Line  2  ( 294 ) and plug Sensor Line  2  ( 282 ) connects to socket Sensor Line  1  ( 292 ) when diode  276  and/or diode  302  (for example) are chosen as the engagement sensing devices. Specific wiring connections through a representative plug and socket pair is depicted in  FIG. 8 . In particular the Sensor Line crossed wiring ( 282  to  292 ,  280  to  294 ) is evident. 
     Importantly, with respect to each of the embodiments shown in  FIGS. 2 and 3 , the present invention is not limited to a sensory circuit using only a simple diode as a sensing means. In particular, it is possible and is contemplated within the scope of the present invention to replace each diode  276  and/or  302  by other electrical circuitry, including current direction-limiting circuitry, so as to permit the sensor circuit to produce a particular electronic signal when specifically sensed at full engagement of the plug  212  and socket  226 . The present invention is not to be limited to circuitry implementing only diodes  276  and  302 .