Abstract:
The technique is disclosed for detecting active ports of an electronic device. The electronic device comprises a plurality of ports, including a first port. A determination is automatically performed as to whether an external component is connected to the first port. The first port is identified as an active port in response to a determination that an external component is connected to the first port. If there no external component is connected to the first port, then the first port is identified as an inactive port. The inactive and active ports may then be distinguished and treated differently during subsequent control and/or management of the plurality of ports. According different implementations the first port may be identified as an active port in response to detection of a capacitive, resistive, and/or inductive load connected to the first port. The first port as may also be identified an active port in response to a determination that a current is flowing through the first port.

Description:
RELATED APPLICATION DATA 
   The present application claims priority under 35 U.S.C. § 119 to U.S. Provisional Application Ser. No. 60/227,944, naming James et al. as inventors, and filed Aug. 25, 2000, the entirety of which is incorporated herein by reference in its entirety for all purposes. 

   BACKGROUND OF THE INVENTION 
   The present invention is related generally to sequencing devices, and more particularly to the sequencing technique with automated active port detection and sequencing. 
   Conventionally, sequencing circuits typically include a relatively small number of ports (e.g. 1–5 ports) for sequencing attached wires according to a predefined sequence. If more wires are needed to be sequenced, then two or more conventional sequencing units may be serially combined to produce a system which is able sequence the desired number of wires. For this reason, conventional sequencers having a relatively large number of sequencing ports (e.g. more than 5) are uncommon, since is it cheaper to combine a few smaller sequencing units than to manufacture a large sequencing unit. 
   One problem with conventional sequencing units is that they are not able to perform automatic wire detection. Without wire-detection, the device controlling the wires would produce “blanks” in the patterns, or points in the sequence where no wires are connected, when the sequencer attempts to turn on those wires. Another problem with conventional sequencing devices relates to zero-crossing issues, as described in greater detail in the Detailed Description Section of this application. 
   Accordingly, it will be appreciated that there exists a present need to improve upon conventional sequencing device designs in order to provide improved features and advantages. 
   SUMMARY OF THE INVENTION 
   According to various embodiment the present invention, methods, systems, and computer program products are disclosed for detecting active ports of an electronic device. The electronic device comprises a plurality of ports, including a first port. A determination is automatically performed as to whether an external component is connected to the first port. The first port is identified as an active port in response to a determination that an external component is connected to the first port. If there no external component is connected to the first port, then the first port is identified as an inactive port. The inactive and active ports may then be distinguished and treated differently during subsequent control and/or management of the plurality of ports. According to one implementation the first port may be identified as an active port in response to detection of a capacitive load connected to the first port. In a different implementation, the first port may be identified as an active port in response to detection of a resistive load connected to the first port. In an alternate implementation, the first port as may be identified an active port in response to a determination that an inductive load is connected to the first port. In yet another implementation, the first port as may be identified an active port in response to a determination that a current is flowing through the first port. 
   Alternate embodiments of the present invention are directed to various methods, systems, and computer program products for sequencing selected ports of an electronic device which includes a first port and a second port. Active ports of the electronic device are automatically identified. According to a specific embodiment, an active port may be characterized as a port which has an external load physically connected to it. Sequencing of only desired, active ports of the electronic device may then be implemented. 
   Additional objects, features and advantages of the various aspects of the present invention will become apparent from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of a sequencing device  100  in accordance with a specific embodiment of the present invention. 
       FIGS. 2A and 2B  shows a block diagrams of active port detection (APD) devices which have been implemented in accordance with specific embodiments of the present invention. 
       FIG. 3  shows a schematic diagram illustrating a specific embodiment of the active port detection device  300  of the present invention. 
       FIG. 4  shows a flow diagram of a Sequencing Initialization Procedure  400  in accordance with a specific embodiment of the present invention. 
       FIG. 5  shows an example of an Active Port Table  500  in accordance with a specific embodiment of the present invention. 
       FIG. 6  shows an example of an alternate implementation of the wire-detect circuit of  FIG. 2A . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a block diagram of a sequencing device  100  in accordance with a specific embodiment of the present invention. As shown in the embodiment of  FIG. 1 , the sequencing device  100  includes a plurality of ports  102 , a sequencer  110 , and an active port detection circuit  130 . In a specific implementation, the active port detection circuit  130  may include a processing device  132  (e.g. a CPU, EEPROM, AISIC, etc.) and memory  134 . As explained in greater detail below, the sequencing device  100  may be configured or designed to automatically detect the presence of one or more wires connected to one or more of the of the ports  102 . Once the sequencing device has determined the specific ports which are active (e.g. ports which have wires connected to them), the sequencing device may then sequence each of the active ports, in a particular order without attempting to include inactive ports in the sequence. According to one implementation, the active port detection circuit  130  may be responsible for sequencing each of the active ports. In an alternate embodiment, the sequencer  110  may sequence the active ports using information provided by the active port detection circuit  130 . 
   According to specific embodiments, the technique of the present invention provides a number of advantages and/or features which are not provided by conventional sequencing devices. For example, the sequencing technique of the present invention may be used to provide automatic wire detection of wires attached to a sequencing unit. Because of this feature, a generic sequencing unit may be manufactured to have a relatively large number of ports (e.g. 5–10 or more ports), and the automatic wire detection feature may then be used to automatically detect and sequence only active ports (e.g. only those ports on the unit where a wire or circuit is physically connected). 
   According to a specific embodiment, and active port detection (APD) device of the present invention may be configured or designed to detect the presence of devices or elements which are physically connected to ports of the APD device. According to a specific implementation, the detection of the presence of an element, component, or device connected to a given port may be based upon detection of a capacitive load connected to the port. Alternatively, the detection of the presence of an element, component, or device connected to a given port may be based upon detection of a resistive load connected to the port. In another implementation, the detection of the presence of an element, component, or device connected to a given port may be based upon detection of current flowing through the port. 
   The APD device may be configured to detect the presence or absence of devices/elements connected to its ports upon power up (or reset). Thereafter, once the active ports have been identified, the APD device may be configured to control only the identified active ports. Empty or no-active ports may be selectively omitted from being controlled by the APD device. In a specific embodiment, the APD device corresponds to a sequencing device which may be configured or designed to sequence only active ports on the sequencing device. However, the technique of the present invention may be applied to applications other than sequencing operations. For example, a device which is configured to implement the active port detection technique of the present invention may be provided to allow a user to connect desired wires and/or circuits to one or more ports on the device, and the device may then automatically determine the active ports on the device, and use this information when performing future operations. 
   According to a specific embodiment, the APD device may be configured or designed to automatically detect the presence of elements which are physically connected to ports of the APD device, even when such elements do not form a closed circuit. For example, using the technique of the present invention, an open wire that is connected to a specific port of the APD device may be detected and the corresponding port identified as being an active port. According to a specific implementation, the determination as to whether a selected port is active or inactive may be achieved by detecting for the presence of a capacitive load connected to that port. This technique is particularly useful in situations where the port is connected to an open circuit such as, for example, a length of electroluminescent wire. Alternatively, according to an alternate implementation, the determination as to whether a selected port is active or inactive may be achieved by detecting for the presence of a resistive load connected to that port. This technique is useful in situations where the port is connected to a closed circuit, as, for example, a light emitting diode (L.E.D). According to yet another implementation, the determination as to whether a selected port is active or inactive may be achieved by detecting for the presence of a current flowing through that port. 
   It will be appreciated that, without the active port detection feature of the present invention, for example, a controller may attempt to control blank or empty ports, or may not differentiate between active ports and non-active ports. For example, in the case of a sequence of devices, empty ports would be included in the sequencing of the ports, thereby resulting visually in “blanks” in the sequencing pattern. 
   According to a specific implementation, the APD device may be configured or designed such that no configuration is required by the user of the invention, other than the user plugging devices/elements into ports on the APD device. The APD device will automatically detect the appropriate active ports. No jumpers or switches need be used to tell the APD device which ports have devices/elements connected to them. In an alternate implementation, the user may at appropriate configuration information such as, for example, the sequencing order of the active ports. Thus, for example, using the technique of the present invention, the APD device may automatically configure itself to be a 2-channel sequencer when 2 elements/devices are connected to ports on the APD device, a 3-channel sequencer when 3 elements/devices are connected to ports on the APD device, etc. 
   Additionally, according to one implementation, logically consecutive active ports on the APD device need not be physically consecutive in terms of the order of servicing each active port. For example, six devices/elements could be connected to a 10-port APD device using any combination of six of the 10 ports. In this example, using the technique of a present invention, the physical sequencing of ports: P 0 , P 3 , P 5 , P 6 , P 8 , P 9 , for example, may be assigned a logical sequence of: Device  0 ,–Device  5 .) 
   Additionally, if ports two and three were physically damaged, the user could connect devices to any of the eight remaining ports to produce a “logically” consecutive sequence of the active ports. Thus, unlike conventional techniques, physically damaged ports do not produce “blanks” in sequences. Moreover, the user can use another connector if a connector (port) aboard the APD device is physically damaged (i.e., pins are broken, electrical continuity to port is broken). 
     FIG. 2A  shows a block diagram of an APD device  200  which has been implemented in accordance with a specific embodiment of the present invention. It will be appreciated that the APD device of  FIG. 2A  may be modified by one having ordinary skill in the art to include additional or fewer components than those illustrated in  FIG. 2A . 
   Referring now to the APD device  200  of  FIG. 2A , a power source ( 201 ) supplies power to a power converter ( 202 ) which converts the power from the power source to a voltage, current, and frequency sufficient to supply a load connected to a port (e.g. port  208   a ) of the circuit with power appropriate to cause this load to function (e.g., power appropriate to cause an L.E.D. to illuminate, power appropriate to cause and electroluminescent wire to light up, etc). In the embodiment of  FIG. 2A , the load may correspond to a particular device, complement, or element, such as, for example, an electroluminescent wire or an L.E.D. 
   According to a specific embodiment, each port of the APD device may include one or more input slots or receptacles for receiving connections to external devices, complements, and/or elements. 
   As shown in  FIG. 2A , one or more switching devices  207  (e.g.  207   a ,  207   n ) may be provided in order to individually control the flow of power and/or current two selected ports. For example, in a specific implementation, a separate switching device may be provided for each respective port of the APD device. Control of each of the switching devices may be accomplished via respective control lines  206  (e.g. control line  206   a ,  206   n ) using one or more control signals. According to a specific embodiment, the control signals may be provided, for example, by the Active Port Detection Circuit  130  or the sequencer  110  of  FIG. 1 . 
   According to a specific implementation, when a load is connected to one of the ports (e.g. port  208   a ), a current may flow from power source ( 203 ) through a switch ( 207   a ) to ground ( 209 ) and back through the Current Sensor ( 205 ), then back to the RETURN ( 204 ) of the power converter. When this current is flowing, the Active-Port Detect Output ( 220 ) may output a signal indicating that a device is connected to port  208   a.    
   According to a specific embodiment, the testing of each port as either an active or inactive port may be accomplished in a serial manner, wherein power is provided to each port, one port at a time. For example, in one implementation, a “turn-on” control signal may be applied to (via control lines  206 ) each switch ( 207 ), one switch at a time. A circuit (e.g. he Active Port Detection Circuit  130 ,  FIG. 1 ) reading the Active-Port Detect Output ( 220 ) may then determine if a device is connected to the port associated with the currently selected switch. In one implementation, the circuit reading the Active-Port Detect Output ( 220 ) may correlate the “turn-on” signal with the Active-Port Detect Output ( 220 ) in order to make a map (e.g. Table  500  of  FIG. 5 ) of the ports to which devices are connected. Using this map, the circuit can commence to control only those ports connected to devices and ignore ports which have no device connected to them. 
   One advantage of detecting the presence of a device connected to a port using a current path (e.g.  204 ) to the power converter ( 202 ) common to all ports is that only one active-port detector circuit is needed to detect a device connected to any ports of the circuit  200 . 
   In one embodiment (as shown, for example, in  FIG. 2A ), the Current Sensor ( 205 ) is placed between RETURN ( 204 ) and ground ( 209 ). In another embodiment (as shown, for example, in  FIG. 2B ), the Current Sensor ( 205 ) is placed between POWER ( 203 ) and DEVICE SUPPLY ( 211 ). The specific embodiment to be used may be determined by the devices used to perform current-detection. In the embodiments of  FIGS. 2A and 2B , each of the APD devices has being configured to utilize a current-detection technique to detect the presence of current flowing from the Power Converter ( 202 ) to one or more of the devices connected to ports  208 . 
     FIG. 3  shows a schematic diagram illustrating a specific embodiment of the active port detection device  300  of the present invention. According to specific embodiments, the active port detection technique of the present invention may be implemented upon the occurrence of a Power ON or Reset of the APD device, or at other times desired by the user. Once the identification of active ports has been accomplished, the APD device may then commence with sequencing and/or controlling the active ports according to a predetermined sequencing pattern. 
   According to a specific implementation, the technique of the present invention may be used to detect the presence of electroluminescent wire connected to one or more ports  315  (e.g.  315   a ,  315   n ) of circuit illustrated in  FIG. 3 . In the embodiment of  FIG. 3 , the circuit  300  is supplied with an AC voltage via a DC to AC inverter ( 302 ). One output terminal of this inverter is connected to input line HVIN  304 . The other output terminal of the inverter is connected to a common line HVCOM  305 . The name HVCOM refers to the fact that current flowing through any wire has a common path to ground ( 314 ) via line  305 , and that, according to a specific embodiment, the current flowing through any port may be detected along this path. 
   As shown in the embodiment of  FIG. 3 , the APD device  300  includes a plurality of ports  315  (e.g.  315   a ,  315   n ). Each of the ports  315  is configured to include one or more receptacles or sockets. In the embodiment of  FIG. 3 , each port is configured to include a receptacle having to connectors (e.g.  317   a ,  317   b ). One of the connectors (e.g.  317   a ) is electrically coupled to the HVIN line  304 , and the other connector (e.g.  317   b ) is electrically coupled to a respective TRIAC  316  (e.g.  316   a ,  316   n ) which provides an electrical path for allowing current to flow from the port to ground  314 . When an electrical connection is made between connectors  317   a  and  317   b , AC current may then flow from HVIN, through the electrical connection, and back to ground ( 314 ). 
   According to a specific embodiment, in order to implement active port detection, an input signal may be applied to a selected control input line, such as, for example, Control Input Line  1  ( 318   a ). According to one implementation, the voltage of the input signal should preferably be sufficient to cause the TRIAC  316   a  to latch. In a specific implementation, the TRIAC may be configured to latch at 5 mA of positive current flowing from power source  301  into the TRIAC through the resistor RW 1 . According to a specific embodiment, the input signal may be applied to one TRIAC at a given instant. Each TRIAC ( 316 ) in the circuit, in turn, is given a turn-on signal in order to test whether its associated port is active or inactive. According to a specific implementation, active port detection may be implemented initially upon the occurrence of a POWER ON or RESET operation. 
   The sensing of the presence or absence of an external load (e.g. external element, component, circuit, etc.) connected to a particular port is commenced after the input signal is applied to its associated TRIAC. Sensing may be performed for each port ( 315   a–n ) by a sensing device, such as, for example, the Active Port Detection Circuit  130  of  FIG. 1 . In one implementation, the sensing device may be configured to sense the current returning to ground ( 314 ) that is flowing through the load, which is connected to the port ( 315   a ) that the TRIAC ( 316   a ) controls. In this way, the sensing device is able to detect a capacitive load across the connectors (e.g.  317   a ,  317   b ) of the port. Additionally, using this technique, the sensing device is also able to detect a resistive load and/or an inductive load across the connectors (e.g.  317   a ,  317   b ) of the port. 
   Current flowing through HV COM ( 305 ) to and from ground ( 314 ) produces a voltage across R 9 , D 1 , and D 2 . Given the fact that there is a slight capacitive coupling of current between the terminals of a socket even with no wire plugged in, a false-detect can occur if this leakage current is not given a sufficient path to prevent the turning on of the detect-transistor (Q 1 ). 
   For example, if R 9  did not exist, then D 1  and D 2  would produce +/−0.7V of potential between HVCOM ( 305 ) and Ground ( 314 ). A +0.7 V potential with regards to ground is sufficient to switch Q 1  on, initiating a wire-detect, which condition is described below. By selecting R 9  so that the leakage current produces much less than 0.7 V of potential via the relation (R=I/V), false wire-detects are thus prevented by the component R 9 . In a specific implementation, the value of R 9  is 470 Ohms. 
   Diodes D 1  and D 2  prevent the voltage between HVCOM ( 305 ) and Ground ( 314 ) from going outside of the range +/−0.7V. This prevents the active port detection circuit from dissipating a significant portion of the power produced by the inverter. For example, with an RMS voltage of 100V AC produced by the inverter, the 0.7V drop across D 1  and D 2  is 0.7V/100V*100 percent=0.7 percent of the inverter power. Without the diodes, perhaps several volts would be present between HVCOM and Ground, leading to a several percent loss in power that is not delivered to the port being tested, and hence the load. 
   When an element is connected to a selected port (e.g.  315   a ) and the TRIAC ( 316   a ) for that port is switched on, the current flow into HVCOM ( 305 ) produces a voltage with respect to Ground ( 314 ) sufficient to switch Q 1  on during the positive part of the current cycle or wave supplied by the inverter ( 302 ). This, in turn, causes C 6  to be discharged to close to zero volts (0 to 0.2V). During the negative part of the current cycle of the inverter ( 302 ), C 6  charges back toward 5V ( 180 ) because Q 1  is now turned off and the capacitor only supplied with current from R 8 . However, C 6  is reset to zero volts before it can rise to a logical  1  (e.g. logical 1=1 volt, reset occurs at about 0.5 volt) by the next positive portion of the current wave. If the capacitor C 6  were not present, the wire output detect voltage at  313  would instantaneously go back to 5V, falsely indicating that no wire is present. According to a specific embodiment, the presence of a wire at the port is sensed by a logical zero (0 to 0.2V with respect to ground) at the wire detect terminal  313 . 
   The absence of a wire is detected by waiting a sufficient amount of time (e.g. 1 ms, 2 wave cycles) for C 6  to be charged up to or past a logical 1 (1.0V or greater with regards to ground  314 ), and hence for the wire-detect output  313  to be at a logical 1. Note that this time should preferably be greater than a half-wave cycle lest a previous wire-detect left capacitor C 6  discharged and it has not charged up to a logical 1 yet. Thus, according to a specific embodiment, 2 or more wave cycles of delay occurs between the turning on of a control signal at  318  and the checking of the Wire Detect output at  313 . 
   According to a specific implementation, the various complements of the circuit illustrated in  FIG. 3  may have the following corresponding values: 
   
     
       
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Reference ID 
               Description 
               Example Value(s)/Types 
             
             
                 
                 
             
           
           
             
                 
               C4 
               capacitor 
               0.0068 uF, 600 V 
             
             
                 
               R9 
               resistor 
               470 Ohms 
             
             
                 
               D1 
               diode 
               1N4007 
             
             
                 
               D2 
               diode 
               1N4007 
             
             
                 
               R6 
               resistor 
               1K 
             
             
                 
               Q1 
               transistor 
               2N2222 
             
             
                 
               R8 
               resistor 
               10K 
             
             
                 
               C6 
               capacitor 
               0.1 uF 
             
             
                 
               316a 
               TRIAC 
               Z01017 
             
             
                 
               316n 
               TRIAC 
               Z01017 
             
             
                 
               RW1 
               resistor 
               1K 
             
             
                 
               RWn 
               resistor 
               1K 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 4  shows a flow diagram of a Sequencing Initialization Procedure  400  in accordance with a specific embodiment of the present invention. The Sequencing Initialization Procedure may be implemented in a variety of manners. For example, in the embodiment of  FIG. 4 , at power on or reset ( 402 ) the device selects ( 404 ) a first port for detection of wire presence, and determines ( 406 ) whether a wire is connected to the port using, for example, the wire detection circuit described previously with respect to  FIGS. 1–3 . If the circuit determines that a wire is connected, the appropriate port ID is added ( 408 ) to a table, such as, for example, the table  500  of  FIG. 5 . Thereafter, or if no wire is detected on the selected port, a determination is made ( 440 ) as to whether all of the ports of the device (e.g. sequencer unit) have been checked. If there are ports that have not been checked, the next port is selected ( 442 ), and the procedure returns to block  406  wherein the newly selected port is checked for the presence of a wire connected to the port. Once all ports have been checked, the port table has been completed, and sequencing of selected ports may then commence ( 414 ), wherein the selected ports correspond to the port IDs in table  500  ( FIG. 5 ). 
   One advantage of this invention is that the device can automatically sequence through patterns appropriate for the number of wires connected. 
   One other embodiment of this flow chart is to return to the wire detect procedure at periodic intervals to maintain a continuous sequence in the event of dynamic hardware failure or alteration. For example, wires could be added or removed, these changes automatically detected, and then the sequence patterns automatically adjusted. Conversely, in the first manifestation, after the table  500  has been constructed, active wires can be removed to create new patterns with blank or “dark” spaces. 
   Zero-Crossing Detection 
   The present inventive entity currently produces several produces which switch electroluminescent wire on at a speeds of 30–100 Hz. Electroluminescent wire (distributed under the tradename Coolneon™ by Funhouse Productions, of Oakland, Calif.) is generally known to one having ordinary skill in the art, and is described in greater detail in U.S. Pat. No. 5,869,930, to Baumberg, et. al, and entitled “Electroluminescent Light Source With A Mixture Layer Filled With A Transparent Filler Substance”, and U.S. Pat. No. 5,485,355, to Voskoboinik, et al., and entitled, “Electroluminescent Light Sources”, the disclosures of which are herein incorporated by reference in their entirety for all purposes. 
   It has been determined that when the load (e.g. electroluminescent wire) is switched sufficiently fast by an inverter (e.g. greater than 10 times per second), the inverter oscillation will periodically attenuate suddenly to 0 VRMS. The inverters recover from this situation and begin oscillating anew after at least 10–20 mS, sometimes more. This time period of 10–20 mS when the inverter is at 0VRMS means that the wire being powered by the inverter perceptibly dims or flickers. This is percieved as ‘wrong’ or ‘irritating’ to a user who is expecting a regular pattern, and is especially enoticable when an inverter is being switched, for example, at 30–60 Hz. 
   The present inventive entity has discovered that when a TRIAC turns on a wire when the inverter is not moving towards 0V, attenuation of its oscillation may occur. 
   It has also been discovered that when a TRIAC is turned on just before the zero-crossing of the current wave (corresponding to the AC current flowing through the wire), that an inverter will not die-down to 0VRMS, but rather will continue oscillating normally. 
   According to a specific embodiment, the wire-detection circuit of  FIGS. 1 and 2  may be modified in the following way so that it not only detects a wire, but also provides zero-crossing information to the device that the wire-detect output goes to. 
     FIG. 6  shows an example of an alternate implementation of the wire-detect circuit of  FIG. 2A . As shown in  FIG. 6 , an optoisolator ( 609 ) is used to detect current flow. Additionally, there are four diodes instead of two. This is because the optoisolator&#39;s photo L.E.D. turn on voltage is around 1.4V, or 2 diode voltage drops, whereas the sequencer circuit of  FIG. 2A  utilizes a normal transistor which turns on at approximately one diode voltage drop. The optoisolator  609  frees up the circuit to sense current off the HV Supply or HV Return, it does not matter which. 
   Referring to  FIG. 6 , when no wire is plugged into any port, a slight leakage current will still flow through the diodes D 1  ( 604 ), D 2  ( 605 ), D 3  ( 606 ) and D 4  ( 607 ). R 1  ( 608 ) is selected by the relation R&lt;&lt;V/I, where V=turn on voltage of photodiode of the optoisolator ( 609 ), and I=maximum leakage current. In this way, false-detects are prevented, the same as in  FIG. 2 . According to a specific implementation, the value of R 1  is 470 ohms. 
   One change to the wire-sense circuit of  FIG. 6  is that the charge capacitor of  FIG. 2A  has been removed from the circuit. The circuit may be the same in all other aspects. The change is in the interpretation of the voltage levels at the Wire Detect Output ( 612 ): 
   When the Wire Detect Output ( 612 ) stays at 5Volts for greater than or equal to one wave-cycle after an attempt is made to turn on a wire, there is no wire plugged into the port for that wire. If, however, the Wire Detect Output is a Square Wave (of 1 wave cycle in duration) swinging, for example, from 0V to 5V, then a wire is detected. In addition, for example, the square-wave gives information about when the zero-current-crossing occurs. By timing the zero crossing of a wire, the next wire can be switched on and the current wire off just before (1–5% of phase before) the actual zero-crossing event occurs. This has been determined to remove inverter die-down (attenuation of inverter oscillation to approximately 0Vrms). 
   When a wire is present and under control (i.e., the TRIAC ( 616   a ) controlling it is turned on via a control signal ( 618   a )), the photodiode of the optoisolator ( 609 ) is excited when the voltage wave of HVSUPPLY is greater than 1.5V, causing the phototransistor of the optoisolator to turn on, bringing the output of the transistor to 0V ( 612 ). When the HVSUPPLY voltage swings back to less than 1.5V, the photodiode and phototransistor both turn off, causing the Wire-detect output ( 612 ) to discurse back to 5.0V. By sensing these transitions, zero-crossing information is obtained by the same facility that is used to detect wires. 
   Although several preferred embodiments of this invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to these precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of spirit of the invention as defined in the appended claims.