Abstract:
Status information is provided from an electronic signaling system to an array of N light emitting diodes (LEDs) connected in series between high and low voltage sources, where N≧2, and where N is selected so that the potential difference between the voltage sources is less than the sum of the cut-in voltages of the N LEDs in the array. Control signals are delivered from the electronic signaling system to the LED array over M control lines (N&gt;M≧1), each of which is connected between two of the LEDs in the array. The control signals cause the LEDs to conduct. The control signals are timed so that the LEDs in the array conduct one or two at a time.

Description:
TECHNICAL FIELD 
     This application relates to electronic signaling and, more particularly, to driving a visual status indicator array in an electronic signaling system, such as those found in network repeaters and switches. 
     BACKGROUND 
     Many computer networks rely on network repeaters and switches to facilitate the exchange of information among the computers in the network. In many networks, such as Ethernet networks, information is exchanged in the form of data packets that pass through each of the repeaters or switches in the network. The repeaters or switches usually monitor the data packets to collect information on the status of network resources. Network administrators then use the status information to manage the network resources. 
     One way of conveying the status information from a repeater to a network administrator is through visual indicators, such as an array of light emitting diodes (LEDs). In many cases, each LED in the array is dedicated to presenting information about a particular status condition on a particular repeater port. The network administrator can determine whether a particular status condition exists on a repeater port by observing whether the corresponding LED in the array is illuminated. One problem with this technique is that additional pins must be added to the repeater chip to deliver status signals to the LED array, thus driving up the cost and complexity of the repeater chip. 
     Sophisticated techniques have been developed to reduce the number of signal lines required to drive an LED indicator array in a network repeater. In one such technique, a 16×5 array of LEDs provides information about five status conditions for each of sixteen repeater ports. The LED array is driven by eight time-multiplexed signals, each of which carries information about all five status conditions for two of the sixteen repeater ports. While this technique for driving the LED array succeeds in placing a great deal of information on very few status lines, the technique requires a relatively sophisticated multiplexing circuit in the repeater chip and an equally sophisticated demultiplexing scheme at the LED array. This technique is much more suited for use with large LED arrays than it is for small arrays, such as a 4×4 or a 6×3 array. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is schematic diagram of a computer network with several workstations connected to a repeater. 
     FIG. 2 is a schematic diagram of a status indicator array. 
     FIG. 3 is a block diagram of a network repeater chip with circuitry to drive the indicator array of FIG.  2 . 
     FIG. 4 is a table showing the operation of the control circuitry of FIG.  3 . 
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     FIG. 1 shows a computer network  100  in which several computers  102 ,  104 ,  106  are connected to a repeater or switch  108 . The repeater  108  includes multiple ports, at least one of which receives data packets from the computers  102 ,  104 ,  106 , and at least one of which distributes the data packets throughout the network  100 . The repeater  108  also includes, or is linked to, a visual display  110 , such as an LED array, that provides a visual indication of various status conditions monitored by the repeater  108 . In general, the visual display  110  responds to status information collected by the repeater  108  from the data packets. The repeater  108  usually collects information about one or more particular status conditions for each of the ports through which data packets travel. For example, a particular repeater might monitor six status conditions for each of six repeater ports, thus producing 36 separate status conditions. In most cases, each of these status conditions has a corresponding LED in the indicator array. Examples of the types of status conditions monitored for individual ports include the standard LINK, PARTITION, ISOLATE, PORT ENABLED, and COLLISION conditions. In some cases, the repeater also monitors status conditions that do not apply to particular ports, but rather apply to the repeater as a whole. Examples of conditions monitored for the repeater as a whole include the RPS FAULT, GLOBAL SECURITY, GLOBAL FAULT, and GLOBAL COLLISION conditions. 
     FIGS. 2 and 3 show a simple LED array  200  and repeater structure  300 , respectively, that allow the repeater to drive N LEDs with fewer than N control lines  205 ,  210 . This LED array  200  and repeater structure  300  are much simpler, much easier to implement, and, for relatively small LED arrays, less costly than previous solutions. 
     The depicted LED array  200 , which in many cases is a portion of a larger LED array, includes three LEDs  202 ,  204 ,  206  connected between a power supply (e.g., +3.3 volts) and ground. Three optional resistors  208 ,  210 ,  212  are included in the array  200  to limit the amount of current drawn through the LEDs. The resistance values of the resistors  208 ,  210 ,  212  depend upon several application-specific factors, including the power supply voltage and the desired maximum current draw. Resistance values on the order of 270 Ω are typical when the depicted LED array  200  is used in a 5.0 volt system, and resistance values on the order of 120 Ω are typical when the array is used in a 3.3 volt system. The power supply voltage and the number of LEDs in the array  200  also vary among applications, but in general these features are selected to ensure that the voltage drop across each LED is not large enough to cause the LED to conduct. In this example, each of the three LEDs  202 ,  204 ,  206  has a cut-in voltage of approximately 1.5 volts, so a power supply of 3.3 volts will not cause any of the diodes to conduct absent input from the control lines  205 ,  210 . 
     Larger arrays are constructed by replicating the structure of FIG.  2 . For example, the LED array  200  is replicated five times to create a 6×3 array. Only 12 control lines are needed to drive the 18 LEDs in the 6×3 array. 
     The control lines  205 ,  210  from the repeater chip  300  connect between adjacent LEDs in the LED array  200 . For example, one of the control lines  205  connects between the first LED  202  and the second LED  204 ; the other control line  210  connects between the second LED  204  and the third LED  206 . If the LED array includes the optional resistors  208 ,  210 ,  212 , each of the control lines connects to the cathode of one of the LEDs  202 ,  204 ,  206  and to one of the resistors  208 ,  210 ,  212 . 
     The repeater chip  300  includes a conventional repeater logic circuit  302  coupled to a logic block  304  that controls the operation of the LED array  200 . The array control logic  304  in turn is coupled to a pair of “tristatable” sink/source buffers  306 ,  308 , each of which drives one of the control lines  205 ,  210 . These “tristatable” sink/source buffers  306 ,  308  are configured to provide three alternative types of output: (1) a logic high value (e.g., +3.3 volts); (2) a logic low value (e.g., 0.0 volts); and (3) a high impedance output. In general, each sink/source buffer sources current to the LED array when providing a logic high output, sinks current when providing a logic low output, and neither sinks nor sources current when providing a high impedance output. 
     The array control logic  304  and the sink/source buffers  306 ,  308  operate as shown in the table of FIG.  4 . None of the LEDs illuminate when both of the sink/source buffers  306 ,  308  provide high impedance outputs. When only the first LED  202  is to illuminate, the first buffer  306  places a low logic output on the first control line  205  and the second buffer  308  places a high impedance output on the second control line  210  [output state (0, Z)]. This forces a voltage of approximately 3.3 volts across the first LED  202 , which causes the first LED  202  to conduct. The current in the first LED  202  flows from the power supply to the first sink/source buffer  306 . The high impedance output provided by the second buffer  308  insures that the second and third LEDs  204 ,  206  do not conduct and therefore do no illuminate. 
     When only the second LED  204  is to illuminate, the first buffer  306  outputs a high logic value and the second buffer  308  outputs a low logic value [output state (1, 0)]. This forces a voltage of approximately 3.3 volts across the second LED  204  and voltages of approximately 0.0 volts across the first and third LEDs  202 ,  206 . In this state, the first buffer  306  sources current to the second LED  204 , and the second buffer  308  sinks this current. The first and third LEDs  202 ,  206  do not conduct. 
     When only the third LED  206  is to illuminate, the first buffer  306  provides a high impedance output and the second buffer  308  provides a high logic output [output state (Z, 1)]. This forces a voltage of approximately 3.3 volts across the third LED  206  and a voltage of approximately 0.0 volts across the first and second LEDs  202 ,  204 . In this state, the second buffer  308  sources current through the third LED  206  to ground. The first and second LEDs  202 ,  204  do not conduct. 
     The repeater usually cycles through the various states, starting with the state in which only the first LED  202  illuminates, then shifting to the states in which only the second LED  204  and only the third LED  206  illuminate. In general, the repeater chip  300  drives the control lines  205 ,  210  at a relatively fast rate and drives the LEDs with high bursts of intensity, so that an illuminated LED appears to illuminate continuously to the human eye. 
     In some embodiments, the repeater chip  300  drives two LEDs at a time by cycling through states that otherwise would be unused. For example, the output state (Z, 0) forces voltages of approximately 1.65 volts across the first and second LEDs  202 ,  204 , causing them to conduct. The third LED  208  does not conduct in this state. Likewise, the output states (0, 1) and (1, Z) cause the first and third LEDs  202 ,  206  and the second and third LEDs  204 ,  206  to illuminate, respectively. In most cases, these states are used only to convey special information, such as at reset to show that the LEDs and control circuitry are functioning properly. 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications are possible without departing from the spirit and scope of the invention. For example, in some cases the LED array  200  includes more than three LEDs driven by more than two lines from the repeater chip. The LED array may even include as few as two LEDs driven by one line from the repeater chip if a sufficiently low supply voltage (e.g., approximately 2.8 volts or less) is present. Also, while the invention has been described in terms of a 3.3 volt power supply, some implementations use power sources greater than 3.3 volts. Other implementations use more than one power source, such as a high voltage source of 1.5 volts and a low voltage source of −1.5 volts. Some implementations use negative logic components that operate between ground and a negative voltage source, such as a −3.3 volt source. Accordingly, other embodiments are within the scope of the following claims.