Abstract:
A circuit and a method are presented which allow for the detection of an active communication device connected to the circuit. The circuit output signal can be used to reduce the power consumption of related equipment when communication is not detected for a predetermined period of time. The circuit includes a plurality of input terminals coupled to respective inactive detection blocks. The circuit also includes an accumulated delay block having delay circuits in serial electrical communication. Each inactive detection block provides a signal to a respective delay circuit that indicates whether the corresponding input terminal is connected to an active driver in the communication device. Each delay circuit provides a delayed output signal to a subsequent delay circuit. The accumulated delay block provides an output signal indicating the presence or absence of the communication device based on the activity at the input terminals over the predetermined period of time.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional U.S. patent application, Ser. No. 60/107,881, filed on Nov. 9, 1998, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of connection detection circuits and more specifically to circuits which detect when a connection detection device is connected to an active communication device over a communication line. 
     BACKGROUND OF THE INVENTION 
     Modern portable electronic equipment put high demands for power on portable power sources such as batteries. To reduce the power consumption of such equipment, circuits have been designed which detect when different features of the equipment are not required and power down those features. 
     The present invention relates to a connection detection circuit which detects when the connection detection circuit, which for example may be incorporated as part of an RS232 receiver circuit, is connected to another communication device and which sets the state of a control line in response thereto. 
     SUMMARY OF THE INVENTION 
     A circuit and a method are presented which allow for the detection of an active communication device connected to the circuit, as distinguished from an inactive communication device connected to the circuit or the absence of a communication device connected to the circuit. The invention relates to a method of determining the presence of an active driver in communication with a receiver input terminal. The method includes the steps of receiving a signal and generating an inactive detection signal in response to the signal. The method also includes the steps of switching either a first voltage or a second voltage onto a conductor in response to the inactive detection signal, delaying the propagation of the switched voltage, and producing the delayed switched voltage as an output signal. In one embodiment, the step of switching either a first or a second voltage includes activating, in response to the inactive detection signal, either a first switch to apply the first voltage or a second switch to apply the second voltage. 
     The invention further relates to a circuit for determining the presence of an active driver in communication with a receiver which has an input terminal and an output terminal. The circuit includes an inactive detection subcircuit which itself includes a first detection input terminal in electrical communication with the output terminal of the receiver, a second detection input terminal which can receive a reference voltage, and a detection output terminal. The circuit further includes a delay stage, which has a delay stage input terminal in electrical communication with the detection output terminal, and a delay stage output terminal which is the output terminal of the circuit. The delay stage generates a delay stage output signal that is presented at the delay stage output terminal in response to a signal from the driver which is received at the input terminal of the receiver. 
     The invention additionally relates to a circuit for determining the presence of at least one active driver of a plurality of drivers which are in communication with a respective one of a plurality of input terminals. Each of the input terminals is capable of receiving one of a plurality of input signals indicative of an active driver in electrical communication with a respective one of a plurality of receivers. The invention additionally includes a plurality of delay stages in serial electrical communication. Each of the delay stages has a delay stage input terminal in electrical communication with a respective one of the input terminals, and each delay stage has a delay stage output terminal. The circuit has an output terminal in electrical communication with the delay stage output terminal of the last one of the plurality of serially electrically communicating delay stages. The last one of the plurality of delay stage generates a circuit output signal which is presented at the circuit output terminal which is indicative of at least an active one of the plurality of drivers in response to signals from the drivers received at the plurality of receivers. 
     The invention still further relates to a circuit for determining the presence of at least one active driver which includes a first delay stage that includes a first signal input terminal which can receive a signal indicating the presence of an active driver in electrical communication with a first receiver and having a first output terminal, such that the first delay stage generates a first output signal at the first output terminal in response to the first input signal. The circuit also includes a last delay stage that includes a last signal input terminal which can receive a signal that can indicate the presence of an active driver in electrical communication with a last receiver, a last accumulated input terminal in electrical communication with the first output terminal of the first delay stage, and having a last output terminal. The last delay stage generates a last output signal at the last output terminal which indicates the presence of at least one active driver in electrical communication with a respective one of the receivers. In another embodiment, the circuit further contains at least one interim delay stage which has an interim signal input that receives an interim input signal indicative of the presence of an active driver in communication with an interim receiver, an interim accumulated input terminal in electrical communication with the output terminal of the first delay stage, and an interim output terminal, such that the interim output signal which is presented at the interim output terminal is responsive to the interim input signal and the signal received at the interim accumulated input terminal. In yet another embodiment, an inverter is in electrical communication with the last output terminal, such that the last output terminal and the inverter provide first and second output signals that are respectively logically complementary. 
     The invention yet additionally relates to a circuit for determining the presence of at least one active driver, including a plurality of receivers which can each respectively receive one of a plurality of input signals, and a plurality of inactive detection blocks, each inactive detection block in electrical communication with a respective one of the plurality of receivers and having an output terminal. The circuit further includes a plurality of delay stages, each of which is in electrical communication with a respective one of the inactive detection blocks, and each one of which is also in serial electrical communication with another of the plurality of delay stages. Each of the plurality of inactive detection blocks generates an inactive detection signal which appears at its output terminal in response to its respective input signal, and which is communicated to a respective one of the delay stages, such that a last delay stage generates a last output signal which is presented at the last output terminal that is indicative of the presence of at least one active driver in electrical communication with a respective one of the receivers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. The advantages of the invention described above, as well as further advantages of the invention, may be better understood by reference to the following description taken in conjunction with the accompanying drawings, in which: 
     FIGS. 1A and 1B form a block diagram of a receive portion of an RS232 device incorporating an embodiment of the connection detection circuit of the invention; 
     FIG. 2 is a block diagram of another embodiment of the accumulated delay circuit shown in FIGS. 1A and 1B; 
     FIG. 3 is a block diagram of an embodiment of an RS232 device constructed in accordance with the invention; 
     FIG. 4 is a block diagram of an embodiment of the online circuit shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In brief overview and referring to FIGS. 1A and 1B, an embodiment of the connection detection circuit  8  of the invention which detects when an active communications device, such as an RS232 transmitter, is connected to it and which sets the state of an Inactive control line in response thereto is shown. In the embodiment shown, the detection connection circuit  8  is incorporated in an RS232 communications device having five receiver input terminals  10 ,  12 ,  14 ,  16 ,  18 . Each of the input terminals  10 ,  12 ,  14 ,  16 ,  18  is capable of providing an input signal, received from an RS232 communications driver, to a respective receiver circuit  20 ,  22 ,  24 ,  26 ,  28  and a respective inactive detection circuit  30 ,  32 ,  34 ,  36 ,  38 . An output terminal  40 ,  42 ,  44 ,  46 ,  48  of each receiver circuit  20 ,  22 ,  24 ,  26 ,  28  is a respective output terminal  40 ,  42 ,  44 ,  46 ,  48  of the RS232 communications device. The output terminals  40 ,  42 ,  44 ,  46 ,  48  pass the corresponding input signals, received from the RS232 communications driver, as output signals of the RS232 communications device. An output terminal  50 ,  52 ,  54 ,  56 ,  58  of each inactive detection circuit  30 ,  32 ,  34 ,  36 ,  38  provides the input signal to a corresponding input terminal  60 ,  62 ,  64 ,  66 ,  68  of a accumulated delay circuit  70 . 
     An output terminal  72  of the accumulated delay circuit  70  is the output terminal of the connection detection circuit  8 , upon which appears an inactive control signal. When an active RS232 communications driver is connected to the input terminals  10 ,  12 ,  14 ,  16 ,  18  of the connection detection circuit, the connection detection circuit  8  of the invention places a first predetermined voltage, as described below, onto its output terminal  72 . When no RS232 communications driver is detected, the connection detection circuit places a second predetermined voltage on its output line  72 . 
     Considering one representative receiver circuit  20  and inactive detection circuit  30 , of the connection detection device, the input terminal  10  is connected to ground through a resistor  74  in the receiver circuit  20 , which in one embodiment is 4.5 K ohms. The input terminal  10  is also electrically connected to a first input terminal  76  of the inactive detection circuit  30 , and the input of a Schmitt trigger  78 , whose output terminal  79  is connected to the input terminal  81  of an inverter  80 . The output terminal  82  of the inverter  80  is connected to a second input terminal  84  of the inactive detection circuit  30  and the input terminal  86  of a second inverter  88 . The output terminal  89  of the second inverter  88  is the output terminal of the device  40 . 
     The first input terminal  76  of the inactive detection block  30  is in electrical connection to one terminal  90  of a FET  94 . The gate  96  of the FET  94  is connected to ground and a third terminal  98  of FET  94  is connected to both the first terminal  100  and the gate  104  of a FET  108 . A third terminal  110  of FET  108  is electrically connected to supply voltage V cc . The common node  114  of FETs  96  and  108  is connected to the input terminal  120  of inverter  124 . The output terminal  126  of inverter  124  is connected to one input terminal  128  of a NOR gate  130 . The other input terminal  132  of NOR gate  130  is the second input terminal  84  of the inactive detection circuit  30 . The output terminal  134  of NOR gate  130  is the output terminal  50  of the inactive detection circuit  30 . The logic which generally is embodied in a NOR gate operating under what is called positive logic is that an output of high or logic 1 is produced only when both inputs to the NOR gate are held at low or logic 0, and the NOR gate produces an output of low or logic 0 if either or both of its inputs are held at high or logic 1. One obtains the rules under negative logic by inverting all output values in the truth table of the NOR gate. 
     The output terminal  50  of the inactive detection circuit  30  is electrically connected to one input terminal  60  of the accumulated delay circuit  70 . This input terminal  60  is the input terminal to one stage  140  of the accumulated delay circuit  70 . There is at least one stage  140 ,  142 ,  144 ,  146 ,  148  in the accumulated delay circuit  70  for each receiver circuit  20 ,  22 ,  24 ,  26 ,  28 . Each stage  140 ,  142 ,  144 ,  146   148  includes at least one PMOS FET  150 ,  152 ,  154 ,  156 ,  158  and at least one NMOS FET  160 ,  162 ,  164 ,  166 ,  168  and a delay capacitor  170 ,  172 ,  174 ,  176 ,  178 . Each input terminal  60 ,  62 ,  64 ,  66 ,  68  is electrically connected to the gate  180 ,  182 ,  184 ,  186 ,  188  respectively of the PMOS FET  150 ,  152 ,  154 ,  156 ,  158  and the gate  190 ,  192 ,  194 ,  196   198  respectively of the NMOS FET  160 ,  162 ,  164 ,  166 ,  168  of its respective stage  140 ,  142 ,  144 ,  146 ,  148 . 
     The first terminal  200 ,  202 ,  204 ,  206 ,  208  of each respective PMOS FET  180 ,  182 ,  184 ,  186 ,  188  of each respective stage  140 ,  142 ,  144 ,  146 ,  148  is connected to supply voltage V cc . The first terminal  210  of NMOS FET  160  is connected to ground. The second terminal  212  of PMOS FET  150  and the second terminal  214  of NMOS FET  160  are electrically connected to each other and one terminal of capacitor  170 . The second terminal of capacitor  170  is connected to ground. The first terminal of capacitor  170  is also electrically connected to the first input terminal  220  of the NMOS FET  162  of the next stage  142 . The second terminal  222  of the NMOS FET  162  of the second stage  142  is electrically connected to the second terminal  226  of the PMOS FET  152  of the second stage  142 , the first terminal of capacitor  172  and the first terminal  230  of NMOS FET  164  of the next stage  144 . Again, the second terminal of capacitor  172  is connected to ground. This pattern in which the second terminals of the PMOS and NMOS FETs of one stage are electrically connected to one terminal of the capacitor of that stage and also are electrically connected to the first terminal of NMOS FET of the succeeding stage is repeated for each stage of the accumulated delay circuit  70  except the last stage  148 . 
     The common connection of the second terminal  260  of PMOS FET  158 , the second terminal  262  of NMOS FET  168 , and the first terminal  264  of capacitor  178  of the last stage  148  is connected to the input terminal  270  of inverter  274 . The output terminal  278  of inverter  274  is connected to the input terminal  280  of inverter  284  and the output terminal  288  of inverter  284  is the output terminal  72  of the accumulated delay circuit  70 . 
     In describing the operation of the device only one representative receiver  20  will be considered for simplicity. Three possible input conditions are herein discussed. The first is the absence of a drive signal, or the presence a noise signal of small magnitude, such as a few millivolt signal. The second is the presence of a drive signal at high or logic 1, which in one embodiment may be of the order of hundreds of millivolts or more above a reference voltage such as ground. The third is the presence of a drive signal at low or logic 0, which in one embodiment may be of the order of hundreds of millivolts or more below a reference voltage such as ground. As will be recognized by those of ordinary skill in the art, these voltages relate simply to one embodiment, and other voltage ranges, which may or may not be symmetrically disposed about a reference voltage such as ground, can equally well be dealt with by a circuit which is another embodiment by the invention. 
     In the first input condition, when there is no driver connected to input terminal  10 , or when a driver is connected to input terminal  10  but is inactive or is floating, the voltage on the input terminal  10  is brought to ground by resistor  74 . This is therefore a low or logic 0 input signal to the Schmitt trigger  78  whose output is therefore high or logic 1. This signal is inverted to low or logic 0 by inverter  80  and again to high or logic 1 by inverter  88 . The high or logic 1 output signal of inverter  88  is presented on the output terminal  40 . 
     The inactive, noise or ground signal on the input terminal  10  which is forced to ground by resistor  74  is also applied to the first terminal  90  of FET  94 , which in con unction with the grounded gate terminal  96  causes FET  94  to be non-conductive. The second terminal  100  and gate  104  of FET  108  being electrically connected in conjunction with the first terminal  110  of FET  108  being electrically connected to supply voltage V cc , turns FET  108  on and brings node  114  high or logic 1. This high signal applied to the input terminal  120  of inverter  124  results in a low or logic 0 output signal being applied to one input terminal  128  of NOR gate  130 . The second input terminal  132  of the NOR gate  130  is connected to the output terminal  82  of inverter  80  of the receiver circuit  20  and is also low or logic 0 as described above. The output terminal  134  of NOR gate  130  is therefore high or logic 1. This voltage level is the inactive signal which is placed on the output terminal  50  of the inactive detection circuit  30 . This high or logic 1 signal is applied to the gates  180  and  190  of FETs  150 ,  160 , respectively, of the first stage  140  of the accumulated delay circuit  70 . 
     When the second input condition, namely an input signal of high or logic 1, which in this embodiment comprises a signal of several hundred millivolts or more above ground, is applied to input terminal  10 , the voltage on the input terminal  10  is not brought to ground by resistor  74  so long as the driver can source or supply sufficient current to sustain the voltage input signal across resistor  74 . There is therefore a high or logic 1 input signal to the Schmitt trigger  78  whose output is therefore low or logic 0. This signal is inverted to high or logic 1 by inverter  80 , which appears at terminal  82  and is communicated to in put terminal  132  of the NOR gate  130 . This high or logic 1 is converted again to low or logic 0 by inverter  88 . The low or logic 0 output signal of inverter  88  is presented on the output terminal  40 . However, since NOR gate  130  has one input which is high or logic 1, it is irrelevant what signal is applied to the other input, because the rules of operation of the NOR gate require that its output be a low or logic 0 in any case. This low or logic 0 NOR gate  130  output voltage level indicates an active signal at input terminal  10 , and is placed on the output terminal  50  of the inactive detection circuit  30 . 
     When the third input condition, namely an input signal of low or logic 0 is applied to input terminal  10 , the voltage on the input terminal  10  is not brought to ground by resistor  74  so long as the driver can source or supply sufficient current to sustain the voltage input signal across resistor  74 . In this embodiment, there is therefore a low or logic 0 input signal of a value which is typically many hundred millivolts or more below reference ground which is applied via first input terminal  76  of the inactive detection circuit  30  to the first terminal  90  of FET  94 . This signal, in conjunction with the grounded gate terminal  96  causes FET  94  to be conductive. This draws node  114  to a low voltage. Second terminal  100  and gate  104  of FET  108  are electrically connected to node  114 . FET  108  therefore is turned off. The low or logic 0 signal at node  114  is applied to the input terminal  120  of inverter  124  which results in a high or logic 1 output signal being applied to input terminal  128  of NOR gate  130 . Once again, because NOR gate  130  has one input which is high or logic 1 its output be a low or logic 0. This low or logic 0 NOR gate  130  output voltage level indicates an active signal at input terminal  10 , and is placed on the output terminal  50  of the inactive detection circuit  30 . 
     The presence of a high or logic 1 signal on the gate  180  of PMOS FET  150  from the output terminal  50  of the inactive detection circuit  30  turns PMOS FET  150  off. The presence of the high or logic 1 signal on the gate  190  of NMOS FET  160  turns NMOS PET  160  on, thereby applying a low or logic 0 signal to one terminal of capacitor  170  and to the first terminal  220  of NMOS PET  162  of stage  142 . 
     For the rest of the discussion it is assumed that all the input terminals  60 ,  62 ,  64 ,  66 ,  68  of the accumulated delay circuit  70  are high or logic 1 indicating that there are no active drivers connected to any of input terminals  10 ,  12 ,  14 ,  16 ,  18  and hence all input terminals  10 ,  12 ,  14 ,  16 ,  18  are at ground due to resistor  74  and its equivalents in each receiver  22 ,  24 ,  26 ,  28 . Because input terminal  62  is high, the gates  182  and  192  of the PMOS PET  152  and NMOS PET  162 , respectively, are high and PMOS PET  152  is off and NMOS PET  162  is on in the next stage  142 . Because of the high or logic 1 value applied to each input terminal  60 ,  62 ,  64 ,  66 ,  68  of the accumulated delay circuit  70 , the PMOS FETs  154 ,  156 ,  158  will be off and the NMOS FETs  164 ,  166 ,  168  will be on for each subsequent stage  144 ,  146 ,  148 . 
     Because a low or logic 0 signal is applied to the first terminal  220  of NMOS FET  162  of stage  142 , the low or logic 0 signal will be propagated to each first terminal  230 ,  232 ,  234  of each respective NMOS FET  164 ,  166 ,  168  of each respective stage  144 ,  146 ,  148  and to the input terminal  270  of inverter  274 . Inverter  274  inverts the signal thereby applying a high or logic 1 signal to the input terminal  280  of inverter  284  and causing the output terminal  72  of the device to be low or logic 0. 
     If conversely one input terminal, for example input terminal  16 , were connected to an active driver, then the output terminal  56  of the inactive detection circuit  36  which is connected to the input terminal  66  of the accumulated delay circuit  70  would be low or logic 0. This signal applied to gates  186  and  196  of PMOS FET  156  and NMOS FET  166 , respectively, of stage  146  will cause NMOS FET  166  to turn off and PMOS FET  156  to turn on, thereby applying V cc  to capacitor  176  and first terminal  234  of NMOS FET  168  of the next stage  148 . Capacitor  176  will therefore charge with a characteristic time constant, delaying the propagation of the high or logic 1 signal to the NMOS FET  168 . 
     The presence of a high or logic 1 signal on the gate  198  of the NMOS FET  168  turns it on thereby applying the V cc  or logic 1 which is on terminal  234  to capacitor  178  and the input terminal  270  of inverter  274 . As the capacitor  178  charges, the application of the V cc  to input terminal  270  is also delayed. 
     The high or logic 1 input applied to the inverter  274  is inverted to a low or logic 0 signal which in turn is applied to the input terminal  280  of inverter  284 . Inverter  284  inverts this signal to high or logic 1 which is then the output signal appearing on device output terminal  72 , indicating that at least one active driver is connected to the receivers  20 ,  22 ,  24 ,  26 ,  28  of the device  8 . 
     Referring to FIG. 2, another embodiment of the accumulated delay circuit  70  of the invention is shown which includes an additional PMOS FETs  300 ,  304 ,  308 ,  312  associated with each stage  140 ,  142 ,  144 ,  146 , but the last stage  148 , respectively. In this embodiment, the input terminals  60 ,  62 ,  64 ,  66 ,  68  of the accumulator delay circuit are connected not only to the gates of each stage  140 ,  142 ,  144 ,  146 ,  148  but also to the gates  320 ,  324 ,  328 ,  332  of the PMOS FETs  300 ,  304 ,  308 ,  312 . The first terminal  340 ,  344 ,  348 ,  352  of each PMOS PET  300 ,  304 ,  308 ,  312  respectively, is electrically connected to the first terminal of each capacitor  170 ,  172 ,  174 ,  176 , respectively. The second terminal  320 ,  324 ,  328 ,  332  of each PMOS FET  300 ,  304 ,  308 ,  312  respectively, is connected to node  380  at the first terminal of capacitor  178 &#39; shown in this embodiment as two capacitors in parallel. In this configuration, when any of the input terminals  60 ,  62 ,  64 ,  66  are low or logic 0, the corresponding PMOS FET  300 ,  304 ,  308 ,  312  turns on reducing the delay caused by the corresponding capacitor stages  170 ,  172 ,  174 ,  176 ,  178 &#39; and allowing the connection of a driver to the input terminals  10 ,  12 ,  14 ,  16 ,  18  of the device  8  to be quickly detected. 
     Referring to FIG. 3, a complete RS232 communication device is shown which is constructed in accordance with the invention. The device includes five receiver units  400 ,  404 ,  408 ,  412 ,  416  and three driver circuits  420 ,  424 ,  426 . Each receiver unit  400 ,  404 ,  408 ,  412 ,  416  includes a respective one receiver circuit  20 ,  22 ,  24 ,  26 ,  28  and a respective one inactive detection circuit  30 ,  32 ,  34 ,  36 ,  38  (FIGS.  1 A and B). The input terminals  10 ,  12 ,  14 ,  16 ,  18  of the receiver units  400 , 404 , 408 , 412 , 416  are the input terminals of the receiver circuits  20 ,  22 ,  24 ,  26 ,  28 . Similarly the output terminals  40 ,  42 ′,  44 ,  46 ,  48  are the output terminals of the receiver circuits  20 ,  22 ,  24 ,  26 ,  28 . The inactive detection output terminals  50 ,  52 ,  54 ,  56 ,  58  are the output terminals of the inactive detection circuits  30 ,  32 ,  34 ,  36 ,  38 . 
     These inactive detection output terminals  50 ,  52 ,  54 ,  56 ,  58  are connected to the input terminals  60 ,  62 ,  64 ,  66 ,  68  of the online device  440  which are also the input terminals to the accumulated delay circuit  70  contained within the online device  440 . The output terminal  72  of the accumulated delay circuit  70  is the output terminal of the ONLINE circuit  440 . This output terminal  72  is the input to an inverter  444  which inverts the output signal of the online circuit  440 . ENABLE-232 (EN232)  450  and NOT-ENABLE-232 (EN232 bar)  454 , as described below, are also output terminals of the ONLINE circuit  440  as is the PUMP-SHUTDOWN line (PUMPSD)  458 . The ENABLE-232 (BN232)  450  and NOT-ENABLE-232 (EN232 bar)  454  terminals provide input signals to a level shifter  480  whose output signal placed on output terminal  490  controls the state of the drivers  420 ,  424 ,  426 . Three additional control lines NOT-SHUTDOWN (SHUTDOWN bar)  460 , NOT-ONLINE (ONLINE bar)  464 , and DRIVER-SHUTDOWN (DRSD)  468 , as described below also control the operation of the ONLINE circuit  440 . 
     The components of the online device  440  shown in FIG. 3 in addition to the accumulated delay circuit  70  are shown in FIG.  4 . The output termninal  72  of the accumulated delay circuit  70  provides one input signal to a NOR gate  500 . The other input signal to the NOR gate  500  is provided by the control line NOT-SHUTDOWN (SHUTDOWN bar)  460 . When NOT-SHUTDOWN (SHUTDOWN bar)  460  is high it prevents the state of the output line  72  of the accumulated delay circuit  70  from propagating and having any effect. That is, the NOT-SHUTDOWN (SHUTDOWN bar)  460  terminal when set high causes the output of the accumulated delay circuit to be ignored or overridden. 
     The output signal of the NOR gate  500  applied to the output terminal  504  is inverted by inverter  510  and the output signal of the inverter is one input signal to a NAND gate  514 . The other input terminal of the NAND gate  514  is provided by control line NOT-ONLINE (ONLINE bar)  464 . The state of NOT-ONLINE (ONLINE bar)  464  therefore provides a second control signal which determines whether the inactive output signal provided by the accumulated delay circuit  70  is propagated on output line PUMP-SHUTDOWN (PUMPSD)  458 . 
     The output signal from NAND gate  514  is one input signal to a second NOR gate  520 . The second input signal to the NOR gate  520  is provided by control line DRIVER-SHUTDOWN (DRSD)  468 . The output signal from NOR gate  520  is provided on output terminal ENABLE-232 (EN232)  450  and is inverted by inverter  530  and placed on output terminal NOT-ENABLE-232 (EN232 bar)  454 . Thus the state of the DRIVER-SHUTDOWN terminal (DRSD)  468  determines, in part, the state of the ENABLE-232 (EN232)  450  and NOT-ENABLE-232 (EN232 bar)  454  and thus provides a way to shut down the drivers  420 ,  424 ,  426 . 
     Variations, modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the invention is to be defined not by the preceding illustrative description but instead by the spirit and scope of the following claims.