Abstract:
The present disclosure relates to a power distribution unit (PDU) having at least one power receptacle for enabling attachment of an AC power cord of an external device thereto. A branch receptacle controller (BRC) has at least one bistable relay and is associated with the one power receptacle for supplying AC power thereto from an AC power source. The BRC monitors a parameter of a line voltage and uses it to detect when AC power is lost, and then toggles the bistable relay, if the relay is in a closed position, to an open position. A rack power distribution unit controller (RPDUC) monitors the bistable relay and commands the BRC to close the bistable relay after AC power is restored.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/152126 filed on Apr. 24, 2015. The entire disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to intelligent power strips with bistable relays, and more particularly to an intelligent power strip which is able to control a plurality of bistable relays in a manner to limit in-rush current as external devices being powered by the intelligent power strip are turned on. 
       BACKGROUND 
       [0003]    This section provides background information related to the present disclosure which is not necessarily prior art. 
         [0004]    Certain models of intelligent power strips use power relays, typically rated less than 250V 20 Arms, to switch a line of a receptacle for the main purpose of rebooting a connected load device, e.g., server. Depending upon the load device&#39;s internal power supply design, substantial in-rush currents may occur while its input bulk capacitors charge up the moment the relay contacts are closed. This brief, but large current surge, can permanently damage the relay contacts, i.e., weld them close, so they are no longer operative. It can also cause the upstream circuit protection device, typically a circuit breaker, to trip. Some relay manufacturers offer more expensive devices that can handle momentary current surges up to four times their design rating. To further supplement the protection of the relay contacts, the in-rush currents can be mitigated by coordinated timing of relay closure according to the voltage zero-crossing of line frequency. 
         [0005]    A number of power strips commonly referred to as Rack PDUs, have switching capabilities associated with all receptacles. PDU stands for power distribution unit and a Rack PDU is used in racks that hold electronic equipment such as servers. The primary reason for the switching capabilities has been two-fold: (a) To be able to remotely recycle power to a connected equipment that is hung up; and (b) to be able to sequentially start up all connected equipment to ensure that upstream breakers do not trip due to all connected loads drawing high in-rush currents concurrently. Typical IT (information technology) loads, for example servers, can draw as much as  5  times their normal current at the time of startup. The above capabilities have typically been addressed in the past through the use of solid state relays at each receptacle. 
         [0006]    Bistable relays are increasingly being used in Rack PDU&#39;s as they are more energy efficient since their coils do not need to remain energized to maintain the state of their contacts. In such a bistable relay, the coil is pulsed to change the state of the contacts from open to closed and vice-versa. The contacts will then remain in their existing state until the coil is pulsed again. In contrast, in a typical normally open relay, when it is desired to close the contacts of the relay, the coil of the relay must be energized and kept energized to keep the contacts closed. When the coil of the typical normally open relay is de-energized, the relay contacts revert to their normally open state. Similarly, in a typical normally closed relay, when it is desired to open the contacts of the relay, the coil of the relay must be energized and kept energized to keep the contacts open. When the coil of typical normally closed relay is de-energized, the relay contacts revert to their normally closed state. In Rack PDU&#39;s having bistable relays, the bistable relays that are closed when there is a loss of power will remain closed. When power is restored, the cumulative in-rush current through the closed bistable relays may cause the upstream circuit protection device, typically a circuit breaker, to trip. 
       SUMMARY 
       [0007]    In one aspect the present disclosure relates to a power distribution unit (PDU) comprising at least one power receptacle configured to enable attachment of an alternating current (AC) power cord of an external device to the power receptacle. A branch receptacle controller (BRC) may also be included which has at least one bistable relay and which is associated with the at least one power receptacle for supplying AC power to the at least one power receptacle from an external AC power source. The bistable relay has contacts able to be set to an open position and to a closed position. The BRC may further be configured for monitoring a parameter of a line voltage from the external AC power source, and to use the monitored parameter to detect when a loss of AC power occurs, and to toggle the bistable relay, if the bistable relay is in a closed position, to an open position upon the detection of an AC power loss condition. A rack power distribution unit controller (RPDUC) may be included which is configured to communicate with the BRC and to monitor a state of the bistable relay, and to command the BRC to selectively close the bistable relay after AC power is restored. 
         [0008]    In another aspect the present disclosure relates to a power distribution unit (PDU) which may comprise at least one power receptacle configured to enable attachment of an alternating current (AC) power cord of an external device to the power receptacle. A branch receptacle controller (BRC) may be included which has a plurality of bistable relays and which is associated with the at least one power receptacle, for supplying AC power to the at least one power receptacle from an external AC power source. Each of the bistable relays may have contacts able to be set to an open position and to a closed position. The BRC may further be configured for monitoring a frequency of a line voltage from the external AC power source, and to use the monitored frequency to detect when a loss of AC power occurs, and to toggle any one or more of the bistable relays which is a closed position to an open position upon the detection of an AC power loss condition. The BRC may further detect a loss of AC power condition by monitoring the frequency of the line voltage and determining, from information relating to a zero crossing of the monitored frequency, that an AC power loss condition has occurred. A rack power distribution unit controller (RPDUC) may also be included which is configured to communicate with the BRC and to control the bistable relays such that the relays that were previously in a closed position prior to the AC power loss condition are all sequentially commanded to again be closed after the AC Power is restored, in a manner that limits an in-rush of current to the PDU. 
         [0009]    In still another aspect the present disclosure relates to a method for monitoring and controlling an application of AC power to a plurality of data center devices. The method may comprise providing at least one AC power receptacle forming a power attachment point for an alternating current (AC) power cord of an independent data center device. The method may also comprise using a branch receptacle controller (BRC) having at least one bistable relay associated with the power receptacle for supplying AC power to the AC power receptacle from an external AC power source, the bistable relay having contacts which are able to be set to an open position and to a closed position. The BRC may be used to monitor a parameter associated with a line voltage of the external AC power source. The monitored parameter may be used to detect when a loss of AC power is about to occur. In response to a detected imminent loss of AC power, the BRC may be used to toggle the bistable relay, if the bistable relay is currently in a closed position immediately before power is lost, to an open position, before power to the BRC is lost. A rack power distribution unit controller (RPDUC) may be used which is configured to communicate with the BRC and to monitor a state of the bistable relay, and to command the BRC to close the bistable relay after AC power is restored. 
     
    
     
       DRAWINGS 
         [0010]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
           [0011]      FIG. 1  is a block diagram of one embodiment of a PDU in accordance with the present disclosure for monitoring and controlling AC power applied to each one of a plurality of AC power receptacles of the PDU; 
           [0012]      FIG. 2  is a high level block diagram of the RPDUC in accordance with one embodiment of the present disclosure; 
           [0013]      FIG. 3  is a block diagram of one embodiment of the CPLD used in the BRC; and 
           [0014]      FIG. 4  illustrates eight (8) bistable relays arranged in two sub banks (i.e., sub bank A and sub bank B). 
       
    
    
       [0015]    Corresponding reference designations indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0016]    Example embodiments will now be described more fully with reference to the accompanying drawings. 
         [0017]    In accordance with an aspect of the present disclosure, bistable relays of an intelligent power distribution strip are managed so that in-rush current is minimized upon restoration of power after a power loss. Since the relays are bistable relays, their last state is persistent regardless of power condition. Upon loss of power, the bistable relays are controlled so that all closed bistable relays are automatically opened. Loss of power can be due to loss of source AC power from an upstream power utility, failure of an uninterruptible power supply (“UPS”) upstream of the intelligent power distribution strip that provides power to the intelligent power distribution strip, or a circuit breaker tripping due to overcurrent conditions. Upon the next power up cycle, the bistable relays that are to be closed are closed sequentially with each one of such bistable relays being closed per every N line cycles to minimize overall in-rush current and prevent the upstream circuit breaker from tripping. The line frequency is monitored and detection of loss of line frequency indicates imminent power loss resulting in the bistable relays being opened. Each bistable relay that is to be closed during the next power up cycle is closed during the power up cycle at a minimum voltage (zero voltage crossing) point of line frequency to minimize contact arcing and in-rush current through its relay contacts. The bistable relays may be opened at a minimum current (zero current crossing) point of line frequency to minimize contact arcing. 
         [0018]    With reference to  FIG. 1  of the drawings, an illustrative Rack PDU in accordance with the present disclosure is described. In the following description, MPH 2   10  represents the Rack PDU, and will hereinafter simply be referred to as “MPH  10 ”. A RPC 2000   12  (hereinafter simply “RPC  12 ”) in this example may be a hot-swappable web card which is installed in the MPH  10 . The RPC  12  may include a microcontroller  14  and preferably also a non-volatile (NV) memory  16 . The RPC  12  may also include a plurality of ports including, but not limited to, a LAN Ethernet port  18 , an Expansion/Management port  20 , a port  22  for coupling to a display module (e.g., “BDM” or “Basic Display Module” available from the assignee of the present disclosure), one or more 1-wire sensor ports  24 , an RD- 232  port  26  and a USB port  28 . The MPH  10  further includes a Rack PDU Controller (RPDUC)  30  having a microcontroller  32  and a non-volatile memory  34 , and one or more branch receptacle controllers (BRC)  36 . Each BRC  36  may have a complex programmable logic device (CPLD)  38  having a voltage and current sensing subsystem  38   a  which senses of a loss of AC input power, a plurality of bistable relays  40 , and an open circuit breaker (OCB) detection subsystem  42  which senses for an open circuit breaker condition. The RPDUC  30  is in bidirectional communication with each of the BRCs  36  via a bus  44 . The RPC  12  is in bidirectional communication with the RPDUC  30  via a bus  46 . A reset switch  48 , which is easily accessible by a user via a faceplate of the MPH  10 , is provided for enabling the user to initiate a hard reset to the BRCs  36  of the system  10 A. 
         [0019]      FIG. 1  also shows a plurality of branch circuit breakers (CB)  50 . By “branch” circuit breaker it is meant that each of the CBs  50  are uniquely associated with one specific BRC  36 . The OCB detection subsystem  42  monitors the CBs  50  to detect when any one or more have been tripped to an open condition. And as explained above, each BRC  36  includes a plurality of bistable relays  40 , which in one specific embodiment comprise eight (8) bistable relays. However, it will be appreciated that a greater or lesser number of bistable relays  40  could be provided per branch. Mechanical bistable relays have coils and mechanical contacts. They can be single coil or dual coil relays. Also, more than one CB  50  may exist for each BRC  36 . For example, each BRC  36  can have its bistable relays arranged in two sub banks, with a separate CB  50  associated with each sub-bank. As used herein, each sub-bank of a BRC  36  is a branch of the BRC. 
         [0020]      FIG. 1  also shows a plurality of AC power receptacles  10   a  each having a first associated optical element  10   a   1  and a second optical element  10   a   2 . Optical elements  10   a   1  may each be an LED having a first color, for example green, that indicates a status of the specific bistable relay  40  associated with its specific AC receptacle  10   a.  The second group of optical elements  10   a   2  may also be, for example, LEDs having a different color, for example red, for providing additional information to the user. Each one of the green LEDs  10   a   1  may indicate, for example, that the bistable relay  40  associated with that specific AC receptacle  10   a  is closed, and an extinguished green LED  10   a   1  would therefore indicate that the associated bistable relay is open. Input power to the MPH  10  may be from an uninterruptible power supply (UPS) or from any other AC power source. 
         [0021]    The RPDUC  30  is shown in greater detail in  FIG. 2 . The RPDUC  30  includes a voltage sensing subsystem  52  and a current sensing subsystem  54 . The subsystems  52  and  54  perform real time RMS voltage measurements and RMS current measurements, respectively, and thus monitor the power input from the AC power source. The monitored power information may be shared with the RPC  12  via bus  44 . As noted above, the voltage and current sensing subsystem  43  of each BRC  36  also monitors for a loss of AC input power, so in this regard there is redundancy of this feature in the RPDUC  30  and the BRCs  36 . The current sensing subsystem  54  of the RPDUC  30  receives an input current signal from each of the branch BRCs (collectively labeled for simplicity in  FIG. 2  with number  36 ) which it uses to perform its current sensing function. Each branch BRC  36  also includes a plurality of current transformers (CTs)  56  for independently measuring a current being drawn by the AC receptacles  10   a  associated with each branch of bistable relays  40 . The signals from each branch CT  56  are input to the current sensing subsystem  54  for analysis. 
         [0022]    The RPC  12  shown in  FIG. 1  manages, monitors and reports information about MPH  10  energy metering and power distribution status obtained from the RPDUC  30  to networked software clients. The RPDUC  30  provides support for the energy metering measurements and calculations, control management, and communications interfaces to the RPC  12 , as described above. The RPDUC  30  communicates with each BRC  36  and, except upon power loss, controls the bistable relays  40  of each BRC by sending command messages to each BRC to independently control each one of its associated bistable relays  40 . 
         [0023]    The BRC  36 , and more particularly its CPLD  38 , directly controls its bistable relays  40 . The BRC  36  also senses individual LED receptacle operational status, and loss of an AC input power signal via line frequency monitoring performed by the voltage and current detection subsystem  38   a,  as well as using the OCB subsystem  42  to detect for an open circuit breaker condition. The bistable relays  40  of each BRC  36  in this example require a nominal 16 msec pulse to their coils to change states, that is, to open or close their contacts. A reference herein to a bistable relay being “open” means that its contacts are open and power is off or interrupted at the receptacle  10   a  to which the bistable relay switches power. As used herein, “power up”, “power down”, “power failure”, and “power cycle” refer to specific conditions of input AC line voltage, which is the AC power provided to the receptacles  10   a  through the bistable relays  40  of each BRC  36 . The term “Configured state”, when used in connection with the bistable relays  40 , means the state that a given bistable relay is configured to be in (i.e., open or closed) when power is on. 
         [0024]    The RPC  12  commands the RPDUC  14  via a SMBus (I2C) communication bus, bus  46  in  FIG. 1  in this example, which in turn, commands the BRC  36  via a SPI communication bus, which is bus  44  in this example, to configure the relay state of each bistable relay  40 . The RPDUC  30  is capable of autonomous behavior without RPC  12  commands. The one or more BRCs  36  are each capable of autonomous behavior without RPDUC  30  commands. 
         [0025]    Referring to  FIG. 3 , the CPLD  38  of one of the BRCs  36  is shown in greater detail. In this example eight bistable relays  40   1 - 40   8  are shown, but it will be appreciated that the MPH  10  may control a greater or lesser number of bistable relays  40 . The CPLD  38  includes a serial-parallel interface (“SPI”) controller  38   b  that manages communications with other subsystems of the CPLD. The CPLD  38  includes suitable logic for generating signals to independently command the bistable relays  40  to each assume a first state (“SET” signals) or a second state (“RESET” signals). The CPLD  38  also includes logic for controlling the green LEDs  10   a   1  and the red LEDs  10   a   2 . The CPLD may control the green LEDs  10   a   1  so that the green LEDs  10   a   1  flash at a first rate when a given bank of bistable relays  40  is drawing a current which is close to an upper predetermined current limit. The CPLD  38  may control the green LEDs  10   a   1  so that the green LEDs  10   a   1  flash at a second rate different from the first rate (e.g., faster rate) when an overcurrent condition arises (i.e., a given bank of bistable relays  40  is drawing more current than allowed). The CPLD  38  may control the red LEDs  10   a   2  so that all of the red LEDs  10   a   2  stay illuminated continuously if an over-current condition arises where a given bank of bistable relays  40  is drawing more current than allowed. The red LEDs  10   a   2  may also be controlled to flash or pulse if an open circuit board condition arises. 
         [0026]    In the BRCs  36 , the voltage and current sensing subsystem  38   a  of each CPLD  38  monitor loss of line frequency on load sides of the respective CB  50  for each of the BRCs  36 . Each BRC  36  allows for two sub-banks of power distribution and the AC power feed can be either same or differently phased. Each sub-bank of bistable relays  40  may optionally have its own CPLD  38  and OCB subsystem  42 .  FIG. 4  illustrates eight (8) bistable relays  40   1 - 40   8  arranged in two sub banks (i.e., sub bank A and sub bank B). 
         [0027]    Each of the one or more BRCs  36  infers imminent power loss by detecting a loss of line frequency of the AC line signal from the AC power source. Each BRC  36  monitors the line frequency and sets true loss of line frequency status after a short period during which less than the expected number of detected voltage zero crossing transitions of the AC line signal has occurred. A true loss of line frequency is defined to be when less than three (3) zero voltage transitions or zero crossings occur over a 32.768 ms interval, satisfying both 50/60 Hz operation. The zero crossing detection hardware of the BRC  36  has built-in hysteresis and digitizes the line frequency. The digitized line frequency is provided to the BRC  36  that uses digital filtering for reliable triggering. In this regard, the BRC  36  counts zero-crossing voltage transitions to make this determination. The number of transitions allows for a single worst case % cycle delay for zero crossing. 
         [0028]    The detection period for detecting loss of line frequency must be small so that the relay coil voltage of each bistable relay  40 , derived from the power supply  30   a  which is powering the entire system  10  (i.e., the RPDUC  30 , the BRC  36  and the RPC  12 ), is maintained sufficiently long enough (typically about 16 ms) for the BRC  36  to pulse the bistable relays  40  that need to be opened into the open state. At a worst case fully loaded condition, i.e., powered at 70 Vac and RPC  12  fully operational, there is approximately 64 ms of power supply hold time. 
         [0029]    The RPDUC  30  has a similar capability to monitor the loss of line frequency on a line side of the CB  50 , as indicated by dashed line  47  in  FIG. 1 . For monitoring redundancy, its loss of line frequency status may be commanded or indicated by dedicated signals via the SPI communication bus  44  going to each BRC  36 , so that each BRC  36  may monitor and compare its own loss of line status, to the status being reported by the RPDUC  30 , before any action is taken by the BRC  36 . Also, the RPDUC  14  may assert a dedicated signal going to the RPC  12  to cause it to enter a low power operating mode in order to extend the power supply hold time. 
         [0030]    The BRC  36  CPLD  38  doesn&#39;t distinguish between power loss due to loss of line power or due to CB  50  open conditions. Therefore, upon power loss, the BRC  36  controls all the bistable relays  40  in the affected sub-bank so that their contacts are switched to (or left in) the open condition. That is, upon power loss, the BRC  36  opens the bistable relays  40  that are closed and leaves open the bistable relays that are open. 
         [0031]    A commanded receptacle  10   a  state overrides autonomous power-up state behavior. That is, if during a power-up cycle a power-up delay for a receptacle  10   a  is pending due to the sequencing of closing the bistable relays  40  that are to be closed, a separate command to power on a receptacle results in immediate processing closure of the bistable relay  40  for that receptacle. 
         [0032]    At initial system startup, all CBs  50  are manually tripped by a user to the open state before power is applied. This results in the RPDUC  30 , upon power-up, autonomously commanding the BRCs  36  to control all the bistable relays  40  to be open immediately to mitigate in-rush currents. Afterwards, the RPDUC  30  queries each BRC  36  to confirm that all of its bistable relays  40  are open and, if they are, alerts a user that the CBs  50  for that BRC  36  may then be closed. The LED  10   a   1  associated with each receptacle  10   a  is on (illuminated) when the bistable relay  40  for that receptacle is then closed, and is off when the bistable relay for that receptacle is open. Although the bistable relays  40  would typically be set to the default “open” position at manufacturing time, the occurrence of excessive shock or vibration during transportation and/or installation may cause a change in state. 
         [0033]    If for the same BRC  36  one CB  50  is detected to be closed at line power loss (true loss of line frequency), all bistable relays  40  are set to their configured receptacle  10   a  power up state by the RPDUC  30 . That is, the bistable relays  40  that are in a closed state at line power loss are set to be re-closed upon power up, and the bistable relays that are in an open state are set to remain open upon power up. 
         [0034]    If for the same BRC  36 , both CBs  50  are detected open at line power loss (true loss of line frequency), all the bistable relays  40  are controlled by the RPDUC  30  so that all of these bistable relays  40  remain open at power up until the CBs  50  are closed. Upon the CBs  50  being closed, the RPDUC  30  proceeds as discussed above during initial system start up. Then upon confirming that all the bistable relays  40  of a BRC  36  are open, the RPDUC  30  then proceeds to command the BRC to close the bistable relays  40  that are to be closed, which the RPDUC may do sequentially as discussed below. 
         [0035]    If the power supply of the RPDUC  30  fails, the RPDUC and the BRCs  36  no longer operate; however, the bistable relays  40  remain in their last configured states, even during subsequent power cycle(s). In this aspect, the power supply of the RPDUC  30  provides power to the BRC  36 . 
         [0036]    The current bistable relay  40  states are immediately updated in the volatile register memory (not shown) of the BRC  36  when configured by the RPDUC  30  and/or when autonomously changed by the BRC  36 , and the volatile register memory can be read by the RPDUC  30  from each BRC  36 . The RPDUC  30  then updates the states for those bistable relays  22  stored in non-volatile memory  34  of the RPDUC. 
         [0037]    Except in the event of a power loss where all the closed bistable relays of each affected sub bank of each affected BRC  36  are opened, only a single bistable relay state per branch of a BRC is permitted to change per N line cycles to mitigate in-rush currents and prevent the CB  50  associated with that particular branch from unexpectedly opening or tripping. For example, during a power up cycle of an affected branch of a BRC  36 , the RPDUC  30  determines which bistable relays  40  of that affected branch are to be closed. It then sequentially sends commands to the BRC  36  to close those bistable relays  40 , one command for a different bistable relay every N line cycles. That is, the RPDUC  30  sends the BRC  36  a command to close one of the bistable relays  40  that are to be closed every N line cycles. This results in one such bistable relay  40  being closed every N line cycles. It should be understood that the non-volatile memory  34  of the RPDUC  30  is used by the RPDUC to store the real time configured states of all the bistable relays  40  of all the BRCs  36 . The RPDUC  30  then determines which bistable relays of an affected branch of an affected BRC  36  are to be closed during a power-up cycle based on the stored configured states. 
         [0038]    The RPDUC  30  may have an all-digital phase-locked loop implemented in firmware. The RPDUC  30  may operate to lock onto the line frequency and precisely coordinate analog-to-digital conversion processes for voltage and current measurements, and to send commands to the BRCs  36  in a deterministic fashion prior to voltage zero-crossing of line frequency. The RPDUC  30  may also command each BRC  36  to close its associated bistable relays  40  according to a synchronized timing to a minimum voltage, which will be at the zero cross of line frequency, to mitigate in-rush current. The RPDUC  30  may synchronize to both line-neutral and line-line voltages. The RPDUC  30  commands each BRC  36  to open its associated bistable relays  40  according to synchronized timing to minimum current zero crossing of line frequency to minimize contact arcing. 
         [0039]    The open and close timings of the bistable relays  40  may be measured during manufacturing functional testing and saved to non-volatile memory, such as the non-volatile memory  34  of the RPDUC  30 . It should be understood that the BRC  36  may have non-volatile memory as well, and that these timings could alternatively or additionally be saved to the non-volatile memory of the BRC and retrieved by the RPDUC  30  as needed. It will also be understood that the bistable relays  40  have an inherent delayed response until release/open states are achieved because of the coils&#39; operate/release times. These timing values are used by the RPDUC  30  to compensate the command execution timing to better synchronize the actual open/close states according to arrival of the voltage and current zero crossing states. For example, if a particular bistable relay  40  was measured to have a 9 msec close time, when the RPDUC  30  is sending a command to the BRC  36  having that bistable relay  40 , to cause it to close, the RPDUC does so 9 msec before the next zero voltage line crossing point. 
         [0040]    The RPDUC  30  may also compensate for relay contact bounce by commanding the closure state ˜1 msec earlier so that a typical 1-2 msec contact bounce occurs around line voltage zero-crossing point. In the foregoing example of the bistable relay  40  having a 9 msec closure time, the RPDUC  14  then sends the command to the BRC  36  to close the bistable relay 10 msec in advance of the next line voltage zero-crossing point. 
         [0041]    The RPC  12  may also report an abnormal operating condition when a bistable relay  40  is commanded to be opened but current is still measured flowing through it. As shown in  FIG. 2 , each receptacle  10   a  has one of the current transformers  56  associated with it that is used to measure current being drawn by the receptacle. This abnormal condition may result from failed or stuck closed relay contacts. The BRC  36  communicates the currents being measured by its current transformers  56  to the RPDUC  30  which then determines whether such an abnormal operating condition exists. If one does exist, the RPDUC  30  communicates this to the RPC  12 . 
         [0042]    As shown in the drawings, each of the RPC  12 , RPDUC  30  and the BRC  36  include the CPLD  38 , the microcontroller  32  and the microcontroller  14 , respectively, which each include appropriate logic for implementing the above described logic functions. It should be understood that other types of devices can be used such as a digital processor (DSP), microprocessor, microcontroller, Field Programmable Gate Array (FPGA), or application specific integrated circuit (ASIC). When it is stated that RPC  12 , RPDUC  30  or BRC  36  have logic for a function, it should be understood that such logic can include hardware, software, or a combination thereof, including the logic implemented in the CPLD. 
         [0043]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.