Abstract:
An apparatus for reducing electromagnetic interference in an electronic system, comprises a switch coupled to a conductive line, and a system management device that can be coupled to the electronic system. The system management device detects whether a device is connected in a particular location in the system, and opens the switch to disable data transmission a long the conductive line to the particular location when the device is not connected. Noise signals are thus prevented from being propagated on transmission lines that are not terminated, and EMI that can be generated by signal reflections on the unterminated conductive line is substantially reduced, if not eliminated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to devices for reducing electromagnetic interference and specifically for devices for reducing electromagnetic interference in an electronic system by controlling transmissions to one or more conductive lines in electronic systems and networks.  
           [0003]    2. Relevant Background  
           [0004]    Whenever an electric charge is accelerated, electromagnetic waves are generated. Typical electric and magnetic fields in electronic circuits are generated by current pulses propagating along a path or a loop within the circuit. Each current pulse that propagates along the path creates a magnetic field perpendicular to the plane of the current path. The resulting voltage drop along the path creates an electric field opposite to the propagation direction and within the same current plane. Most common current paths within a personal computer consist of I/O cables, printed circuit board (PCB) signal traces, power supply cables, and power-to-ground loops. These paths can act as antennae, radiating electric and magnetic fields that cause EMI by interacting with other signals. The magnitude of EMI is a function of several characteristics of the transmitted signal, such as its frequency, duty cycle, edge rate, and voltage swing (amplitude). This EMI may result in erroneous transmission of data, lost data, or a reduction in the amount of acceptable noise for that system.  
           [0005]    As the computer market evolves, increasingly higher-speed data processing and transmission technologies are being developed. Electronic components and circuits, such as microprocessors, operate at increasingly higher frequencies and lower voltages and are increasingly more susceptible to electromagnetic interference (EMI). Unfortunately, nearly any computer system has the potential for causing EMI during operation.  
           [0006]    Another source of EMI, aside from I/O cables, PCB signal traces, power supply cables, and power-to-ground loops, can arise when high-speed data is transmitted to the pins of an unterminated connector. In this situation, the open pins act as small antennae that radiate the transmitted signals. These open pins have been observed to generate up to 10 decibels or more of EMI. The EMI can interfere with other components within the computer system as well as other susceptible electronic systems that may be nearby. Thus, whether the open pins reside within or outside of a computer system housing, it is desirable, and in some situations necessary, to reduce these emissions to acceptable levels.  
           [0007]    In the prior art, various techniques are recommended to reduce EMI in data transmission lines. See “Characteristics and Measurement Techniques of the Spectral Content of Signals Generated by High-Performance ICs”, Fairchild Semiconductor Application Note, June 1992 (AN-831), revised November 1999 (AN010998). One technique known as the parallel termination scheme matches the effective impedance of the transmission line with a resistor coupled in parallel. Another technique known as the series termination scheme places a resistor in series with the output driver and the transmission line. The resistor value is selected such that when added to the integrated circuit (IC) output resistance, the total equals the effective impedance of the transmission line. This effectively forms a voltage divider with the transmission line producing a half-voltage level at the source which doubles upon reflection at the end of the line. These techniques are applicable to distributed or point-to-point data transmissions, respectively, but do not address the issue of open connector portions at the end of the transmission medium.  
           [0008]    Similarly, other components such as ferrite cores and beads, feedthrough capacitors, connector shields, gaskets, and conductive tapes can all prevent unwanted EMI signals, as known in the art. These techniques are not suitable, however, for use on connector pins because the components would interfere with mating the pins to a corresponding female connector. It is therefore desirable to provide a device for reducing, and even eliminating, EMI propagated by signals being transmitted to unterminated connectors.  
         SUMMARY  
         [0009]    In one embodiment, an apparatus for controlling transmissions to reduce electromagnetic interference in an electronic system comprises a switch coupled to a conductive line, and a system management device that can be coupled to the electronic system. The system management device detects whether a device is connected in a particular location in the system, and opens the switch to disable data transmission along the conductive line to the particular location when the device is not connected. Noise signals are thus prevented from being propagated on transmission lines that are not terminated, and EMI that can otherwise be generated by signal reflections on the unterminated conductive line is substantially reduced, if not eliminated.  
           [0010]    In accordance with one aspect of the apparatus, the system management device tracks inventory of a plurality of devices connected to a corresponding plurality of locations in the system.  
           [0011]    In another aspect, the system management device detects when one of the plurality of devices is disconnected from the corresponding location in the system.  
           [0012]    In a further aspect, the system management device and the plurality of devices can be coupled to a communication bus.  
           [0013]    In still another aspect of the apparatus, one of the plurality of devices is a hub comprising a second plurality of switches. The system management device can communicate signals to the hub to open and close each of the second plurality of switches.  
           [0014]    In yet another aspect of the apparatus, the hub utilizes an arbitrated loop protocol.  
           [0015]    In another aspect of the apparatus, the hub utilizes a fiber channel arbitrated loop protocol.  
           [0016]    In another aspect of the apparatus, an identifier module on the device can indicate to the system management device whether the device is connected to the particular location.  
           [0017]    In another aspect of the apparatus, a terminating device can indicate to the system management device whether the device is connected to the particular location.  
           [0018]    In another aspect of the apparatus, the terminating device can pull a designated pin on a connector portion to a designated state to indicate to the system management device whether the device is connected to the particular location.  
           [0019]    In another embodiment, a computer system includes a connection plane with a plurality of connector portions and a communication bus. A system management device is coupled to one of the connector portions. The system management device includes a logic module to detect when other devices are connected and disconnected to the plurality of connector portions via the communication bus. The logic module can also indicate whether the other devices are part of an arbitrated loop network, and transmit a signal to disable transmission to at least one of the connector portions when the device is disconnected.  
           [0020]    In one aspect, the computer system includes a hub with a port bypass circuit. The hub can support arbitrated loop capability, such as fiber channel arbitrated loop (FC-AL).  
           [0021]    In an aspect of a computer system that supports FC-AL, the hub can receive data via optical fiber and transmit data via electrically conductive wire. The hub includes one or more port bypass circuits that each include a switch. The switches can be opened and closed by a system management device that communicates with the hub via a communication bus.  
           [0022]    In another aspect, an identifier module indicates to the system management device whether one of the other devices is connected.  
           [0023]    In another aspect of the apparatus, a terminating device indicates to the system management device whether one of the other devices is connected by setting the state of a designated pin in the connector portion, to which the terminating device is connected, to a designated value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The features of the described embodiments believed to be novel are specifically set forth in the appended claims. However, embodiments of the invention relating to both structure and method of operation, may best be understood by referring to the following description and accompanying drawings.  
         [0025]    [0025]FIG. 1A is a block diagram of an example of a server system that can utilize an apparatus for controlling transmissions to reduce electromagnetic interference in accordance with an embodiment of the present invention.  
         [0026]    [0026]FIG. 1B is a block diagram of examples of functions performed by a system management blade that can be utilized in the server system shown in FIG. 1A.  
         [0027]    [0027]FIG. 2A is a diagram of an example of a fiber channel arbitrated loop network in which various embodiments of the present invention can be utilized.  
         [0028]    [0028]FIG. 2B is a diagram of an example of a dual port bypass circuit which can be utilized in the fiber channel arbitrated loop network shown in FIG. 2A.  
         [0029]    [0029]FIG. 3A is a block diagram of an example of a system management blade that includes a function to set registers in a port bypass circuit in accordance with an embodiment of the present invention.  
         [0030]    [0030]FIG. 3B is a flow diagram of an embodiment of a Set PBC Registers function in the system management blade shown in FIG. 3A.  
         [0031]    [0031]FIG. 4A is a diagram of an example of an airflow guide on which a module for identifying a “null” device to the system management blade is provided in accordance with an embodiment of the present invention.  
         [0032]    [0032]FIG. 4B is a side cross-sectional view of the airflow guide shown in FIG. 4A.  
         [0033]    [0033]FIG. 4C is a flow diagram of an embodiment of an enable/disable transmit function in the system management blade shown in FIG. 4A.  
         [0034]    [0034]FIG. 5A is a diagram of an example of a terminating device coupled to communicate with a system management blade in accordance with an embodiment of the present invention.  
         [0035]    [0035]FIG. 5B is a flow diagram of an embodiment of a Set PBC Registers function in the system management blade shown in FIG. 5A. 
     
    
     DETAILED DESCRIPTION  
       [0036]    Referring now to FIG. 1A is a block diagram of an example of a server system  100  that can utilize an apparatus for controlling transmissions to reduce electromagnetic interference in accordance with an embodiment of the present invention. Server system  100  includes slots in which removable blades can be inserted. When one or more of the blades is disconnected from mid-plane  108 , connector portion  104  on mid-plane  108  is left unterminated. As described hereinabove, EMI can propagate on the unterminated connector portions  104 , which can cause problems such as missing or erroneous data in blades connected to mid-plane  108  or other susceptible components outside of server system  100 . To help reduce this EMI, a device for controlling transmissions to unoccupied slots can be included in one or more of the blades.  
         [0037]    An example of a blade that can include a function or device to control transmission to unoccupied slots is system management blade  110 , which performs a central role including event reporting, configuration and inventory management, hot-swap control, and provides local panel and network operations center (NOC) console user interfaces.  
         [0038]    [0038]FIG. 1B is a block diagram of examples of functions typically performed by an embodiment of system management blade  110  that can be utilized in the server system  100  shown in FIG. 1A. The functions are performed for blades connected to mid-plane  108  and can include Power Supply Control  150 ; Inventory Tracking And Reporting  152 ; Maintaining Property Pages  154 ; Maintaining Control, Action, And Configuration Information  156 ; Reporting, Logging, And Responding To Events And Alarms  158 ; Monitoring And Reporting Blade Performance  160 ; Controlling Hot-Swaps  162 ; and Network Console User Interface  164 . The functions of system management blade  110  can be implemented in hardware, software, firmware, or a combination of hardware, software, and firmware components.  
         [0039]    In the embodiment shown, server system  100  supports various components attached to various types of blades connected to mid-plane  108 . In some embodiments, a chassis for server system  100  can support dual power grids (not shown), redundant paths to system management blade  110 , FC storage blade  111 , server blade  112 , redundant fiber channel busses via FC-AL hub blade  114 , Integrated Drive Electronics (IDE) storage blade  116 , cooling fans (not shown); redundant network blades  118 ; and load-balanced power supplies (not shown).  
         [0040]    Server system  100  supports a variety of configurations of different types of blades, or entirely of one type of blade. One such chassis to support server system  100  is the commercially available compact peripheral component interconnect (cPCI) Blade Server Chassis, Model Number bh7800, from Hewlett-Packard Company in Palo Alto, Calif. While server system  100  is used as an example herein, it is anticipated that various embodiments of the present invention can be utilized in various types of systems where unterminated connector portions can emit EMI.  
         [0041]    Mid-plane  108  can support and/or include one or more communication buses  120  for the blades in server system  100  and includes one or more connector portions  104  for each slot in the chassis. For example, when server system  100  utilizes the cPCI bus standard, connector portion  104  is included in each slot of mid-plane  108  for all power, ground, 32 bit, and 64 bit PCI signals. Components on the blades are coupled to corresponding connector portions  106 . These optional connectors can be used for a variety of purposes such as a bridge to other communication buses  120  in mid-plane  108 . In some embodiments, one of communication buses  120  conform to the compact Peripheral Component Interconnect (cPCI) bus standard, and another of communication buses  120  conform to the Inter-IC (I 2 C) bus standard. Other suitable bus structures and protocols can be utilized in addition to, or instead of, the cPCI and I 2 C bus on communication buses  120 .  
         [0042]    In some embodiments, mid-plane  108  also includes an EEPROM that allows mid-plane  108  to identify itself to system management blade  110  for inventory and configuration tracking, and an FET (field effect transistor) for each slot that allows the blades to operate when system management blade  110  is removed. Industry-standard Ethernet, SCSI, and Fiber Channel (FC) interfaces to mid-plane  108 , as well as other interfaces, can be utilized.  
         [0043]    FC storage blade  111  provides storage medium that can be accessed by devices on nodes that are part of FC-AL network  200  (FIG. 2A).  
         [0044]    Server blades  112  can include a range of components from a complete server with on-board storage memory to one or more high-performance reduced instruction set computing (RISC) processors.  
         [0045]    Fiber Channel Arbitrated Loop (FC-AL) hub blade  114  enables the use of fiber channel buses embedded in mid-plane  108  and a FC connection to via connector portions  104 . FC-AL hub blade  114  can be implemented with port bypass circuits, such as PBC  240  (FIG. 2B) as described herein to provide fiber channel arbitrated loop capability.  
         [0046]    Integrated Drive Electronics (IDE) storage blade  116  provides redundant arrays of independent disks (RAIDs) to store the same data redundantly on multiple hard disks, thereby improving fault tolerance and reliability. IDE storage blade  116  can typically store large amounts of data and can be accessed via mid-plane  108  by server blades  112  having an appropriate interface.  
         [0047]    Network blade  118  provides an interface between a local area network and a wide area network, typically via an Ethernet interface. Network blade  118  includes components that perform tasks such as routing, prioritization, security, bandwidth management, and network management. A console connected to network blade  118  can provide user interfaces to monitor and control hubs, switches, ports, and traffic over a network.  
         [0048]    Referring now to FIG. 2A, a block diagram of an example of a fiber channel arbitrated loop (FC-AL) network  200  is shown with which various embodiments of the present invention can be utilized. While FC-AL network  200  is used as an example herein, it is anticipated that various embodiments of the present invention can be utilized with any type of device, server, network (including peer-to-peer and wide area networks), or other systems where unterminated connector portions can cause EMI. Various embodiments of the present invention can also be utilized in any type of system that utilizes data transfer infrastructure and protocols instead of, or in addition to, fiber channel.  
         [0049]    FC-AL network  200  can provide high bandwidth data transfer between up to one-hundred and twenty-six devices. In some embodiments, FC-AL network  200  allows multiple devices, each called “a node,” to be connected together. A node may be any device or group of devices, such as computer workstations (not shown), FC storage  111 , server  112 , storage disk arrays  116 , tape libraries (not shown), and/or printers (not shown), having an interface allowing it to be connected to FC-AL network  200 .  
         [0050]    Each node communicates with all other nodes on FC-AL network  200 . During initialization of FC-AL network  200 , each device is assigned an address. These addresses may be assigned in various ways including manually, dynamically, or by wiring the rear of the rack where the devices are installed. When a device is ready to transmit data, the device transmits its address onto FC-AL network  200 . When the sending device receives its own address, the device becomes the master of the FC-AL network  200  and can communicate with the addressee. FC-AL network  200  therefore supports one active connection between two devices at a time, so control of the FC-AL network  200  must be arbitrated, usually according to priority, when more than one device requests a connection.  
         [0051]    Each node has at least one port, referred to as node-loop (NL) port  216 , to provide access to other nodes. NL ports  216  are the connections in a fiber-channel node through which data may pass over the fiber channel to NL ports  216  of other nodes. A typical fiber-channel drive has two NL ports  216  packaged within the drive&#39;s node. Each NL port  216  includes a pair of “fibers”—one to carry information into NL port  216  and one to carry information out of NL port  216 . Each “fiber” is a serial data connection, and, in one embodiment, each fiber is a coaxial wire (e.g., coaxial copper conductors, used when the nodes are in close proximity to one another); in other embodiments, a fiber is implemented as an optical fiber over at least some of its path (e.g., when nodes are separated by an appreciable distance, such as nodes in different cabinets or, especially, different buildings). The pair of fibers connected to each NL port  216  is referred to as a link  218 . Links  218  carry information or signals packaged in “frames” between nodes. Each link  218  can handle multiple types of frames (e.g., initialization, data, and control frames). One example of a link is bus  120  (FIG. 1A)  
         [0052]    Each node is directly attached to one of hub ports  220  of FC-AL hub blade  114  by link  218 . Arbitrated loop  224  is typically implemented inside FC-AL hub blade  114 . Generally, FC-AL hub blade  114  will have between seven to ten ports  220 , and a maximum number of devices, e.g.,  126  devices, can be connected to arbitrated loop  224  by linking several hubs  114  together.  
         [0053]    An advantage of FC-AL hub blade  114  is that each hub port  220  includes port bypass circuit (PBC)  240 , such as shown for example in FIG. 2B. If hub port  220  detects that a device is absent or not responding, hub port  220  closes PBC  240 , thereby preserving the continuity of arbitrated loop  224 . PBC  240  prevents a failing device or connection from bringing down the entire arbitrated loop  224  and also allows hot-swapping, which is the ability to add and remove devices while arbitrated loop  224  is active. An example of PBC  240  suitable for use in arbitrated loop  224  is port bypass circuit model number VSC7148, which is commercially available from Vitesse Semiconductor Corporation in Camarillo, Calif.  
         [0054]    In the example of PBC  240  shown in FIG. 2B, PBC  240  includes a multiplexer  242  that is controlled by the SEL1 line. When an operational device  258  is in communication with hub port  220  (FIG. 2A), the SEL1 line is set HIGH, and external input line  244  is selected. Otherwise, the SEL1 line is set LOW and output line  246  of previous PBC  250  is selected since there is no connected or functional device that can provide input to hub port  220 .  
         [0055]    FC-AL hub blade  114  and device  258  interface with bus  252  via connectors  254 ,  256 , respectively. Transmit line  248  transmits data to the corresponding device  258  via bus  252 . PBC  240  includes several registers that can be set via an application programmer interface (API) to PBC  240  to control operation of components in PBC  240  such as transmit enable switch  260  and receive enable switch  262 . In general, FC-AL hub blade  114  toggles SEL1 to bypass device  258  when device  258  is disconnected, while transmit enable switch  260  and receive enable switch  262  remain closed.  
         [0056]    One problem that arises when output line  246  of previous PBC  250  is selected is that the data is transmitted not only to multiplexer  242 , but also along transmit line  248 . Lines coupled to connector  254 , such as transmit line  248 , carrying data with fast edge rates or that are continuously active, such as clocks or data lines, should be terminated. Additionally, a line may pick up and transmit noise from other lines. When device  258  is not connected to bus  252 , transmit line  248 , as well as other lines coupled to connector  254  that are capable of conducting noise signals, should be terminated when they are “long” compared to the wavelength of the applied frequency of the signal. If transmit line  248  is not terminated in its characteristic impedance, a signal reflection will occur. The amplitude of the reflection depends on the amount of impedance mismatch between transmit line  248  and the load, which is infinite when transmit line  248  is not terminated. The amplitude of the reflection also depends on the rise time of the signal as well as the rise time of the signal compared to the length of the conductor in transmit line  248 . It is also desirable to terminate other lines coupled to connector  254 , such as receive line  262 , that are capable of conducting noise signals.  
         [0057]    When device  258  is disconnected from connector  256 , the portion of connector  256  coupled to bus  252  is typically left open. In the presence of signals at the appropriate frequency and amplitude, conductive parts, such as pins, in the open portion of connector  256  can act as antennae, radiating EMI that can disrupt operation of other devices within susceptible range.  
         [0058]    Referring now to FIGS. 3A and 3B, FIG. 3A is a block diagram of an example of system management blade  110  that performs Set Port Bypass Circuit (PBC) Registers function  304  in accordance with an embodiment of the present invention, to reduce EMI in an electronic system or network. Some devices that connect to mid-plane  108  include a Field Replaceable Unit Identifier (FRU-ID) module (not shown) that sends signals over communication bus  120  to system management blade  110  that allow Track and Report Inventory function  152  keep an accurate and timely record of devices connected to and disconnected from mid-plane  108 . Connector portion  302  is coupled to mid-plane  302  to communicate with system management blade  110  via bus  120 . When a slot for supporting a device is vacant, connector portion  302  is left open.  
         [0059]    In some embodiments Track and Report Inventory function  152  can use a Serial Presence Detect (SPD) mechanism, as known in the art, to detect the presence of a blade or other device in a slot. When a device is initially connected or disconnected to mid-plane  108 , Report, Log, and Respond to Events and Alarms function  158  records the event and performs any functions needed to accommodate the change to server system  100  (FIG. 1A). Track and Report Inventory function  152  can also retain information regarding slots that are capable of interfacing with FC-AL hub blade  114  (FIG. 1A) to provide fiber channel functionality.  
         [0060]    In accordance with an embodiment of the present invention, a function such as Set PBC Registers function  304  can be performed when Track and Report Inventory function  152  detects that a blade has been connected to or disconnected from mid-plane  108 . Note that Set PBC Registers function  304  can be a standalone function, or included as part of another function, such as Reporting, Logging, And Responding To Events And Alarms function  158  as shown in FIG. 1B. Additionally, Set PBC Registers function  304  can be implemented in hardware, software, firmware, or a combination of hardware, software, and firmware components.  
         [0061]    [0061]FIG. 3B is a flow diagram of an embodiment of Set PBC Registers function  304 . In the embodiment shown, function  318  determines whether the slot is occupied based on information from Track and Report Inventory function  152 . Note that not all blades in a fiber channel enabled slot may be capable of interfacing with FC-AL hub blade  114  (FIG. 1A), therefore Set PBC Registers function  304  can access information maintained by Track and Report Inventory function  152  to determine whether the device has fiber channel capability in function  320 . The information in Track and Report Inventory function  152  can include a pre-programmed list of device identifiers and corresponding indicators of whether the device includes fiber channel capability. In other embodiments, the device can send an indicator of whether it has fiber channel capability when it is connected.  
         [0062]    Referring to FIGS. 2A, 2B, and  3 B, if the slot is occupied and the device occupying the slot has fiber channel capability, function  322  sets one or more registers to include the device in the FC-AL network  200 . Function  324  sets one or more registers and to enable (close) transmit switch  260  in PBC  240 .  
         [0063]    If the slot is not occupied, or the device occupying the slot does not have fiber channel capability, function  326  sets one or more registers to bypass the device in the FC-AL network  200 . Function  328  sets one or more registers and to disable (open) transmit switch  260  in PBC  240 .  
         [0064]    As described for FIG. 2B, PBC  240  includes registers that can be set via an application programmer interface (API) to open and close transmit switch  260  and receive switch  262 . When transmit switch  260  is open, signals from previous PBC  250  are not conducted past transmit switch  260 . The reflections that can occur when transmit line  248  is unterminated are minimized, and as a result, there are no noise signals to be radiated by open connector portion  302  (FIG. 3A).  
         [0065]    Referring to FIGS. 2B and 4A, FIG. 4A is a diagram of another embodiment of the present invention showing a null device, such as airflow guide  402 , with Field Replaceable Unit Identifier (FRU-ID) module  404  for identifying the null device to system management blade  110 . FIG. 4B is a side cross-sectional view of airflow guide  402  shown in FIG. 4A, that includes air blocking members  410  to prevent cooling air from flowing past airflow guide  402 . The cooling air is redirected to flow past blades with active components and circuits that require cooling. Airflow guide  402  also includes connector portion  406 , which mates with connector portion  408 . Note that connector portion  406  may only mate with part of connector portion  408 , which leaves the remaining connector portion open to radiate EMI as described hereinabove. Thus, it is desirable to open transmit switch  260  to prevent any signals on transmit line  248  from being broadcast by the open part of connector portion  408 .  
         [0066]    To determine when to open transmit switch  260 , FRU-ID module  404  transmits signals to identify airflow guide  402  to system management blade  110 . Thus, when airflow guide  402  is inserted in an open slot, a function such as Track and Report Inventory function  152  detects the slot as being occupied by a null device, i.e., airflow guide  402 , and reports the event to Report, Log, and Respond to Events and Alarms function  158 . A function such as Enable/Disable Transmit function  412  to enable or disable transmissions to the slot (and connector portion  408 ) can then be invoked.  
         [0067]    In some embodiments, Enable/Disable Transmit function  412  can open or close transmit switch  260  associated with PBC  240 , similar to the embodiment of Set PBC Registers function  404  shown in FIG. 4B. In other embodiments, switches associated with transmit lines can be controlled regardless of whether the lines are coupled to PBC  240  or the device has fiber channel capability.  
         [0068]    Referring to FIGS. 2B and 4C, an embodiment of Enable/Disable Transmit function  412  is shown in FIG. 4C. Function  418  determines whether a slot is occupied by accessing information maintained by Track and Report Inventory function  152  (FIG. 4A). If the slot is not occupied, function  422  disables transmissions on transmit lines associated with the slot. Function  420  determines whether the slot is occupied by a null device, such as airflow guide  402  connected to connector portion  408  (FIG. 4A). If a null device is connected, function  422  disables transmissions on transmit lines associated with the slot. If the slot is occupied by an operational device (i.e., not a null device), function  424  enables transmissions on transmit lines associated with the slot.  
         [0069]    Thus, a system configured in accordance with an embodiment of the present invention can provide the ability to control transmissions on a variety of transmit lines, in addition to transmit lines associated with PBC  240  (FIG. 2B). This capability can greatly reduce EMI in the system.  
         [0070]    Referring now to FIG. 5A, a diagram of an example of terminating device  502  coupled to communicate with system management blade  110  in accordance with an embodiment of the present invention is shown. Terminating device  502  can be an electronic logic circuit mounted on support structure  504 , such as a null device. In other embodiments, terminating device  502  can be implemented in an active device such as a printed circuit board using hardware, software, or a combination of hardware and software components. Connector portion  506  on support structure  504  interfaces with at least a portion of connector portion  508 , which is coupled to mid-plane  108  and communicates with system management blade  110  via bus  120 .  
         [0071]    To determine when to open transmit switch  260 , terminating device  502  includes a circuit component, such as a pull-up transistor (not shown), to pull a designated, unused pin in connector portion  508  HIGH. When system management blade  110  detects the designated pin being pulled HIGH, a function such as Track and Report Inventory function  152  detects the slot as being occupied by terminating device  502 , and reports the event to Report, Log, and Respond to Events and Alarms function  158 .  
         [0072]    When the slot is fiber channel enabled, Set PBC Registers function  512  can be invoked to open or close transmit switch  260  (FIG. 2B).  
         [0073]    Referring to FIGS. 2B, 5A, and  5 B, FIG. 5B is a flow diagram of an embodiment of Set PBC Registers function  512  in accordance with an embodiment of the present invention for fiber channel enabled slots. Function  518  determines whether the slot being occupied is fiber channel enabled based on information from Track and Report Inventory function  152 .  
         [0074]    Function  520  determines whether the slot is occupied by terminating device  502  by detecting the state of the designated pin. If the slot is fiber channel enabled and the state of the designated pin is HIGH, function  522  sets one or more registers to disable transmissions to the slot, such as, for example, by opening transmit switch  260 . If the slot is fiber channel enabled and the state of the designated pin is not HIGH, function  524  sets one or more registers to enable transmissions to the slot, such as, for example, by closing transmit switch  260  in PBC  240 .  
         [0075]    Note that terminating device  502  can be configured with one or more various types of components to affect the state of the designated pins. Further, the state of the pins can be set to HIGH or LOW by terminating device  502  to indicate when transmit switch  260  should be opened.  
         [0076]    Note also that in some embodiments, a device similar to terminating device  502  and a function similar to function  512  (FIG. 5C) can be implemented in systems that do not support fiber channel capability, but in which it is still desired to prevent transmissions to lines that are not terminated. In such embodiments, function  520  can check the status of the designated pin set by terminating device  502  to determine whether to enable or disable transmissions.  
         [0077]    The ability to prevent signals from being transmitted by one or more lines coupled to a connector portion by opening transmit switch  260  in port bypass circuit  240  (FIG. 2B) provides a very effective solution to the problem of EMI propagated by open connector portions. A function for detecting whether a slot is open, and to set transmit switch  260  accordingly, can be implemented as a standalone function or included with other functions performed by system management blade  110  (FIG. 1A). Various embodiments of the present invention can be utilized in systems that do not include FC-AL hub blades  114  or utilize arbitrated loops. Further, various embodiments of the present invention can be implemented in systems that utilize an arbitrated loop, but do not transmit or receive signals via fiber channels.  
         [0078]    It is also important to note that a female connector portion can be coupled to conductive lines may also propagate EMI. In situations where transmissions to the female connector portion can be controlled by port bypass circuit  240 , a function to set transmit switch  260  accordingly can be implemented as described for preventing transmissions on lines coupled to the female connector portion. Such would be the case, for example, where connector portions  302  is a female connector portion coupled to mid-plane  302 .  
         [0079]    Further, a function in accordance with the present invention, such as Set PBC Registers functions  512 , can also include instructions to disable or enable switches on other lines, such as receive switch  262 . In this manner, EMI can be reduced in a system by disabling transmissions along lines that are coupled to unterminated connectors. Additionally, in some embodiments, functions similar to Track and Report Inventory  152 ; Report, Log, and Respond to Events and Alarms  158 ; and Set PBC Registers function  512 , can include instructions to detect whether a device is installed in a slot and control switches on transmission lines which are coupled to unterminated connectors whether or not the lines are coupled to PBC  240 .  
         [0080]    While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions and improvements of the embodiments described are possible. For example, those having ordinary skill in the art will readily implement the steps necessary to provide the structures and methods disclosed herein, and will understand that the process parameters, materials, and dimensions are given by way of example only. The parameters, materials, and dimensions can be varied to achieve the desired structure as well as modifications, which are within the scope of the invention. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.  
         [0081]    In the claims, unless otherwise indicated the article “a” is to refer to “one or more than one”.