Abstract:
A fan control module is provided for a system unit. The fan control module includes power outputs for supplying power to a plurality of fan. It also includes a temperature sensor for giving a temperature signal. It further includes a control unit connected to receive the temperature signal and including preprogrammed control information for determining power signals to be supplied to each of the fan units for controlling the speed thereof. The fan control module can control the fan units in a coordinated manner enabling reliable and effective cooling of the system unit under widely varying parameters. It can mean that existing system components can be employed in harsher temperature environments that they were originally designed for, without needed a complete redesign thereof. The fan control module can be provided with electrical noise isolation circuitry to isolate other components from electrical noise generated by the fan units. The system unit can, for example, be a computer system unit for rack mounting in a telecommunications application.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to cooling system units. In particular, the invention relates to providing controlled cooling for a computer system for use in environments and applications that place high demands on system reliability, for example in the telecommunications industry. 
     Deregulation and privatization is causing unprecedented competition in the worldwide telecommunications market. This climate of fierce competition has meant that service providers must introduce new, more sophisticated and user-friendly services at an accelerated pace to retain or attract subscribers, while not compromising traditional telecommunications company (telco) service quality. 
     These pressures of competition have also placed high demands on Network Equipment Providers (NEPs). Traditionally, NEPs have designed, built and supported proprietary computing equipment, as the strict telco requirements could not be met by the commercial computing sector. Those requirements include the so-called Telcordia Technologies Network Equipment Buildings Systems (NEBS) tests. However, due to the lead times required to design and test such proprietary equipment, and the cost of supporting such equipment, there is a need to find another route, at least for the supply of the more cost and performance sensitive sectors within the telco industry. 
     A major concern of the telco sector is the reliability of systems under environment conditions as set by the NEBS tests. 
     In order to keep up with the ever-increasing demands of the telco industry, and in order to provide equipment at reasonable cost and within reasonable time scales, it would be desirable to use as many off-the-shelf computer system components as possible, rather than having to design and test each system in its entirety from scratch. For example, it would be desirable to select components designed for the commercial computing sector. However, such equipment is typically not designed with the stringent requirements of the telco industry in mind. Accordingly, it is an aim of the present invention to address the provision of cost-effective equipment that can meet technical demands of the telco environments, for example as regards providing reliable operation under adverse operating temperatures, while also meeting the modern commercial demands of that environment. 
     SUMMARY OF THE INVENTION 
     Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Combinations of features from the dependent claims may be combined with features of the independent claims as appropriate and not merely as explicitly set out in the claims. 
     In accordance with one aspect of the invention, there is provided a fan control module for a system unit. The fan control module comprises power outputs for supplying power to a plurality of fan. It also includes a temperature sensor for giving a temperature signal. It further includes a control unit connected to receive the temperature signal. The control unit includes preprogrammed control information for determining power signals to be supplied to each of the fan units for controlling the speed thereof dependent upon the temperature signal. 
     The provision of a separate fan control module for controlling the fan units in a coordinated manner enables reliable and effective cooling of the system unit under widely varying parameters. It also means that existing system components can be employed in harsher temperature environments than they were originally designed for, without needing a complete redesign thereof. 
     Moreover, where the fan control module includes one or more power inputs from a power supply that is also used to power the other components of the system unit, the fan control module can be provided with electrical noise isolation circuitry to isolate other components of the system unit, from electrical noise generated by the fan units. 
     In order to limited the power handling requirements of thc fan control module circuits, in an embodiment of the invention the fan control module can be logically split into two parts. A first part controls a first pair of fan units and the second part controls a second pair of fan units. Each part of the fan control module can be provided with respective inputs, outputs and control units. The control information programmed in the control unit of each part can be identical. Preferably, one temperature sensor is be employed by both parts to provide a co-ordinated ramp for the fan speeds. Also, where more than four fans are provided, more than two fans per part could be controlled and/or more parts could be employed, as appropriate. 
     The fan control module is preferably configured on a single circuit board. This provides particular advantages where the fan control card is to be provided as an addition to a system. The temperature sensor is preferably mounted on the circuit board, although it could be placed at some another part of the system as appropriate. Preferably one temperature sensor is used as this facilitates the provision of a controlled and co-ordinated ramp up of the fan speeds. However, more than one temperature sensor could be used, if desired, with each temperature sensor providing respective signals and control of the individual fans being dependent upon individual temperature signals or a function of some or all of the temperature signals. 
     Preferably speed signals, for supply to an alarms module, are directed via the fan control module and a power distribution board. The fan control module does not process these signals, but the feeding of the signals via the fan control module enables an efficient wiring loom to be made, with a single bundle of wires and a single connector being connected to a fan unit. 
     In accordance with another aspect of the invention, there is provided a system unit including a fan control module, the fan control module comprising power outputs for supplying power to a plurality of fan units, a temperature sensor for giving a temperature signal, and a control unit connected to receive the temperature signal and including preprogrammed control information for determining power signals to be supplied to each of the fan units for controlling the speed thereof dependent upon the temperature signal. 
     In a particular embodiment the system unit is a computer system unit including at least one processor module. It may contain anywhere between one and four processor modules. This puts further demands on the cooling requirements, as these will vary in accordance with the number of processors present. Accordingly, the power supply signals output by the control unit can be made dependent upon to the number of processor modules present. 
     In accordance with a further aspect of the invention, there is provided a method of controlling cooling of a system unit, the method comprising: 
     a fan control module receiving a temperature signal from a temperature sensor; 
     the fan control module determining power outputs to the fan units for controlling the speed thereof dependent upon the temperature signal from the temperature sensor and preprogrammed control information for determining power signals to be supplied to each of the fan units for controlling the speed thereof. 
     In the particular embodiment mentioned above, the system unit is a computer server intended to be rack-mounted for a telecommunications application. It will be appreciated that this puts further strain on the cooling requirements, due to different possible configurations of adjoining equipment in a particular installation, and the possible proximity of other heat generating elements. It will be appreciated that the present invention provides particular and important technical advantages when applied to the adaptation of systems to meet the strict reliability and temperature requirements of, for example, telecommunications applications and that it is ideally suited to such telecommunications applications. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention will be described hereinafter, by way of example only, with reference to the accompanying drawings in which like reference signs relate to like elements and in which: 
     FIG. 1 is a perspective view from the front of an embodiment of the invention including sacrificial transport brackets; 
     FIGS. 2A and 2B are plan and front views, respectively of the embodiment of FIG. 1 with alternative mounting brackets, and 
     FIG. 2C is a side view showing the mounting holes for alternative types of mounting arrangements; 
     FIG. 3 is perspective view from the rear of the embodiment FIGS. 1 and 2 illustrating a removable top cover; 
     FIG. 4 is an exploded view of the aforementioned embodiment; 
     FIG. 5 is a front view of the aforementioned embodiment; 
     FIG. 6 is a rear view of the aforementioned embodiment; 
     FIG. 7 is a plan view of a computer motherboard; 
     FIG. 8 is schematic block diagram of and example of the architecture of an embodiment of the invention; 
     FIG. 9 is perspective view from the rear of the embodiment FIGS. 1 and 2 illustrating the removal of a power supply unit; 
     FIGS. 10A,  10 B,  10 C and  10 D are rear, top, front and perspective views of a power sub-frame for receiving three power supply unit, and 
     FIG. 10E illustrates connections for various connectors of a power sub-frame assembly; 
     FIG. 11 is a schematic diagram of circuitry from a power distribution board of the power sub-frame of FIG. 10; 
     FIG. 12 illustrates the location of an alarm circuit; 
     FIG. 13 is a schematic block diagram of the logic of the alarm circuit; and 
     FIG. 14 is a schematic diagram illustrating the configuration of a fan control module. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a particular embodiment of the invention will be described by way of example only. 
     FIG. 1 is a perspective view of a system unit  10  for use in a rack-mountable system. In a particular example described herein, the system unit is a computer system unit for forming a computer server for a telecommunications application, for example an Internet server. As shown in FIG. 1, the unit  10  has a front surface  12  formed by a front wall, a rear surface  14  formed by a rear wall, a left end surface  16  formed by a left side wall, a right end surface  18  formed by a right side wall, a lower surface  20  formed by a base wall and an upper surface  22 , in the present example formed by a cover  30 . As shown in FIG. 1, the system unit  10  is provided with sacrificial transport flanges  24 , which extend above and below the system unit. This optional feature is removed before installation of the system unit  10  in a rack. 
     The system unit  10  is constructed with an extremely robust chassis  11 , with the various walls  12 - 20  and the cover  30  forming the casing of the chassis  11  as well as internal walls (not shown) being formed of heavy gauge steel. The walls of the chassis can be made, for example, from electroless nickel-plated mild steel with a thickness of, for example, 1.5 to 2.0-mm. 
     The steel chassis  11  is pre-formed with mounting holes for the attachment of mounting flanges or a slide mechanism to enable the system unit  10  to be provided with a wide variety of mounting options and rack sizes. Mounting flanges can be provided to suit standard 19-inch, 23-inch, 24-inch or 600-mm nominal frame widths. (One inch=approximately 25.4 mm). 
     FIG. 2A is a plan view of the unit  10  showing the upper surface  22 /cover  30  and various options for flanges  26  with the displacements from the front surface indicated in mm. 
     FIG. 2B is a front view of the unit  10  showing the front surface  12  and two different examples of mounting flanges  26 . The mounting flange shown to the left (as seen in FIG. 2B) is provided with a handle to facilitate insertion and removal of the unit  10  from the racking system, whereas the flange  26  to the right (as viewed in FIG. 2B) is not provided with a handle. 
     In the present example, the mounting flanges can be attached using screws which pass through the mounting flange into threaded holes in the end walls  14 ,  16  at either side of the chassis  11  of the unit  10 . FIG. 2C is a side view of the system unit  10 , showing the holes in the side of the system unit  10  for the mounting of flanges or a slide mechanism. Vertical rows of holes are for the attachment of flanges to be attached to vertical rack components, and horizontal rows of holes provide for the attachment of a runners for permitting a slideable mounting of the system unit in a rack. 
     FIG. 3 is a perspective rear view of the unit  10  showing the cover  30  that forms the top surface  22  of the unit  10 . As can be seen, the cover  30  is provided with front locating flanges  32  that, in use, engage a co-operating front flange  31  of the body of the chassis  11 . Side flanges  33  engage either side of the end walls forming the left and right ends  16  and  18  of the chassis  11 . Detents  34  on those end walls engage within L-shaped slots  35  in the side flanges  33  so that the cover may be lowered onto the top of the chassis  11  and then moved forwards so as to cause the detents  34  to latch within the slots  35 . At the rear of the cover  30 , a rear flange  36  with a lower lip  37  engages over an abutment  38  at the top of the rear end wall  14  of the casing  10 . The cover can be secured to the remainder of the chassis  11  by means of a screw  39  that passes through this rear flange into a threaded hole in the abutment  38 . 
     FIG. 4 is an exploded perspective view from the front of the system unit  10 . This shows a motherboard  40  that is mounted on a horizontal mounting plane  41  within the chassis  11 . Mounted on the motherboard  40  are between one and four processor modules  42 . A riser card  44  can receive a plurality of dual in-line memory modules (DIMMs)  46 . Further DIMMs  46  can be received directly in slots in the motherboard. A slideable carriage  48  is provided for receiving one or more media drives. 
     As shown in FIG. 4, the slideable carriage  48  can receive up to two media drives. In the present instance, two media drives including a digital audio tape (DAT) drive  50  and a CD-ROM drive  52  are provided. Appropriately configured metal cover plates  54  and  56  are provided for the media drives  50  and  52 . A disc bay assembly  58  provides a small computer system interface (SCSI) backplane and cables for receiving one or more SCSI media drives, such as a SCSI disc drive  60 . Although, in the present instance, the drives are controlled via a SCSI-type interface, it will be appreciated that another media drive interface (e.g., IDE) could be used. A SCSI card (not shown) is located within the chassis to the front of the motherboard. A bezel (decor panel)  62  is provided for covering ventilation holes  63  in the front wall  12  of the chassis  11 . A bezel  64  is provided for covering the media drives  50 ,  52  and  60 . 
     A fan control module  66  controls the operation of processor fans  68  and system fans  70 . A power sub-assembly that includes a power sub-frame  72  with a power distribution board assembly, is provided for receiving three separate power supply units  74 . An alarms module in the form of an alarms card  78  enables the signalling of alarms to the outside world, and is also connected to an LED card  2  for signalling alarms locally on the front of the unit  10 . A power switch  82  is also provided on the front surface of the unit  10 . FIG. 4 also illustrates one PCI card  84  to be received within a PCI slot  85  on the motherboard  40 . 
     FIG. 5 is a front view of the unit  10  showing the bezels  62  and  64 , a power and alarm panel  90  which includes the power switch  82  and a number of status light emitting diodes (LEDs)  92 . FIG. 5 also illustrates the slots  86  and  88  for the media drives such as media drives  50  and  52  shown in FIG.  4 . 
     FIG. 6 is a rear view of the unit  10  in a configuration with three DC power supply units  74 A,  74 B and  74 C. Each of the power supply units  74 A,  74 B and  74 C is the same, and provides redundant power for the unit  10 . However, as will be seen later, one or more of the DC power supply units could be replaced by AC (mains) power supply units. The power supplies are hot swappable (i.e., while the system is running), as long as they are swapped one at a time. 
     With regard to power supply unit  74 A, it can be seen that this is provided with a handle  94  that is used for inserting and removing the power supply unit  74 A. The handle  94  includes a flange portion that is able to receive a screw  95  for securing the power supply unit to the chassis  11 . First and second power cable sockets  96  and  98  are shown. 
     Also shown is a grounding plate  100  that is secured by knurled nuts  102 ,  104  and  106  to grounding studs  103 ,  105  and  107 . Grounding stud  103  provides a connection directly to the chassis  11  of the unit  10 . Grounding studs  105  and  107 , on the other hand are electrically isolated from the chassis by an insulating board and are instead connected to logic ground (i.e. the ground of the electronic circuitry). By means of the grounding plate  100 , logic ground can be connected directly to chassis ground. The provision of this grounding plate provides for optional tying of logic ground to chassis ground. It will be noted that each of the power supply units  74  is provided with a similar grounding plate  100 , for connection to corresponding grounding studs. If it is desired to isolate logic ground from chassis ground, it is necessary to remove the grounding plate  100  from each of the power supply units  74 A,  74 B and  74 C. 
     An isolated ground system is needed in some telco applications when operating in a Regional Bell Operating Company (RBOC) mode. When operating in such a mode, the chassis and logic ground are connected at a remote location to provide, for example, lightning protection. In this case two-hole lugs  101  having a pair of holes  111  to fit over the pair of grounding studs  105  and  107  are provided for each of the power supply units  74  and are secured over the studs using nuts  104  and  106 . A similar two-hole lug  101  is secured to the grounding studs  108  and is secured with similar nuts. Earthing wires  109  from each of the two-hole lugs  101  on the power units and the chassis then are taken to the remote, earthing location. The studs  103   105 ,  107  and  108  are of a standard thread size (M5). The studs  105 / 107  and the studs  108  are at a standard separation (15.85 mm). The studs  105 / 107  are self-retaining in the insulated board on the power supply units. The stud  103  is selfretaining in the casing of its power supply unit  74 . The suds  108  are also selfretaining in the system unit chassis. 
     In a non-isolated ground situation, chassis ground can simply be tied to a desired ground potential (for example, to the racking system) by connecting a grounding cable to grounding studs  108  provided on the rear of the chassis. A further earth connection is provided via the power cables for the power supplies. 
     FIG. 6 also illustrates rear ventilation holes  110  through which air is vented from the system. FIG. 6 also shows the alarms module  78  with a serial connector  112  enabling connection of the alarms module to a network for the communication of faults and/or for diagnostic operations on the unit  10  to be performed from a remote location. FIG. 6 also shows a number of PCI cards  84  received within respective PCI slots  116 . A number of further external connections  114  are provided for connection of serial connections, parallel connections and SCSI connections, and for the connection of a keyboard or a Twisted-Pair Ethernet (TPE) connector. 
     FIG. 7 is a plan view of the motherboard  40  shown in FIG.  4 . Four CPU module slots  120  are provided. Each of these slots is able to receive one processor module  42 , and any number between one and four slots may be occupied by a processor module  42 . A connector arrangement  122  is provided for receiving a riser card  44  as shown in FIG.  4 . Also, connectors  124  (in four banks) are provided for receiving DIMMs  46  as mentioned with reference to FIG.  4 . Edge connectors  126  are provided for connecting the motherboard to connectors mounted on the mounting plane  41 . Also shown in FIG. 7 is the slot  128  for the alarms module  78  and various ports  130  for the connectors  114  shown in FIG.  6 . 
     FIG. 8 is a schematic overview of the computer architecture of the system  10 . As shown in FIG. 8, various components within the system are implemented through application-specific integrated circuits (ASICs). The system is based round a UltraSparc Port Architecture (UPA) bus system that uses a Peripheral Component Interconnect (PCI) protocol for an I/O expansion bus. The CPU modules  40 . 0 ,  40 . 1 ,  40 . 2 ,  40 . 3 , and a UPA-TO-PCI (U2P) ASIC  154  communicate with each other using the UPA protocol. The CPU modules  40  and the U 2 P ASIC  154  are configured as UPA master-slave devices. An Address Router (AR) ASIC  154  routes UPA request packets through the UPA address bus and controls the flow of data to and from memory  150  using a Data Router (DR) ASIC  144  and a switching network  148 . The AR ASIC  154  provides system control. It controls the UPA interconnect between the major system components and main memory. 
     The DR ASIC  144  is a buffered memory crossbar device that acts as a bridge between six system unit buses. The six system unit buses include two processor buses, a memory data bus and to I/O buses. The DR ASIC  144  provides crossbar functions, memory port decoupling, burst transfer and First-in-First-Out (FIFO) data read functions. Clock control for the operation of the processor is provided by a Reset, Interrupt, Scan and Clock (RISC) ASIC  152 . 
     The PCI bus is a high performance 32-bit or 64-bit bus with multiplexed address and data lines. The PCI bus provides electrical interconnection between highly integrated peripheral controller components, peripheral add-on devices, and the processor-memory system. A one-slot PCI bus  155  connects to a PCI device  156 . 0 . A three-slot PCI bus connects to three PCI slots  156 . 1 ,  156 . 2  and  156 . 3 . Two controllers are also connected to the second PCI bus  157 . These include a SCSI controller  174  and a PCI-TO-EBus/Ethernet controller (PCIO)  158 . The SCSI controller  174  provides electrical connection between the motherboard and separate internal and external SCSI buses. The controller also provides for SCSI bus control. The PCIO  158  connects the PCI bus to the EBus. This enables communication between the PCI bus and all miscellaneous I/O functions as well as the connection to slower, on board functions. Thus, the PCIO enables the connection to an Ethernet connection via a Transmit/Receive (Tx/Rx) module  161  and a network device (ND) module  162   
     An EBus2  159  provides a connection to various I/O devices and internal components. Super I/O  164  is a commercial off-the-shelf component that contains two serial port controllers for keyboard and mouse, an IEEE 1284 parallel port interface and an IDE disk interface. The super I/O drives the various ports directly with some electromagnetic interference filtering on the keyboard and parallel port signals. The alarms module  78  interfaces with the motherboard and provides various alarm functions. The NVRAM/TOD  168  provides non-volatile read only memory and the time of day function. Serial port  170  provides a variety of functions. Modem connection to the serial port  170  enables access to the Internet. Synchronous X.25 modems can be used for telecommunications in Europe. An ASCII text window is accessible through the serial port on non-graphics systems. Low speed printers, button boxes (for computer aided design applications) and devices that function like a mouse are also accessible through the serial port. The serial port includes a serial port controller, line drivers and line receivers. A one-Mbyte flash programmable read only memory (PROM)  172  provides read only memory for the system. 
     FIG. 9 is a perspective rear view of the system  10  showing the withdrawal and/or insertion of a power supply unit  74  in a non-isolated ground situation. In this example, AC power supply units  74  are shown. It can be seen that the power supply unit  74  is provided with the handle  94 . As shown in FIG. 9, the handle  94  is provided with a grip  184 , a pivot  182  and a latch  180 . To insert the power supply unit  74  it is necessary to slide the power supply unit into the power sub-frame  72  with the grip  184  of the handle  94  slightly raised so that the detent  180  can be received under the top  184  of the power sub-frame  72 . As the power supply unit  74  reaches the end of its movement into the power sub-frame  72 , connectors (not shown) provided on the power supply unit  74  make connection with a corresponding connector on the power distribution board at the rear of the power sub-frame  72 . Also, at this time, the handle can be pushed down into the position shown in FIG.  9 . This causes the detent  180  to latch behind the upper portion  184  of the power sub-frame  72 . The handle  94  can then be secured in place by tightening the screw  95 . The AC power supply unit  74  shown in FIG. 9 has a single power socket  97 , whereas the DC power supply units  74  shown in FIG. 6 have two power sockets  96  and  98 . Irrespective of whether the arrangement is as shown in FIG. 6 with two DC power sockets  96  and  98 , or as shown in FIG. 9 with one AC power socket  97 , the configuration of the power socket(s) and the lever  94  is such that the lever cannot be moved, and therefore the power supply unit cannot be released from the power sub-frame  72  and the chassis  11  with a plug  186  of a power cable  188  in place in one of the power sockets  96 / 97 / 98 . The removal operation is achieved by releasing the screw  95 , removing the power plug, and lifting and pulling on the handle  94 . 
     In an isolated ground situation, in order to hot-swap a power supply unit  74 , it is merely necessary to remove the two-hole lug  101  with its connecting earth wire  109  from the studs  105 ,  107  of the power supply unit to be removed, to remove the old power supply unit  74 , to replace a new power supply unit  74  and then to reconnect the two-hole lug  101  and connecting earth wire  109  to the studs  105 ,  107  of the new power supply unit  74 . These operations can all be performed with the system under power from the other power supply units  74  and with the two-hole lugs  101  and earth wires  109  in place over the chassis studs  108  and the studs  105 ,  107  of the other power supply units  74 . 
     The isolated ground situation is not shown in FIGS. 6 and 9. In the non-isolated ground situation shown in FIGS. 6 and 9, hot-swapping of a power supply unit is even easier, as it is merely necessary to remove the selected power supply unit  74  and to replace it with the new power supply unit  74 . 
     FIGS. 10A,  10 B,  10 C and  10 D are rear, top, front and perspective views of a power sub-frame for receiving three power supply units: 
     The power sub-frame  72  comprises a rectangular, box-shaped frame  191 , with four exterior walls on four sides (the top, bottom and two lateral surfaces), one open side  195  for receiving three power supply units and a power distribution circuit board  190  opposite to the open side. In the present instance, the walls are made of electroless nickel-plated mild steel. 
     FIG. 10A shows the power distribution board at the “rear” of the power sub-frame (i.e. opposite to the open side). When inserted in the chassis of the system unit, this “rear” of the power sub-frame is actually the forward-most side of the power sub-frame when viewed with respect to the system unit. The power distribution board  190  is formed with ventilation holes  194  and carries circuit tracks and components (not shown). FIG. 10A also illustrates the flanges  198  with screw holes  199  for securing the power sub-frame to the rear chassis wall. FIG. 10B shows the top of power sub-frame. It will be noted that the power sub-frame body  196  is provided with apertures  197  for lightness and for ventilation purposes. 
     FIG. 10C shows the open (front) side  195  (see FIG. 10B) of the power sub-frame. When inserted in the chassis of the system unit, this “front” of the power sub-frame is actually the rear-most side of the power sub-frame when viewed with respect to the system unit. Within the power sub-frame  72 , connectors  192 A,  192 B and  192 C for the three power supply units  74 A,  74 B and  74 C, respectively, can be seen. These connectors are mounted on the power distribution board  190  inside the power sub-frame  72 . FIG. 10C also shows the flanges  198  with screw holes  199  for securing the power sub-frame to the rear chassis wall. 
     FIG. 10D is a perspective view of the power sub-frame  72 , which shows that this in fact forms part of a power sub-assembly  71 . Internal walls  200  separate three compartments, each for a respective one of the three power supply units  74 . Cables  202  connect standby power and signal lines from the power distribution board  190  to a connector  204  for connection to an alarms module. Cables  206  connect main power and signal lines from the power distribution board  190  to various connectors  208 ,  210 ,  212  and  214 . FIG. 10E shows the various connector types  192 ,  204 ,  208 ,  210 ,  212  and  214  and the electrical signal connections thereto. 
     FIG. 11 is a schematic representation of some of the logic connections on the power distribution board. For ease of explanation, only those connections relevant for an understanding of the present invention are described. 
     At the left of FIG. 11, the three connectors  192 A,  192 B and  192 C for the three power supply units  74 A,  74 B and  74 C are shown. For reasons of clarity and convenience only those connections relevant for an understanding of the present invention as shown. For example, as illustrated with respect to FIG. 10E, the connectors  192  have many pins and pass many signals via respective lines. However, as not all of these lines are necessary for an understanding of the present invention, and as it would be confusing to illustrate all of the signal pathways on a diagram, only selected pathways are shown in FIG.  11 . It is to be noted from FIG. 10E, that the power supply units output ground, +3V3, +5V, +12V, −12V and +5V standby potentials as well as control signals such as PSU OK, PSU ON, etc. The +5V standby voltage is used for powering the alarm module  78 . The other voltages are for powering the motherboard and other main system components. The various lines could be configured using bus bars, wires, printed circuit or thick film conductors as appropriate. 
     Firstly, the two-of-three circuit  232  will be explained. This circuit is powered by the +5V standby voltage  231  provided from each of the power supply units  74 . Each of the power supply units outputs a PSU OK signal via a pin on its respective connector to a corresponding PSU OK line  230 A,  230 B and  230 C when the power supply unit is operating correctly. Each of these PSU OK lines  230  is connected to the two-of-three circuit  232 . This comprises three AND gates  234 ,  236  and  238 , each for comparing a respective pair of the PSU OK signals. The outputs of the AND gates are supplied to an OR gate  240 . 
     If the output of this OR gate is true, then at least two of the power supply units  74  are operating correctly, and power can be supplied to the motherboard of the computer system. This can be achieved by closing the main power line  245 . An output signal  242  could be supplied to a gate  244  (for example a power FET) to enable current to pass to the motherboard and other system components. Additionally, or alternatively, a power OK signal  246  for controlling some other form of switch mechanism (not shown). 
     If alternatively the output of the OR gate  242  is false, then this indicates less than two of the power supply units  74  are operative. In this case power is prevented from being passed to the motherboard  40  of the computer system. This can be achieved by interrupting the main power line  245 . An output signal  242  could be supplied to a gate  244  (for example a power FET) to prevent current being passed to the motherboard and other system components. Additionally, or alternatively, a power fault signal  246  could be passed to the alarms module and/or for controlling some other form of switch mechanism (not shown). 
     One-of-three power control is effectively provided by the alarms module  78  to be described later. However, with reference to FIG. 11, input A/B signals  268  and output sense signals  270  are passed to the alarms module for standby operation, and control signals  272  could be returned for turning off of a power supply unit, if required. 
     FIG. 11 further illustrates a protection circuit  256  that is able to detect an overcurrent representative of a current greater than 2*Imax, where Imax is the maximum current that can be output by a power supply, 2*Imax being the maximum current which should be required by the system unit. If a current greater than 2*Imax is detected, this is representative of a fault in the system unit. In accordance with telco requirements, in such a situation the system should be powered down. By providing for overcurrent detection on the power distribution board, where the maximum drawable current should be 2*Imax, it is possible to test for a fault at a lower overall current than if this test were made within each power supply unit. If the test were made in each power supply unit, each power supply unit would need to be tested for an overcurrent in excess of Imax, whereby one would be testing for a total current drain of 3*Imax. This could lead to faults not being detected or not detected early enough and the system could incorrectly be drawing up to 3*Imax, which could damage components and traces (tracks). 
     Thus, as shown in FIG. 11, each of the main power lines (e.g., +12V)  250 A,  250 B and  250 C from the power supply units  74 A,  74 B and  74 C, respectively is connected to form a common power supply line  254 . An overcurrent detector  258  detects a current in excess of 2*Imax. If such a current is detected (for example as a result of a fault represented by the box  266 ), then a signal  261  is provided to the connectors  192 A,  192 B and  192 C for shutting down the power supplies  74 A,  74 B and  74 C, respectively. Also, a signal  262  is passed to a switchable shunt  260  (e.g., a silicon controlled rectifier (SCR), a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), an Insulated Gate Bipolar Transistor (IGBP), etc) to shunt the power supply line  254  to ground. This will cause any energy stored in the power supplies and also in the system (for example as represented by the capacitor  264 ) to drain to ground, thus protecting the system. 
     The use of the two-of-three circuit described above means that redundant power supply operation is provided in that the system can remain powered even if one of the three power supply units fails. As only two-of-three power supply units are needed to power the system the third power supply unit can be hot swapped while the other two power supply units power the system. 
     FIG. 11 illustrates the location of an alarms card forming the alarms module  78  in the rear of the system unit  10 . 
     FIG. 12 is a functional block diagram for illustrating the alarm sub-system on the alarms module  78 . The alarms sub-system provides lights out management or remote management of the system over a serial connection. The alarms module  78  interfaces with the motherboard through an EBus edge connector slot  298  (connected to EBus 2  as shown in FIG.  8 ). A PCI-style bracket is attached to one edge of the alarms module (as seen in FIG. 11) and provides the external interfaces at the rear of the chassis  11 . Internal interfaces provide connections to the power supply assembly and to the LED card  80  located at the front panel of the system unit  10 . The alarms module is powered by the standby, or reserve, power supply. The alarms module only requires power from a single power supply to remain operable. Accordingly, the alarms module can remain operable even in a situation where the system has been powered down due to there being only one power supply unit operable. 
     The alarms sub-system comprises a logic device  280  which receives inputs  298  from the EBus, inputs  286  from the fans, input  290  from general purpose events, input  270  from the power supply unit output rails and inputs  268  from the A and B power inlets. The logic circuit samples, or multiplexes, the inputs to the microcontroller  296  in response to multiplex signals from the microcontroller  296 . The microcontroller  296  processes the sampled (multiplexed) inputs. The microcontroller  296  provides power control signals  272  for controlling the power supply units, and alarm outputs for the output of alarm signals. The microcontroller  296  also outputs power supply unit status signals  304  and fault signals  306 . The micro controller  296  can further output a system reset signal  310 , when required. Alarm signals to be passed to a remote location can pass via a remote serial connection  112 . Diagnostic and remote control signals can be passed from the network via the serial connection  112  to the microcontroller  296 . Control signals can thus be provided via the remote serial connection over the network for powering on and powering off the system. Examples of other commands that can be sent to the microcontroller via the remote serial connection  112  are to turn alarms off, to reset the monitoring of all failures, to display the status of all fans, power supply units, alarms and fault Light Emitting Diodes (LEDs), to display an event log, etc. 
     The microcontroller is programmed to report any fan failures or changes in power supply units status by means of the LEDs  92  (FIG. 5) on the system front and optionally to report the faults via the remote serial connection  112 . The microcontroller  296  is programmed to maintain the event log that was referenced above. 
     FIG. 14 illustrates the configuration of the fan control module  66  shown in FIG.  4 . The fan control module is subdivided into two halves  66 A and  66 B. One half  66 A handles one processor fan  68 A and one system fan  70 B and the other half  66 B handles the other processor fan  68 B and the other system fan  70 B. The fans are connected to the fan control module  66  by respective power lines  320  so that the fans receive their power via the fan control module. The fan control module receives +12V power via power lines  324 A/B from the power distribution board  190  and supplies voltages to the fans via the power lines  320  in a controlled manner. 
     For convenience, tacho (speed) signals from the fans pass via the alarms control module  66 . The speed signals are not processed by the fan control module, but are instead forwarded via tacho sense  326  to the power distribution board  190 . The power distribution board then routes the tacho sense signals to the alarms module  78  to form the signals  286  shown in FIG.  13 . This routing is convenient as it enables simpler wiring looms to be used. Also, when replacing a fan unit, the maintenance engineer only needs to remove a single bundle of wires from the fan to the fan control module  66 , rather than having to locate a number of different connectors connected to the fan. The fan control module thus has four fan connectors, each for receiving a connector connected to a bundle of wiring from a respective fan, plus a further connector for receiving a connector with a bundle of wires from the power distribution board. 
     As shown in FIG. 14, each half  66 A/ 66 B of the fan control module receives respective power lines  324 A/B from the power distribution board. Each half of the fan control module includes electrical noise isolation circuitry  340 A/B. This electrical noise isolation circuitry  325  A/B, which can be of conventional construction, prevents dirty power signals on the lines  320 A/B caused by electrical noise from the fans being passed back along the power lines  324 A/B and potentially contaminating the otherwise clean power supply to the electronics of the system unit (e.g., the components on the SCSI bus. The provision of clean power supply signals in a telco application is important in order to ensure reliability of operation. Although in the present example the noise isolation circuitry is located in the fan control module, it could be located elsewhere as long as it is effective to isolate the main power lines from fan-related electrical noise. 
     As further shown in FIG. 14, each side  66 A/B of the fan control module comprises control logic  342 A/B which receives a temperature signal from a temperature sensor  344  and adjusts the speed of the fans by adjusting the voltage supplied thereto in accordance with pre-programmed parameters in order to provide a desired degree of cooling. The control logic  342 A/B can be implemented by an ASIC, a programmable logic array, or any other appropriate programmable logic. Alternatively, it could be implemented by software running on a microcontroller or microprocessor module. 
     It should be noted that the fan control module could be implemented in a unitary manner, rather than being divided into two halves. Although in the present instance the fan control module is preferably configured on a single circuit board, this need not be the case. Also, although the temperature sensor is also mounted on the same circuit board, it could be mounted elsewhere. Moreover, although it is preferred that a single temperature sensor is used, with the advantage that the fan speeds of the respective fans can be ramped up in parallel in a controlled manner, more than one temperature sensor could be used. Ideally, in this case they would be located close together and control of the individual fans could be dependent on individual signals but would more preferably be dependent on a function of some or all of the temperature signals. As a further feature, the control logic could be provided with different sets of programmed parameters depending on the number of processors present and could be responsive to the number of processors present. 
     It will be appreciated that although particular embodiments of the invention have been described, many modifications/additions and/or substitutions may be made within the spirit and scope of the present invention. Accordingly, the particular example described is intended to be illustrative only, and not limitative.