You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
This is a continuation of U.S. Application Ser. No. 09/713,892 filed Nov. 16, 2000 now U.S. Pat. No. 6,353,942. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to toilets and, more particularly, to vacuum toilet systems. 
     BACKGROUND OF THE INVENTION 
     Vacuum toilet systems are generally known in the art for use in both vehicle and stationary applications. A vacuum toilet system typically comprises a bowl for receiving waste having an outlet connected to a vacuum sewer line. A discharge valve is disposed between the bowl outlet and vacuum sewer line to selectively establish fluid communication therebetween. The vacuum sewer line is connected to a collection tank that is placed under partial vacuum pressure by a vacuum source, such as a vacuum blower. When the discharge valve is opened, material in the bowl is transported to the sewer pipe as a result of the pressure difference between the interior of the bowl and the interior of the sewer line. Conventional vacuum toilet systems also include a source of rinse fluid and a rinse fluid valve for controlling introduction of rinse fluid into the bowl. 
     The components of a conventional vacuum toilet are typically provided separately and are overly difficult to assemble. The discharge valve is typically mounted in a first position, while the rinse valve is mounted in a second, separate position. A flush control unit (FCU) is mounted remote from both valves and provides control signals to the discharge and rinse valve actuators. Accordingly, various mounting brackets, tubing, and wires are needed to interconnect the various components, making assembly overly complicated and time-consuming. 
     In addition, the separate components used in conventional vacuum toilets make repair and maintenance overly time consuming and labor intensive. Maintenance concerns are particularly significant in aircraft applications, in which a number of sub-systems are installed on board. According to general practice in the airline industry, each sub-system includes one or more components which must be replaced in the event of failure, such replacement components being commonly referred to as line replaceable units (LRUs). Presently, the entire vacuum toilet is defined as the LRU for the vacuum toilet system. As a result, an airline must stock one or more replacement toilets in the event of a toilet failure, so that the replacement toilet may be swapped in for the faulty toilet. A “bench test” is then performed on the faulty toilet to determine which components have failed in the toilet. The faulty components are then repaired or replaced (which may include significant disassembly and reassembly of the toilet) so that the toilet may be reused on another aircraft. 
     Each of the steps performed during a toilet repair is overly difficult and time consuming. To remove an entire toilet assembly from an aircraft requires disassembly of at least four self-locking mounting fasteners, an electrical connection, a grounding strap, a potable water line connection, and a waste discharge pipe connection. Each connection may be difficult to access, and may require a particular tool in order to loosen and disconnect. The same connections must then be reconnected for the replacement toilet. 
     Even if it were possible to remove and replace a single toilet component, it would be overly difficult and time consuming to do so. Removal of a component would require disconnection of several wires and pipes, and the components are often located in areas which are difficult to access. Furthermore, it would be difficult to diagnose whether one component or several components had failed. There exists a multitude of combinations of simultaneous component failures, which may lead to trouble-shooting errors and the replacement or repair of non-faulty components. 
     In view of the foregoing, it is apparent that the replacement and repair of conventional toilets is overly time consuming, and requires an airline to maintain a large stock of replacement toilets in the event of equipment failure. 
     Other repairs, which may not require substantial amounts of trouble shooting to identify the failed components, still require significant amounts of disassembly and reassembly. The toilet bowl, for example, is typically formed of stainless steel covered with a non-stick coating that is subject to failure. In conventional toilets, the bowl is a structural, load bearing component that is attached to a base support. In some toilets, the base support is permanently attached to the bowl and therefore the entire toilet must be removed to replace the coating. In other toilets, the bowl is removable from the support base, and therefore fasteners must be removed and the bowl must be disconnected from the rinse fluid and discharge lines. In addition, the rinse ring or nozzle used to direct rinse fluid into the bowl must be removed. Furthermore, if the non-stick coating fails, the bowl must be removed from all of the other toilet components for a re-coating process, steps of which are performed at high temperature to remove the old coating and apply a new coating to the toilet bowl surface. Accordingly, the replacement of a conventional bowl is overly complicated and time consuming. 
     From the foregoing, it will be appreciated that a number of toilets must be kept in stock for replacement in the event of a faulty toilet. The number of stock toilets is further increased due to the left-handed and right-handed discharge configurations of conventional vacuum toilets. Typically, the component layout of a conventional vacuum toilet must be modified according to the type of discharge configuration desired. In addition, different components maybe required, such as a toilet bowl with a left-handed or right-handed discharge. As a result, an airline must have both left- and right-handed discharge replacement toilets on hand, thereby increasing the number of stock parts required. 
     From the above, it will be appreciated that a need exists for a vacuum toilet that is easier to maintain and which reduces the number of stock parts required. 
     SUMMARY OF THE INVENTION 
     In accordance with certain aspects of the present invention, a modular vacuum toilet is provided for use in a vacuum toilet system having a sewer line placeable under partial vacuum pressure and a source of rinse fluid. The modular vacuum toilet comprises a support structure, and a removable bowl supported by the support structure, the bowl defining an outlet and having a rinse fluid dispenser associated therewith. A valve set module is provided having a discharge valve with an inlet in fluid communication with the bowl outlet, an outlet in fluid communication with the sewer line, and a movable discharge valve member disposed between the discharge valve inlet and outlet; a rinse fluid valve having an inlet in fluid communication with the source of rinse fluid, an outlet in fluid communication with the rinse fluid dispenser, and a movable rinse fluid valve member disposed between the rinse fluid valve inlet and outlet; and a flush control unit having a circuit board operably connected to the discharge. valve and rinse fluid valve for controlling actuation of the discharge valve member and rinse fluid valve member. 
     In accordance with additional aspects of the present invention, a method of servicing a vacuum toilet is provided, in which the vacuum toilet is attached to a vacuum toilet system having a sewer line placeable under partial vacuum pressure and a source of rinse fluid, and in which the vacuum toilet includes a waste receptacle defining an outlet and having a rinse fluid dispenser associated therewith. The method comprises providing a first valve set module having a discharge valve with an inlet adapted to engage the receptacle outlet and an outlet adapted for releasable connection to the sewer line, a rinse fluid valve with an inlet adapted for releasable connection to the source of rinse fluid and an outlet adapted for releasable connection to the rinse fluid dispenser, and a flush control unit adapted to control operation of the discharge valve and rinse fluid valve. The discharge valve is detached from the bowl outlet, discharge valve outlet from the sewer line, the rinse fluid valve inlet from the rinse fluid source, and the rinse fluid valve outlet from the rinse fluid line, and the valve set module is removed from the vacuum toilet. A second valve set module is inserted into the vacuum toilet, the second valve set module including a discharge valve with an inlet adapted to engage the receptacle outlet and an outlet adapted for releasable connection to the sewer line, a rinse fluid valve with an inlet adapted for releasable connection to the source of rinse fluid and an outlet adapted for releasable connection to the rinse fluid dispenser, and a flush control unit adapted to control operation of the discharge valve and rinse fluid valve. The second valve set discharge valve inlet is then attached to the bowl outlet, the discharge valve outlet to the sewer line, the rinse fluid valve inlet to the rinse fluid source, and the rinse fluid valve outlet to the rinse fluid line. 
     In accordance with further aspects of the present invention, a method of servicing a vacuum toilet is provided wherein the toilet has a receptacle for receiving waste defining an outlet and includes a rinse fluid dispenser associated therewith. A discharge valve has an inlet in fluid communication with the receptacle outlet, an outlet in fluid communication with a sewer line placeable under partial vacuum pressure, and a moveable discharge valve member disposed between the discharge valve inlet and the discharge valve outlet. A rinse fluid valve has an inlet in fluid communication with a source of rinse fluid, an outlet in fluid communication with the rinse fluid dispenser, and a moveable rinse fluid valve member disposed between the rinse fluid valve inlet and the rinse fluid valve outlet. A flush control unit is adapted to control actuation of the discharge valve member and rinse fluid valve member, in which at least one of the discharge valve, rinse fluid valve, flush control unit, and waste receptacle is a line replaceable unit. The method comprises removing the faulty line replaceable unit from the toilet, and installing a new line replaceable unit into the toilet. 
     In accordance with still further aspects of the present invention, a valve set is provided for use in a vacuum toilet system having a sewer pipe placeable under partial vacuum pressure. The valve set comprises a discharge valve having an outlet, and an outlet pipe attached to the discharge valve outlet and defining a branch. A discharge pipe has a first end adapted to releasably engage the sewer pipe and a second end releasably attachable to the branch in at least a first position corresponding to a left-handed discharge configuration and a second position corresponding to a right-handed discharge configuration. 
     Other features and advantages are inherent in the apparatus claimed and disclosed or will become apparent to those skilled in the art from the following detailed description and its accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are front and rear perspective views, respectively, of a modular vacuum toilet in accordance with the present invention. 
     FIG. 2 is a schematic diagram of the vacuum toilet of FIG.  1 . 
     FIG. 3 is an enlarged view of a tab used to secure a bowl to the frame. 
     FIG. 4 is an enlarged perspective view of the valve set incorporated into the vacuum toilet of FIG.  1 . 
     FIGS. 5A and 5B are perspective views of a discharge valve and actuator incorporated into the valve set. 
     FIG. 6 is a side elevation view, in cross-section, of a rinse valve assembly incorporated into the valve set. 
     FIGS. 7A-D are side elevation views, in cross-section, of the rinse valve assembly showing the various stages of a rinse cycle. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIGS. 1A,  1 B, and  2 , a modular vacuum toilet suitable for use in a vehicle, in accordance with the present invention, is generally referred to with reference numeral  10 . The modular vacuum toilet  10  generally includes a valve set  8 , a frame  20 , and a bowl  36 . The vehicle is provided with a sewer line  11 , a vacuum tank  13  connected to the sewer line  11 , and a vacuum source (not shown) for placing the vacuum tank  13  under partial vacuum pressure. The vehicle further includes a source of rinse fluid  15  connected to a rinse fluid supply line  19 . 
     The frame  20  is provided for supporting the components of the vacuum toilet  10 . As best shown with reference to FIGS. 1A and 1B, the frame  20  includes a bottom member  24  adapted for attachment to a support surface of the vehicle. Vertical supports  26  extend upwardly from the bottom member  24 , and a top member  28  is attached to the vertical supports  26 . The top member  28  is formed with an opening  30  near the front and two slots  29  near the rear thereof. In the illustrated embodiment, an intermediate support  32  is attached between adjacent vertical supports  26 , and a bracket  27  is attached to the bottom member  24 . The bottom member  24 , top member  28 , and bracket  27  are preferably formed of sheet metal, while the vertical supports  26  and intermediate support  32  are preferably formed of tube steel, both of which are readily available and inexpensive. Other materials having sufficient rigidity, however, may also be used. 
     The bowl  36  is provided for receiving waste material. The bowl  36  has a curved sidewall  38  and an out-turned flange  40  extending about an upper edge of the sidewall (FIGS.  1 A and  1 B). The out-turned flange  40  further includes tabs  39  sized for insertion through the slots  29  formed in the frame top member  28 , as best shown in FIG. 3. A bottom of the sidewall is formed in an outlet  42 , and the sidewall  38  is sized for insertion into the opening  30  of the frame top member  28 . The outlet  42  fluidly communicates with a discharge valve  70  through a transfer pipe  44 . The transfer pipe  44  preferably includes a collar  47  sized to frictionally engage and seal with the outlet  42 . 
     To attach the bowl  36  to the frame  20 , the bowl  36  is inserted through the opening  30  and positioned so that the tabs  39  are aligned with the slots  29  and the outlet  42  is aligned with the collar  47 . The bowl  36  is lowered so that the tabs  39  pass through and lock with the slots  29 . Simultaneously, the outlet  42  is inserted into and engages the collar  47 . In this position, the out-turned flange  40  closely overlies the frame top member  28  so that downward forces applied to the bowl  36  are transferred to the frame  20 . As a result, the bowl  36  is not a load-bearing component, and may be made of non-structural materials such as plastic, thin-walled metal (defined herein as less than approximately 0.040″ thick), or other known alternatives. In addition, the bowl  36  is separable from the frame  20  and therefore may be replaced independently from the rest of the toilet  10 . Still further, the tabs  39  may be manipulated manually, and therefore no tools are required to install or remove the bowl  36 . 
     At least one rinse fluid dispenser, such as nozzles  46 , is provided inside the bowl  36  for directing rinse fluid over the surface of the bowl. As best shown in FIGS. 1A and 1B, a plurality of nozzles  46  are spaced about the bowl sidewall  38  and are oriented to direct rinse fluid over portions of the bowl surface. The number of nozzles may be more or less than that shown, depending on the size of the bowl surface to be rinsed. As used herein, the phrase “rinse fluid dispenser” includes the illustrated nozzles  46 , as well as known substitutes, such as spray rings. 
     A vacuum breaker  33  is positioned above the top edge of the bowl  36 , and a first rinse fluid pipe  35   a  extends from the nozzles  46  to the vacuum breaker  33 . A second rinse fluid pipe  35   b  extends from the vacuum breaker  33  to a rinse valve  72 . Quick-disconnect couplings  108   a,    108   b  are provided to connect the first and second rinse fluid pipes  35   a,    35   b  to the vacuum breaker  33 . 
     The separate frame  20  advantageously allows the bowl  36  to be a line replaceable unit (LRU). When the bowl  36  becomes worn or otherwise needs replacement, a maintenance person may simply disconnect the first rinse fluid pipe  35   a  using the quick disconnect coupling  108   a,  manipulate the tabs  39  so that the are disengaged from the slots  29 , and pull upward on the bowl  36  to remove the bowl  36  from the frame  20 . A new bowl  36  may then be inserted into the frame  20  as described above, and the first rinse fluid pipe  35   a  maybe connected to the vacuum breaker  33  using the quick-disconnect coupling  108   a.  As a result, the entire toilet need not be removed and serviced. In addition to facilitating bowl removal and replacement, the frame  20  allows a wider range of materials to be used for the bowl  36 , since the frame  20 , rather than the bowl  36 , supports the load. 
     As best shown in FIG. 1A, the valve set  8  is mounted to the frame bracket  27 . The valve set  8  is preferably attached to the bracket  27  using fasteners that may be manipulated by hand, such as knurled screws  37 . The valve set  8  includes four sub-components: a discharge valve  70 , a rinse valve  72 , a flush control unit (FCU)  74 , and an actuator  76  (FIG.  4 ). The discharge valve  70  includes a discharge valve housing  78  divided into two halves  78   a ,  78   b . As best shown in FIGS. 5A and 5B, the housing  78  includes a pair of inlets  79 ,  80  formed in the housing half  78   a  aligned with a pair of outlets  81 ,  82  formed in the housing half  78   b.    
     The housing  78  further defines a chamber for receiving a discharge valve member, such as valve disk  83 . An axle  84  is attached to the valve disk  83  has two ends  84   a,    84   b.  Holes are formed in the housing halves  78   a,    78   b,  sized to receive the axle ends  84   a,    84   b,  respectively, so that the disk  83  is supported for rotation about the axle  84 . The periphery of the disk  83  is formed with gear teeth  85 , and a pair of apertures  86 ,  87  are formed through the disk  83 . The apertures  86 ,  87  are spaced so that both register simultaneously with the associated inlet/outlet pairs  79 / 81 ,  80 / 82  as the disk  83  rotates. In the illustrated embodiment, the apertures  86 ,  87  and associated inlet/outlet pairs  79 / 81 ,  80 / 82  are spaced 180 degrees apart. 
     According to the illustrated embodiment, the inlet  79  is connected to one end of the transfer pipe  44 , with the other end of the transfer pipe  44  being attached to the bowl outlet  42 . An air intake check valve  45  is attached to the other inlet  80 , and is oriented to allow fluid to flow into the inlet  80  while preventing fluid from discharging out of the check valve  45  (FIGS.  1 A and  2 ). A U-shaped outlet pipe  12  (FIG. 1B) has a first end connected to the outlet  81  and a second end connected to the outlet  82 . The outlet pipe  12  further has a branch  17  leading to a discharge pipe  21 . 
     In accordance with certain aspects of the present invention, the branch  17  and discharge pipe  21  are adapted to provide both right- and left-handed discharge configurations. As best shown in FIG. 1B, the branch  17  includes a pair of spaced pins  160  (only one shown in FIG. 1B) and the discharge pipe  21  a pair of spaced J-shaped slots  162  (only one shown in FIG. 1B) positioned to engage the pins, so that the discharge pipe  21  is removably attached to the branch  17 . The pins  160  and J-shaped slots  162  are preferably spaced 180 degrees apart, so that the discharge pipe  21  maybe positioned for either right- or left-handed discharge simply by rotating the discharge pipe  21  before attachment, without requiring changes to the other toilet components. The free end of the discharge pipe  21  is adapted for releasable connection to the sewer line  11 , such as with a clam shell coupling (not shown). 
     In operation, when the disk apertures  86 ,  87  are aligned with the inlet/outlet pairs  79 / 81 ,  80 / 82 , the discharge valve  70  not only transfers waste from the transfer pipe  44  to the sewer line  11 , but also pulls additional air into the sewer line  11  through the air intake check valve  45 . The additional air intake reduces noise that is normally generated during a flush. 
     The actuator  76  is provided for driving the valve disk  83 . As best shown in FIG. 5A, the actuator  76  includes a spur gear  90  enmeshed with the gear teeth  85  formed about the periphery of the disk  83 . The spur gear  90  is mounted to a rotatable shaft  92 , and a drive is provided for rotating the shaft  92 . The FCU  74  is operably coupled to the actuator  76  to control operation of the actuator. According to the illustrated embodiment, the disk  83  may be rotated in a single direction by ninety degree increments to open and close the discharge valve  70 . Alternatively, the disk  83  may also be reciprocated back and forth across a ninety degree arc to open and close the valve  70 , or the disk  83  may be controlled in other manners according to other disk designs and layouts. 
     The rinse valve  72  is provided for controlling flow of rinse fluid to the bowl  36 . As best shown in FIG. 6, the rinse valve  72  comprises a housing block  100  formed with an inlet bore  101 , defining an inlet  102 , and an outlet bore  103 . The inlet bore  102  is adapted for connection to the rinse fluid line  19  via a quick-disconnect coupling (not shown). An insert  104  is positioned in a downstream portion of the outlet bore  103  and defines an outlet  105 . The outlet end of the insert  104  is barbed to secure one end of the second rinse fluid pipe  35   b  thereto, while the opposite end of the second rinse fluid pipe  35   b  has the quick-disconnect coupling  108   b  (FIGS.  1 A and  1 B). A poppet valve bore  106  is also formed in the housing block  100 , and fluidly communicates with the inlet bore  101 . An annular recess  107  is formed in the housing block  100  concentric with the poppet valve bore  106  to establish fluid communication between the poppet valve bore  106  and the outlet bore  103 . 
     The rinse valve  72  includes a rinse valve member, such as a ball valve  110 , which is disposed in the outlet bore  103  for selectively establishing fluid communication between the outlet bore  103  and the outlet  105 . The ball valve  110  includes a shaft  111  and a valve member  112  having a flow passage  113  extending therethrough. A seal  114  is provided downstream of the valve member  112  for preventing leakage between the valve member  112  and the downstream portion of the outlet bore  103 . As shown in FIG. 6, the flow passage  113  is perpendicular to the outlet bore  103 , thereby preventing fluid flow. The ball valve  110  is rotatable, however, to align the flow passage  113  with the outlet bore  103 , thereby establishing fluid communication between the upstream portion of the outlet bore  103  and outlet  105 . 
     In the preferred embodiment, the top of the shaft  111  is adapted to mechanically engage the axle end  84   a,  as best shown in FIG. 4, so that rotation of the disk  83  also rotates the ball valve  110 . In the illustrated embodiment, the shaft  111  is formed with a key  115 , while the axle end  84   a  has a slot  116  sized to receive the key  115 . As a result, a separate actuator is not required to actuate the ball valve  110 , thereby reducing cost and space requirements for the toilet. 
     The rinse valve  72  further includes a fuse valve  120  for metering rinse fluid flow through the rinse valve when the ball valve  110  is open. As used herein, the phrase “fuse valve” indicates a valve that actuates after a set value of fluid has passed therethrough. As best shown in FIG. 6, a bonnet  121  is attached to the housing block  100  to close off the poppet valve bore  106  and the recess  107 . A flexible diaphragm  122  is attached between the housing block  100  and the bonnet  121  to define a pilot chamber  117  above the diaphragm  122  and a flow chamber  118  below the diaphragm  122 . As illustrated at FIG. 6, the diaphragm  122  is in a closed position, in which the diaphragm  122  engages an annular intermediate wall  123  extending between the poppet valve bore  106  and recess  107 , thereby closing off fluid communication between the poppet valve bore  106  and recess  107 . A poppet valve  124  is disposed inside the poppet valve bore  106  and is attached to the diaphragm  122 , so that the poppet valve  124  moves with the diaphragm  122 . The top of the poppet valve  124  is formed with a pilot port  125 , and flow ports  126  extend radially through a sidewall of the poppet valve  124 . A spring  127  is disposed in the poppet valve port for biasing the diaphragm  122  away from the intermediate wall  123  toward an open position, in which fluid communication is established between the poppet valve bore  106  and the recess  107 . 
     The fuse valve  120  limits the amount of rinse fluid allowed to flow through the rinse valve  72  when the ball valve  110  is open. During operation, the ball valve  110  is normally in a closed position to prevent flow of rinse fluid through the rinse valve  72 . The rinse fluid flows through both the pilot port  125  to register at the pilot chamber  117 , and through the flow ports  126  to register in the flow chamber  118 . Because there is no rinse fluid flow, the rinse fluid pressure is the same in both the pilot chamber  117  and the flow chamber  118 , so that the spring  127  urges the diaphragm  122  and poppet valve  124  to the open position, as shown in FIG.  7 A. 
     In response to a flush command, the ball valve  110  is rotated to the open position so that the ball valve flow passage  113  communicates the outlet bore  103  to the outlet  105 , thereby creating fluid flow through the valve  72  (FIG.  7 B). During fluid flow, the rinse fluid experiences a pressure drop as it passes through the flow ports  126 , thereby reducing the fluid pressure in the flow chamber  118  while the pressure in the pilot chamber  117  stays substantially the same. The resulting pressure differential across the diaphragm  122  ultimately overcomes the force of the spring  127  so that the diaphragm  122  and poppet valve  124  move to the closed position, as shown in FIG.  7 C. When the diaphragm is in the closed position, fluid flow through the rinse valve  72  is again cut off, this time by the engagement of the diaphragm  122  with the intermediate wall  123 . Because of the fuse valve  120 , the volume of rinse fluid passing through the open ball valve  110  is substantially constant from flush to flush, regardless of the rinse fluid pressure supplied to the rinse valve  72 . It will also be appreciated that the fuse valve  120  provides a redundant shut-off, so that the ball valve  110  or the fuse valve  120  may be used to stop rinse fluid flow should the other fail. 
     The rinse valve  72  further includes a face valve  130  for returning the diaphragm  122  back to the open position after the ball valve  110  is subsequently closed. Referring to FIG. 6, a bypass bore  131  is formed in the housing block  100  that connects the inlet bore  101  to an auxiliary bore  132 . A reset bore  134  intersects the bypass bore  131  and communicates with a ball valve bore  135  formed in the housing block  100 . A reset insert  136  is inserted in the reset bore  134  and has a top surface adapted to engage a bottom of the ball valve  110 . The ball valve  110  is formed with reset passages  137  extending into the ball valve  110  to a transverse passage  138  extending entirely through the ball valve  110 . The reset passages  137  are located on the ball valve  110  so that they align with the reset insert  136  only when the ball valve  110  is in the closed position. The seal  114  prevents rinse fluid from leaking from the transverse passage  138  to the outlet  105 . No seal is provided upstream of the ball valve  110  so that, when one of the reset passages  137  is aligned with the insert  136 , fluid communication is established from the inlet bore  101 , through the bypass and reset bores  131 ,  134  and one of the reset passages  137  to the flow chamber  118 . 
     According to the illustrated embodiment, the rinse valve  72  also includes a drain valve  133  disposed in the auxiliary bore  132  to provide freeze protection, as is well known in the art. 
     In operation, the diaphragm  122  moves to the closed position while the ball valve  110  is open, thereby stopping rinse fluid flow through the rinse valve  72  (FIG.  7 C). With the ball valve  110  in the open position, neither reset passage  137  is aligned with the reset insert  136 . The ball valve  110  is subsequently closed, thereby aligning one of the reset passages  137  with the insert  136  and establishing fluid communication from the inlet bore  101  to the flow chamber  118  (FIG.  7 D). The incoming rinse fluid pressure registers at the flow chamber  118 , so that the flow chamber reaches the same pressure as the pilot chamber  117 . With the differential pressure across the diaphragm  121  removed, the spring  127  is again allowed to urge the diaphragm  121  to the open position, thereby resetting the fuse valve  120  to the position shown in FIG.  7 A. 
     In the preferred embodiment, a position sensor is used to provide feedback regarding poppet valve position feedback. In the illustrated embodiment, a magnet  140  is attached to the poppet valve  124 , and a hall effect switch  141  is located outside of the bonnet  121  in a switch enclosure  142  attached to the bonnet  121  (FIG.  6 ). The hall effect switch  141  provides a signal that varies according to the position of the magnet  140  to indicate the position of the poppet valve  124 . The poppet valve position signal may be used for diagnostic purposes such as fault detection by comparing the position signal to the position of the disk  83  or ball valve  110 . 
     The FCU  74  comprises a housing  150  attached to the discharge valve housing half  78   b  opposite the rinse valve  72  (FIG.  4 ). The housing  150  encloses one or more circuit boards (not shown) for controlling operation of the toilet  10 . In addition to the typical inputs and outputs, the FCU  74  also receives feedback from the poppet valve position sensor  141 . 
     The FCU housing  150  further houses a position sensor for determining the position of the disk  83 . As best shown in FIG. 5A, magnets  152  are attached to the axle end  84   b  of the disk  83 . The axle end  84   b  extends into the FCU housing  150 , so that the magnets  152  are positioned proximal the control board. Hall effect switches  154  are provided directly on the circuit board for sensing the magnets  152  and thus determining the rotational position of the disk  83 . In the illustrated embodiment, a pair of magnets  152  are attached to the axle end  84   b,  and a pair of hall effect switches  154  are attached to the circuit board. The switches  154  actuate between on and off positions depending on the proximity of the magnets, thereby indicating the position of the disk  83 . As a result, the position of the disk  83  is directly sensed rather than inferring disk position based on actuator position. In addition, the switches  154  are located inside the FCU housing  150  and are therefore isolated from contamination due to lubrication or other material. 
     With the above construction, the valve set  8  is quickly and easily removed and replaced. To remove the valve set  8 , the discharge pipe  21  is disconnected from the sewer line  11 , the rinse valve inlet  102  is disconnected from the rinse supply line  19 , and the quick-disconnect coupling  108   b  of the second rinse fluid pipe  35   b  is disconnected from the vacuum breaker  33 . The knurled screws  37  are then removed from the bracket  27  and the valve set  8  with attached transfer pipe  44  is lowered so that the transfer pipe disengages the bowl outlet  42 . Thus the valve set  8  is removed with the transfer pipe  44 , outlet pipe  12 , discharge pipe  21 , and second rinse pipe  35   b.  A new valve set  8 , also having a new transfer pipe  44 , outlet pipe  12 , discharge pipe  21 , and second rinse pipe  35   b  may then be attached to the bracket  27  and reconnected. 
     From the foregoing, it will be appreciated that the valve set  8  of the present invention incorporates all of the valve and control apparatus. The rinse valve  72 , FCU  74 , and actuator  76  are all mounted to the discharge valve  70  to create an LRU, wherein a single module may be targeted for maintenance in the event of a valve or control failure. The wiring between the components may remain in place so that, in the event of a valve or control failure, only the piping connections between the valve set  8  and the drain, sewer, and rinse water piping need be undone to remove the valve set  8 . 
     Maintenance of the modular vacuum toilet  10  is entirely different from that of conventional vacuum toilets. Instead of defining the entire toilet as an LRU, the toilet  10  defines individual components or groups of components as LRUs. The bowl  36  may be independently removed from the toilet  10  and replaced. Similarly, the valve set  8  may be separately removed from the toilet  10 . Furthermore, the individual components may be quickly removed with the use of few or no tools. 
     The branch  17  and discharge pipe  21  of the valve set  8  are adapted to provide both right- and left-handed discharge configurations without additional modifications to the other toilet components, thereby further reducing the number of parts needed in stock. 
     The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications would be obvious to those skilled in the art.

Summary:
A modular vacuum toilet, and a method of servicing such a toilet, are disclosed. The toilet is used with a vacuum toilet system having a sewer line placeable under partial vacuum pressure and a source of rinse fluid. The modular vacuum toilet comprises a frame and a removable bowl engaging and supported be the frame, the bowl defining an outlet and having a rinse fluid dispenser associated therewith. The modular vacuum toilet also has a valve set module. The valve set module includes a discharge valve having an inlet in fluid communication with the bowl outlet, an outlet in fluid communication with the sewer line, and a movable discharge valve member disposed between the discharge valve inlet and outlet. A rinse fluid valve is also incorporated into the valve set module and has an inlet in fluid communication with the source of rinse fluid, an outlet in fluid communication with the rinse fluid dispenser, and a movable rinse fluid valve member disposed between the rinse fluid valve inlet and outlet. The valve set module further includes a flush control unit having a circuit board operably connected to the discharge valve and rinse fluid valve for controlling actuation of the discharge valve member and rinse fluid valve member.