Abstract:
A selectable mode field transmitter is configurable to operate in one of a plurality of operating modes having different combinations of function, performance, and power consumption. The selectable mode field transmitter includes a housing, a sensor located within the housing, and transmitter circuitry for transmitting data provided by the sensor to a receiver external to the housing. The transmitter circuitry includes a controller that electrically configures the transmitter circuitry to one of a plurality of operational modes in response to mode selection data received from a source external to the housing. Therefore, the selectable mode field transmitter can be configured based on the needs or requirements of a particular application.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to field transmitters used for process control. In particular, the invention relates to field transmitters having selectable modes of operation. 
         [0002]    Processing plants, such as chemical, petroleum, gas, and pharmaceutical plants require careful control and monitoring of a variety of process variables. Examples of process variables include pressure, temperature, flow, conductivity, and pH. To monitor process variables located throughout a processing plant, devices known as field transmitters have been developed. A field transmitter includes a transducer that responds to a measured process variable with a sensing element and converts the variable to a standardized transmission signal (e.g., an electrical or optical signal) that is a function of the measured process variable. 
         [0003]    Depending on the application, a large number of transmitters may be required to monitor process variables throughout a processing plant. In addition, the application or functionality required of each transmitter may be different, depending on the application. To meet each of the variety of applications and functionality, a manufacturer typically offers a variety of unique transmitters, each providing different functionality and performance. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    A selectable mode field transmitter is configurable to operate in one of a plurality of operating modes having different combinations of function, performance, and power consumption. The selectable mode field transmitter includes a housing, a sensor located within the housing, and transmitter circuitry for transmitting data provided by the sensor to a receiver external to the housing. The transmitter circuitry includes a controller that electrically configures the transmitter circuitry to one of a plurality of operational modes in response to mode selection data received from a source external to the housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a perspective view of a mode selectable transmitter connected to provide sensed process data to a control room. 
           [0006]      FIG. 2  is a simplified block diagram view of a mode selectable transmitter. 
           [0007]      FIG. 3  is a perspective view of a pair of mode selectable transmitters connected to a feature board to provide sensed process data to a control room. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    This disclosure describes a mode selectable transmitter that provides a variety of selectable modes of operation. Each mode provides a different allocation of power or functionality within the mode selectable transmitter. For example, the mode selectable transmitter may have a universal mode that allocates power to provide good overall performance. The mode selectable transmitter may also have a fast response mode, in which power is allocated such that updates to a measured process variable are provided at a faster rate than the universal mode (at the expense of some other functionality of the transmitter). One of the benefits of a mode selectable transmitter is that a single transmitter may be used to meet a variety of unique applications. 
         [0009]      FIG. 1  is a diagram of a process measuring system, which includes mode selectable field transmitter  10 , process pipe  12 , and control room  14 . Mode selectable field transmitter  10  is coupled to monitor a process variable (such as temperature, flow, or pressure) of a fluid or gas contained within pipe  12 . In this embodiment, the monitored process variable is communicated to control room  14  via twisted wire pair  16 , and control room  14  provides power to mode selectable field transmitter  10  via twisted wire pair  16 . 
         [0010]    In other embodiments, mode selectable field transmitter  10  is connected to control room  14  via a digital multi-drop network, which also provides power from control room  14  to mode selectable field transmitter  10 . Examples of digital communication standards used in digital multi-drop networks include Foundation Fieldbus and Profibus. In yet another embodiment, mode selectable field transmitter  10  communicates wirelessly with control room  14 . In this embodiment, mode selectable field transmitter  10  may be powered by a battery system. For purposes of this description, mode selectable field transmitter  10  is described in terms of a common embodiment, in which mode selectable field transmitter  10  is connected to control room  14  via twisted wire pair  16 , although mode selectable transmitter  10  may be employed in a variety of architectures. 
         [0011]    In this embodiment, mode selectable field transmitter  10  may communicate monitored process variables to control room  14  using analog or digital means of communication. Mode selectable field transmitter  10  communicates analog data to control room  14  by controlling the magnitude of the loop current (typically 4-20 mA) to reflect the value of the monitored process variable. The 4-20 mA range of loop current reserved for communicating sensed process variables to control room  14  means that all transmitter operations must function on less than 4 mA of current. Mode selectable field transmitter  10  provides a means for allocating the limited current (i.e., limited power) in order to tailor the performance of mode selectable field transmitter  10  to a particular application. 
         [0012]      FIG. 2  is a simplified block diagram of one embodiment that illustrates components within mode selectable transmitter  10 . Components include sensor  20 , analog-to-digital (A/D) converter  22 , A/D bias circuit  24 , controller  26 , clock  28 , signal processor  30 , local operator interface (LOI)  32 , digital communication circuit  34 , digital-to-analog (D/A) converter  36 , memory device  38  and current measuring circuit  39 . Controller  26  provides control signals to connected components to alter or modify the operation of the components. By selectively controlling the operation of each of the connected components, mode selectable field transmitter  10  can be controlled to operate in a number of different modes. 
         [0013]    Sensor  20  is a transducer connected to monitor a process variable and provide an electrical signal representative of the monitored process variable. Sensor  20  provides the electrical signal to A/D converter  22 , which converts the analog signal provided by sensor  20  to a digital signal that can be communicated to controller  26 . A/D bias circuit  24  regulates the power provided to A/D converter  22 . Controller  26  provides the digital signal representing the measured process variable to signal processor  30 , which performs a series of mathematical operations on the digital signal. For instance, signal processor  30  may execute a signal compensation algorithm to modify the sensed processor variable to account for variances in sensor  20  caused by temperature changes or other factors. 
         [0014]    Signal processor  30  provides a processed signal to controller  26 , which communicates the processed signal to control room  14  using analog or digital means. For analog communication, the measured process variable (following processing by signal processor  30 ) is provided by controller  26  to digital-to-analog (D/A) converter  36 , which modulates the loop current between 4-20 mA based on the measured process variable. For digital communication, controller  26  provides the processed signal (or other data such as diagnostic data) to digital communication circuit  34 , which communicates the data to control room  14 . Digital communication circuit  34  may also receive digital communications such as requests and instructions from control room  14 . 
         [0015]    In addition, controller  26  may provide data (such as the measured process variable) to LOI  32 . In one embodiment, LOI  32  may include a simple display unit (such as an LCD output) that displays the measured process variable locally. In other embodiments, LOI  32  may include functionality that is more complex. For example, LOI  32  may include a display unit and a user interface that allows a user to provide requests and instructions locally to mode selectable field transmitter  10 . 
         [0016]    Modes of operation provided by mode selectable field transmitter  10  are selected by controlling or modifying the operation of components within the transmitter, such as A/D converter  22 , A/D bias  24 , clock  28 , signal processor  30  and LOI  32 . In the embodiment shown in  FIG. 2 , controller  26  provides mode selection instructions to the connected components in order to implement a desired mode of operation. Controlling the components may include selecting between two or more hardware configurations or controlling or modifying software executed by the components. That is, mode selection may be implemented through hardware or software configurations. 
         [0017]    In general, the trade-off between higher performance and power allows some elements to be run in higher performance modes at the expense of other components. By selecting a particular mode of operation, mode selectable field transmitter  10  can be configured to meet the needs of a particular application. 
         [0018]    As shown in  FIG. 2 , controller  26  provides mode selection instructions to A/D converter  22 . Mode selection instructions provided to A/D converter  22  configure the update rate associated with AID converter  22 . The update rate is defined as the rate at which A/D converter  22  provides an electronic signal representing a measured process variable to controller  26 . 
         [0019]    There are several ways to affect the update rate of A/D converter  22 . In one embodiment, a faster update rate is achieved by increasing the internal rate at which A/D converter  22  operates. The internal update rate may be controlled by controller  26  using hardware or software modifications to A/D converter  22 . In addition, controller  26  may configure the internal update rate of A/D converter  22  by selectively increasing or decreasing the clock frequency generated by clock  28 . Increasing the internal update rate associated with A/D converter  22  generally requires an increase in power consumption in order to preserve the accuracy of data provided by A/D converter  22 . Increasing the power consumption of A/D converter is discussed below with respect to A/D bias circuit  24 . 
         [0020]    In another embodiment, the update rate of A/D converter  22  is increased without increasing the internal update rate of A/D converter  22 . In this embodiment, A/D converter  22  is configured to provide updates using less internal data. The net result is increased update rates at the same or similar power consumption requirements, but with less overall accuracy in the data provided by A/D converter  22 . Therefore, controller  26  may configure the update rate associated with A/D converter  22  in a variety of ways. 
         [0021]    In addition to controlling the update rate of A/D converter  22 , controller  26  may also configure the amount of time it takes A/D converter  22  to provide a first update following start-up of mode selectable transmitter  10 . A/D converter  22  may be configured by controller  26  to operate at an increased internal rate or by reducing the data used to form the update (as described above). After the first update is provided, the update rate of A/D converter  22  may revert to a normal update rate. 
         [0022]    In conjunction with mode selection instructions provided to A/D converter  22  to alter the update rates associated with A/D converter  22 , controller  26  may also provide mode selection instructions to A/D bias circuit  24  to regulate the amount of power A/D converter  22  draws from a power supply (not shown). For example, if the internal rate of A/D converter  22  is increased while preserving the accuracy of A/D converter  22 , then A/D converter  22  will typically require additional power consumption. In this scenario, controller  26  configures A/D bias circuit  24  increases the amount of power A/D converter  22  draws from the power supply. If A/D converter  22  is configured to operate at a slower update rate, in which less power is consumed, then A/D bias circuit  24  is configured to decrease the amount of power A/D converter  22  draws from the power supply. 
         [0023]    Controller  26  provides mode selection instructions to clock  28  to control the frequency of clock  28 . The frequency generated by clock  28  determines the speed with which a number of components within mode selectable field transmitter  10  operate, such as signal processor  30  and A/D converter  22 . For example, a higher clock frequency allows signal processor  30  to increase the number of instructions processed or A/D converter  22  to operate at an increased update rate. A higher clock frequency may therefore be used to allow signal processor  30  to run more complex signal conditioning algorithms to improve the accuracy of sensed process variables, or may allow microprocessor to process data at a higher overall pace to allow for higher update rates. As described above with respect to A/D converter  22 , a higher clock frequency may also be used to increase the update rate of A/D converter  22 . However, the trade-off associated with higher clock frequencies is increased power consumption by selectable mode transmitter  10 . 
         [0024]    In addition to affecting performance of signal processor  30  by modifying the clock frequency, performance of signal processor  30  can also be modified by altering the algorithms it executes. Controller  26  may instruct signal processor  30  to execute a particular signal compensation algorithm, or may communicate a new or modified algorithm to signal processor  30 . The signal compensation algorithm executed by signal processor  30  affects the accuracy of the signal compensation provided by signal processor  30 . Typically, a signal compensation algorithm takes into account individual characteristics of sensor  20  and current operating parameters, such as temperature, to provide signal compensation. Complex algorithms may be used to provide better (i.e., more accurate) signal compensation. More complex signal compensation algorithms require additional computing (i.e., execution of more instructions) by signal processor  30 , and therefore requires additional power (or some other tradeoff, such as reduced update rates). 
         [0025]    Controller  26  also provides mode selection instructions to LOI  32 . Control of LOI  32  may be as simple as turning LOI  32  “on” or “off.” In other embodiments, LOI  32  may include a variety of additional functions. For instance, LOI  32  may include an LCD screen that provides advanced functionality, such as graphing capabilities or a user interface. Additional functionality typically requires additional power requirements. Therefore, based on the selected mode of operation, controller  26  controls which features or functions of LOI  32  are made available. For example, to conserve power, controller  26  may turn off a number of the advanced features provided by LOI  32 . 
         [0026]    Finally, based on the selected mode of operation, controller  26  controls how data is communicated to control room  14 . Controller  26  provides mode selection instructions to digital communications circuit  34  and D/A converter  36 . Oftentimes, digital communication is employed only intermittently (e.g., every six months). In this case, controller  26  may conserve power by selectively controlling whether digital communication circuit  34  is “on” or “off.” In addition, controller  26  may provide mode selection instructions regarding how digital communication circuit operates. For example, digital communication circuit  34  may be configured to operate in a high signal mode in which the strength of the digital signal generated by digital communication circuit  34  is increased. This may be beneficial if mode selectable field transmitter  10  is operating in electronically noisy conditions. 
         [0027]    Controller  26  may also provide mode selection instructions to D/A converter  36  to affect analog communications of mode selectable field transmitter  10 . In one embodiment, the response time of D/A converter  36  is selectable configured. The response time of D/A converter  36  defines the amount of time D/A converter  36  takes to respond to a changing input variable. A fast responding D/A converter  36  may be beneficial in applications in which the update rate of A/D converter  22  is increased. However, by increasing the response time of D/A converter  36 , the amount of power consumed by D/A converter  36  increases, and the amount of output noise generated by D/A converter  36  increases. Conversely, power consumption is reduced by decreasing the response time of D/A converter  36 . 
         [0028]    Therefore, in the embodiment shown in  FIG. 2 , controller  26  provides mode selection instructions to one or more of the connected components in order to implement a desired mode of operation. 
         [0029]    Communication of a selected mode of operation to mode selectable field transmitter  10  (and therefore, to controller  26 ) may implemented in a variety of ways. In one embodiment, the desired mode of operation is loaded onto memory device  38 , which may be implemented with Electronically Erasable Programmable Read Only Memory (EEPROM). Controller  30  accesses memory device  38  and provides control signals to each of the necessary components to implement the desired mode of operation. This type of mode selectable field transmitter would typically be programmed only once, and would be programmed to meet the needs of a specific application. For instance, a customer may order a field transmitter with specific operational requirements. A manufacturer or distributor would program memory device  38  with the mode of operation that meets the operational requirements provided by the customer. The benefit of this approach is that a single field transmitter may be easily programmed by a manufacturer or supplier to meet the needs of a particular application. 
         [0030]    In another embodiment, mode selectable transmitter  10  is capable of being switched between different modes of operation while in service. For instance, in one embodiment, mode selection is digitally communicated from control room  14  to controller  26  using the digital communication capabilities of mode selectable field transmitter  10 . The digital communication may include the necessary information for controller  26  to implement the desired mode of operation, or may provide identification of the desired mode of operation stored within memory device  38 . Based on a received digital communication indicating the desired mode of operation, controller  26  provides mode selection instructions to the necessary components. 
         [0031]    In other embodiments, a user communicates the desired mode of operation using LOI  32 , or physical switches connected to provide mode selection data to controller  26 . Based on instructions received locally from LOI  32  or from physical switches, controller  26  provides mode selection instructions to the necessary components. 
         [0032]    In another embodiment, mode selectable field transmitter  10  makes mode selection decisions automatically based in part on any detectable excess or unused power. Field transmitter  10  may be designed to consume less power than specified based on safe design margins to account for manufacturing variances and operating conditions. The net result is that field transmitter  10  may operate under conservative estimates of the power available to field transmitter  10  and the power consumed by different functions of field transmitter  10 . By detecting excess or unused power, field transmitter  10  can select an operating mode to take advantage of the excess or unused power. 
         [0033]    In one embodiment, mode selectable field transmitter  10  measures the quiescent current (i.e., the current presently consumed by field transmitter  10 ) using current measuring device  39  and provides the measured quiescent current to controller  26 . Based on the measured quiescent current used by field transmitter  10 , any unused current may be allocated to various components within mode selectable field transmitter  10 . 
         [0034]    In addition, the quiescent current will typically vary depending on the operating temperature of field transmitter  10 . Therefore, information regarding the operating temperature of field transmitter  10  may be used in conjunction with quiescent current measurements to determine the power requirements of field transmitter  10 , as well as whether any excess or unused current is available to field transmitter  10 . In one embodiment, the quiescent current and operating temperature are measured at start-up. Based on known relationships between quiescent current and operating temperature, current operating requirements of field transmitter  10  may be derived, and the presence of any excess or unused current can be determined. In another embodiment, the quiescent current and operating temperature are measured continuously, wherein excess or unused current determinations are made based on both the quiescent current and the operating temperature. Based on determinations of excess or unused current, controller  26  may provide mode selection instructions to various components to take advantage or utilize any excess or unused power. 
       EXAMPLE MODES OF OPERATION 
       [0035]    The following examples are used to illustrate possible modes of application of mode selectable field transmitter  10 . The following examples are not meant to be exhaustive, but merely illustrative of the variety of different operating modes available to mode selectable field transmitter  10 . 
       Universal Mode 
       [0036]    Performance of selectable mode transmitter operating in universal mode provides good overall performance. Power is allocated to individual components to provide good updates rates, good signal compensation, and at least some LOI operations. This mode of operation is likely to meet a wide range of applications. 
       Fast Response Mode 
       [0037]    As compared to the universal mode, the fast response mode provides faster update rates. This mode of operation may be useful in applications in which the sensed process variable may vary rapidly with time. 
         [0038]    The fast response mode of operation may be implemented in a number of ways. In one embodiment, the fast response mode of operation is implemented by configuring the internal rate of A/D converter  22  to increase the update rate at which data is provided to controller  26 . Controller  26  may increase the internal update rate of A/D converter  22  directly, or may configure clock  28  to provide an increased or higher frequency clock signal to A/D converter  22 . The increase in update rates provided by A/D converter  22  will typically result in an increase in power allocation to A/D converter  22 . To increase the power allocated or drawn by A/D converter  22 , controller  26  configures A/D bias circuit  24  to provide or allow A/D converter  22  to draw additional power from the power supply (now shown). 
         [0039]    The increase in power allocated to these components results in a decrease of power being made available for other functions. For instance, in the fast response mode, power may be decreased to components like LOI  32  or digital communication circuit  34 . The reduction of power to LOI  32  results in a decrease in functionality provided by LOI  32 . Digital communication circuit  34  may be selectively turned “on” and “off”, as required, to reserve power. In addition, power provided to digital communication circuit  34  may be conserved at the expense of the associated reliability of signals provided/received by digital communication circuit  34 . 
         [0040]    In addition, power consumed by signal processor  30  may be reduced by selecting a less complex signal compensation algorithm, at the expense of accuracy provided by the signal compensation algorithm. 
         [0041]    Therefore, in one embodiment the fast response mode allocates additional power to A/D converter  22  and A/D bias  24  to provide increased update rates. The increase of power allocated to these components is based on a reduction of power provided to one or more of the other connected components. Therefore, there may be many sub-modes of operations to select from within the fast response mode. For instance, fast response mode may be implemented by re-allocating power previously consumed by LOI  32 . In another embodiment, power is re-allocated in part from LOI  32 , and in part from a decrease in signal compensation provided by signal processor  30  (resulting in a decrease of power consumed by signal processor  30 ). 
         [0042]    In another embodiment, the fast response mode is implemented not by re-allocating power to A/D converter  22 , but by configuring A/D converter  22  to provide faster update rates at the expense of accuracy provided by A/D converter  22 . That is, controller  26  configures A/D converter  22  to form updates based on less internal A/D converter data than would be used in the universal mode. This allows the update rate of A/D converter  22  to be increased without having to allocate additional power to A/D converter  22 . 
       Fast Start Mode 
       [0043]    The fast start mode measures and communicates a first measured variable as quickly as possible following start-up of field transmitter  10 . Fast start mode may be useful in several applications. For instance, in one application field transmitter  10  is a wireless device (battery operated) that measures process variables at a reduced rate (e.g., data measured once per second). To conserve battery power, field transmitter  10  may power down or “sleep” between measurements. In this application, by reducing the amount of time following start-up required to measure and communicate the process variable, the overall amount of power required to measure and communicate the process variable is reduced, resulting in conservation of battery life. 
         [0044]    In another application, it may be critical to measure and communicate the process variable as quickly as possible following start-up of field transmitter  10 . In this embodiment, a two-step initialization process is employed in which the fast start mode is employed to measure and communicate the process variable at an increased rate upon start-up, and then standard or full initialization of field transmitter  10  is employed after the fast start mode to make subsequent measurements of process variables. 
         [0045]    Similar to the fast response mode, in the fast start mode controller  26  configures A/D converter  22  to operate at a higher update rate. In addition, controller  26  may configure clock  28  to operate at a higher frequency. Because only the first measurement or update is being made at the increased update rate or clock frequency, both the update rate and clock frequency may be run at speeds that would be otherwise unsustainable over longer periods of time. 
         [0046]    Operating in fast start mode (in either of the embodiments described above) requires the allocation of power to components such as A/D converter  22 . Because the fast start mode is typically only implemented for a short amount of time following start-up of field transmitter  10 , the available power can initially be distributed to the necessary components such as A/D converter  22  and signal processor  30  to provide a fast, initial measurement of the process variable. After the first initial measurement is made, power may be reallocated to other components with field transmitter  10  such as LOI  32  and digital communication circuit  34 . 
       High Performance Signal Compensation Mode 
       [0047]    As compared to the universal mode, the high performance signal compensation mode provides the most accurate data signal possible. Signal compensation accounts for individual sensor characteristics that affect the accuracy of the sensed process variable. The accuracy of data processed by signal processor  30  is dependent on the complexity of the signal compensation algorithm. To provide a more accurate data signal, signal processor  30  makes use of more complex signal compensation algorithms that increases the number of instructions (and therefore processing time) executed by signal processor  30 . This mode of operation may be useful in applications in which the accuracy of the sensed process variable is of the utmost importance. 
         [0048]    In one embodiment, the increase in processing time required for the more complex signal compensation algorithm is accompanied by a decrease in the update rate. The decrease in the update rate allows signal processor  30  the necessary time to process each measured process variable using the more complex signal compensation algorithm. In another embodiment, the update rate remains unchanged, but the frequency of clock  28  is increased such that signal processor  30  is able to execute additional instructions required in the more complex signal compensation algorithm without having to reduce the update rate. The increase in the frequency at which signal processor  30  operates increases the power consumed by signal processor  30 . As discussed above, the increase allocation of power to signal processor  30  must be accompanied by a decrease in power allocation elsewhere. 
       Advanced LOI Mode 
       [0049]    In the advanced LOI mode of operation, additional power is allocated to LOI  32  in order to provide additional, or more complex, functionality. For example, additional functionality provided by LOI  32  may include functions such as graphing of acquired measured process variables. To provide the additional functionality provided in the advanced LOI mode, additional power is allocated to LOI  32 . To accommodate the additional power provided to LOI  32 , power is decreased to other components within field transmitter  10 . For example, the re-allocation of power to LOI  32  may come at the expense of update rates or signal compensation. 
       On-Demand LOI Mode 
       [0050]    The on-demand LOI mode is power conversation feature that may be used in conjunction with any of the other listed modes of operation. The on-demand LOI mode maintains LOI  32  in a sleep mode in which no or very little power is allocated to LOI  32 . Upon request, power is temporarily supplied to LOI  32  for a short amount of time (e.g., 30 seconds) to allow a user to view or interact with field transmitter  10  via LOI  32 . This requires controller  26  to re-allocate power temporarily within field device  10  to compensate for the temporary increase in power provided to LOI  32 . This re-allocation of power may result in temporary slowing of update rates or temporary reduction in the accuracy of signal compensation provided by signal processor  30 . The benefit of this mode of operation, is the ability to provide increased functionality or performance of field transmitter  10  during the period in which LOI  32  is in sleep mode. 
       High Speed Analog Mode 
       [0051]    In the high speed analog mode, additional power is allocated to D/A converter  36 , such that the response rate (i.e., the rate at which D/A converter  36  responds to changes in measured process variables) is increased. In one embodiment, additional power allocated to implement the high speed analog mode is provided by operating digital communication circuit  34  in a sleep mode in which little or no power is allocated to digital communication circuit  34 . This mode may be beneficial in applications in which process variables change rapidly, and additional power has already been allocated to increase update rates associated with AID converter  22 . 
         [0052]    Operating digital communication circuit  34  in sleep mode allows power to be reallocated to other components within field transmitter  10 . In addition, digital communication circuit  34  is able to monitor the loop current for digital communications while in sleep mode. If digital activity is detected, then digital communication circuit  34  is switched to an operational mode and power is re-allocated to digital communication circuit  34 . 
       High Signal Mode 
       [0053]    As compared to the universal mode, the high signal mode provides an improved digital communication signal. This mode of operation may be useful in applications in which field transmitter  10  is operating in a electronically “noisy” condition that might otherwise prevent field transmitter  10  from communicating digital information. Once again, additional power allocated to digital communication circuit  34  means that power is reduced to one of the other components within field transmitter  10 . 
       Half Power Mode 
       [0054]    This mode of operation is useful in distributed field device architectures, such as the distributed architecture shown in  FIG. 3 . In the configuration shown in  FIG. 3 , two mode selectable field transmitters  40   a  and  40   b  are connected to monitor process variable data in pipes  42   a  and  42   b , respectively. Mode selectable field transmitters  40   a  and  40   b  may be essentially identical to field transmitter  10 , described with respect to  FIGS. 1 and 2 , except that instead of communicating with control room  14  using analog or digital means, mode selectable field transmitters  40   a  and  40   b  communicate data to feature board  48  using controller area network (CAN) bus  46 . Feature board  48  then communicates data provided by mode selectable field transmitters  40   a  and  40   b  to control room  14  via typical analog or digital communication using twisted wire pair  50 . 
         [0055]    In a distributed application such as the one shown in  FIG. 3 , mode selectable transmitters  40   a  and  40   b  must share the power provided by control room  14 . Thus, the half power mode allows mode selectable transmitters  40   a  and  40   b  to be configured to operate on half as much power as would otherwise be allocated to a single or standalone field transmitter (such as mode selectable field transmitter  10  shown in  FIG. 1 ). 
         [0056]    Typically, in distributed architectures like the one shown in  FIG. 3 , field transmitters are designed to operate in a distributed system in which power is limited. The ability to operate in half power mode allows a generic field transmitter, such as mode selectable field transmitters  40   a  and  40   b , to be used in a distributed environment. In half power mode, each mode selectable field transmitter  40   a  and  40   b  is constrained to operating on half of the power as would otherwise be allocated to a single or standalone field transmitter. The power provided to each mode selectable field transmitter  40   a  and  40   b  may be distributed within each field transmitter as desired. 
         [0057]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.