Abstract:
An I/O circuit for use with an industrial controller provides a photovoltaic optical isolator communicating between a controller and a field-side of the I/O circuit, the photovoltaic optical isolator eliminating the need for high wattage power dropping circuits for powering the field-side of the I/O circuit from high-voltage field signals. The field effect transistor type transistors permit use of the low-power photovoltaic optical isolator while allowing flexible connections to various field circuit types.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to I/O modules for industrial control systems and in particular to a low heat dissipation I/O module that may flexibly accommodate the digital control of AC or DC power. 
         [0002]    Industrial controllers are specialized computer systems used for the control of industrial processes or machinery, for example, in a factory environment. Industrial controllers differ from conventional computers in a number of ways. Physically, they are constructed to be substantially more robust against shock and damage and to better resist external contaminants and extreme environmental conditions. The processors and operating systems are optimized for real-time control and execute languages allowing ready customization of programs to comport with a variety of different controller applications. Further, the controllers have a highly modular architecture that allows different numbers and types of input and output modules to be used to connect the controllers to the process or machinery to be controlled. This modularity is facilitated through the use of special “control networks” suitable for highly reliable and available real-time communication. Such control networks (for example, Ethernet IP) differ from standard communication networks (e.g. Ethernet) by guaranteeing maximum communication delays, for example, as obtained by pre-scheduling the bandwidth of the network, and/or providing redundant communication capabilities to high-availability. 
         [0003]    As part of their enhanced modularity, industrial controllers may employ I/O modules dedicated to a particular type electrical signal and function, for example, detecting input AC or DC signals or controlling output AC or DC signals. Each of these I/O modules may have a connector system allowing them to be installed in different combinations in a housing or rack along with other selected I/O modules to match the demands of the particular application. Multiple racks may be located at convenient control points near the controlled process or machine to communicate with a central industrial controller via a special control network such as Ethernet IP. 
         [0004]    Normally different I/O modules are required for the control of AC and DC signals reflecting differences in the control devices used for the signals (e.g. triacs versus transistors). Different I/O DC modules may be required depending on whether the DC signal is “sourced” from the DC output module or “sinked” into the DC output module. 
         [0005]    Normally the circuitry of the I/O module communicating with the industrial controller (controller-side) is optically isolated from the circuitry communicating with the controlled process or machine (field-side) in order to prevent faults in the field-side from damaging the industrial control system or other I/O modules. For output circuits (such as AC output or DC output circuits), it can be necessary to provide a source of low voltage DC for operation of the field-side circuitry independent of the power available on the controller-side. Often this is done through the use of a voltage dropping resistance converting the high voltages controlled by the I/O module into a lower voltage suitable for powering the field-side circuitry. Such voltage dropping resistors are often physically large to handle the necessary power dissipation and must be spaced apart to aid in dissipating the incident heat. 
         [0006]    The control of AC voltages is often provided by a triac or thyristor that may be switched at a zero crossing of the AC signal (to reduce electrical switching noise and power dissipation) by a conventional trigger circuit relying on a phase shifted version of the AC signal being controlled. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides an output circuit for an I/O module eliminating the need for high wattage, power-dropping resistors for generating field-side power through the use of a photovoltaic optical isolator which generates its own electrical power through optical energy conversion. The use of the photovoltaic optical isolator is rendered practical by switching the controlled power using high-voltage field effect transistors instead of conventional triacs. Slow turnoff times incident to the low power output of a photovoltaic optical isolator and high impedance of the field effect transistors are managed by an active pulldown transistor detecting the beginning of control voltage reduction. The use of FETs allows the circuit to be used both for AC and DC sourcing and DC sinking operation. 
         [0008]    Zero crossing detection may be obtained by the use of an independent zero crossing detector allowing flexible control of AC waveforms by switching the waveform based on a variable phase relationship with the zero crossing of the waveform. 
         [0009]    More specifically, one embodiment of the present invention provides an I/O module circuit for use in an industrial control system and having a housing supporting terminals for receipt of electrical power to be controlled by the I/O module circuit. A circuit contained in the housing includes: at least one field effect transistor having a source and drain in series between the terminals to control current flow through the terminals according to a signal received by a gate of the field effect transistor, a photovoltaic voltage optical isolator having a photovoltaic stack connecting to the gate of the field effect transistor and optically isolated from a light emitting diode, and a releasable electrical connector also supported by the housing receiving a signal indicating a desired control of electrical power at the terminals of the I/O module, the signal controlling current through the light emitting diode. 
         [0010]    It is thus a feature of at least one embodiment of the invention to provide an output I/O module that does not require voltage dropping resistors for developing field-side power. It is a feature of at least one embodiment of the invention to provide an I/O module with reduced heat dissipation allowing more compact design and/or the elimination of cooling features such as fans or fans subject to failure or environmental contamination. 
         [0011]    The circuit contained in the housing may include two series connected FETs and the photovoltaic stack is connected from a junction at connected sources of the two series connected FETs to gates of the two series connected FETs. 
         [0012]    It is thus a feature of at least one embodiment of the invention to provide a versatile output that can be used to control AC and DC sinking and DC sourcing control configurations. It is a feature of at least one embodiment of the invention to provide for a high impedance solid-state device that may be powered practically from a photovoltaic stack. 
         [0013]    The I/O module circuit may further include an active pulldown transistor connected between the gates of the first and second FETs and the junction at the two series connected FETs. 
         [0014]    It is thus a feature of at least one embodiment of the invention to provide rapid turn off to the FETs to reduce power dissipation in the FETs during turnoff. 
         [0015]    The I/O module circuit may further include a diode separating the photovoltaic stack from a connection of the active pulldown transistor to the gates of the FETs and a control input of the active pulldown transistor may be connected to the photovoltaic stack on opposite sides of the diode from the gates of the FETs. 
         [0016]    It is thus a feature of at least one embodiment of the invention to provide a system for rapid turn off of the FETs that does not provide substantial load (or power dissipation) when the FETs are in the on state. It is a further feature of at least one embodiment of the invention to preserve a full on state of the FET transistors as driving voltage decays until a rapid switch off is initiated, thereby further reducing unnecessary power dissipation in the FETs. 
         [0017]    The I/O module circuit may further include a pulldown resistor across photovoltaic stack. 
         [0018]    It is thus a feature of at least one embodiment of the invention to provide a mechanism to flexibly control and provide certainty in switch off speed of the system. 
         [0019]    The I/O module circuit may further include a zero crossing detector connected to the terminals for detecting a zero crossing of an AC waveform connected to the terminals to control a switching of the FET at a time related to a zero crossing of the AC waveform. 
         [0020]    It is thus a feature of at least one embodiment of the invention to provide a circuit that can switch rapidly on or off during or a predetermined offset in time from zero crossings of the control power for reducing power dissipation and electrical interference or for providing modulation of an AC signal for power control and the like. 
         [0021]    The I/O module circuit may further include an electronic circuit receiving an input of the zero crossing detector and the signal indicating a desired control of electrical power at the terminals of the I/O module and operating to control application of the signal indicating a desired control of electrical power at the terminals of the I/O module to the light emitting diode to be coincident with a zero crossing of the AC waveform. 
         [0022]    It is thus a feature of at least one embodiment of the invention to permit flexible change in the phase relationship between zero crossing and the switching of the transistors for novel control applications. 
         [0023]    In one embodiment, the invention provides for field-side power that is floating with respect to the control side power allowing the output transistors to be used in either sink or source mode without a special jumper or the like. 
         [0024]    Specifically, the invention may provide an I/O module circuit for use in an industrial control system having a housing, with terminals supported by the housing for receipt of electrical power to be controlled by the I/O module circuit and a circuit contained in the housing. The circuit may provide at least one field effect transistor having a source and drain in series between the terminals to control current flow through the terminals according to a first electrical signal received by a gate of the field effect transistor. A releasable electrical connector supported by the housing receives a second electrical signal indicating a desired control of electrical power at the terminals of the I/O module and a power isolator receives at a first controller-side the second signal, and providing at a second field-side output power and the first signal communicating information of the second signal. The output power is derived from input electrical power received at the first controller-side of the power isolator but isolated and floating with respect to that input power and the first signal is isolated and floating with respect to the second signal. 
         [0025]    It is thus a feature of at least one embodiment of the invention to permit an output FET be used in sinking or sourcing configuration by floating the gate control with respect to external field-side power and its connection. 
         [0026]    In one embodiment of the invention, the invention provides an isolated source of field-side power using magnetic coupling. 
         [0027]    Specifically, the invention may provide an I/O module circuit for use in an industrial control system having a housing, with terminals supported by the housing for receipt of electrical power to be controlled by the I/O module circuit, and a circuit contained in the housing. The circuit may include at least one field effect transistor having a source and drain in series between the terminals to control current flow through the terminals according to a first signal received by a gate of the field effect transistor and at least one transformer-coupled isolator receiving input electrical power referenced to a second signal isolated with respect to the second signal to provide output electrical power referenced to the first signal of floating with respect to the second signal. 
         [0028]    It is thus a feature of at least one embodiment of the invention to provide for substantial isolated power transfer from the control side to the field-side using magnetic coupling to provide for rapid turn off and turn off times of the FET commensurate with low impedance driving. 
         [0029]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a perspective view of a remote I/O rack of a type suitable for use with the present invention showing a rack housing having multiple I/O modules contained therein for communication with an industrial process via terminals and a remote industrial controller via a network cable; 
           [0031]      FIG. 2  is a block diagram showing the I/O rack of  FIG. 1  in the context of the industrial control system incorporating multiple remote I/O racks and an industrial controller; 
           [0032]      FIG. 3  is a block diagram of the remote I/O rack showing connectors for attaching I/O modules installed in the rack to a common bus communicating with the network cable and electrical power, and showing one I/O module providing a power control circuit and a zero crossing circuit controlled by a microcontroller; 
           [0033]      FIG. 4  is a detailed schematic of the power control block of  FIG. 3  such as may receive a control signal to control an AC or DC signal at output terminals; 
           [0034]      FIG. 5  is graphs of two waveforms at points within the power control block during a turning off of the controlled power and showing the action of an active pulldown transistor to provide for sharp turn off of controlled power; 
           [0035]      FIGS. 6   a - c  are different connection configurations possible between the I/O module of the present invention and the controlled circuitry showing configurations for controlling AC and DC signals in sourcing and sinking configurations; 
           [0036]      FIG. 7  is an exploded perspective view of a circuit board realizing a portion of the present invention showing a replaceable FET module; 
           [0037]      FIG. 8  is a figure similar to  FIG. 3  showing a DC output module; 
           [0038]      FIG. 9  is a detailed schematic of the power control block of  FIG. 8  such as may receive a control signal to control a DC signal at output terminals; 
           [0039]      FIG. 10  is a simplified block diagram of a magnetic isolator used in the circuit of  FIG. 9 ; 
           [0040]      FIGS. 11   a  and  11   b  are figures similar to  FIGS. 6   b  and  6   c  showing possible sinking and sourcing configurations of the output of the power control block; and 
           [0041]      FIG. 12  is a figure similar to  FIG. 7  showing a replaceable FET module for the configuration of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0042]    Referring now to  FIGS. 1 and 2 , an I/O rack  10  may provide for a rack housing  12  having multiple openings  14  for orienting and supporting corresponding replaceable I/O modules  16 . The I/O modules  16  generally provide an electrical interface between an industrial controller  17  and an industrial process or machine  18 , the latter having actuators presenting loads  20  as powered by external field-side power sources  22  which may be AC sources (as shown) or DC sources as will be described. Each of the I/O modules  16  may have exposed screw terminals  24  or similar releasable terminals for connecting circuitry of the I/O module  16  via electrical conductors  26  to the load  20  and field-side power source  22  of the industrial process or machine  18 . 
         [0043]    The housing  12  of the I/O rack  10  may be supported, for example, on a DIN rail  29  of a type known in the art or may provide for other mounting arrangements allowing it to be located close to the desired controlled loads  20 . The I/O rack  10  may connect via a control network  28 , for example, Ethernet IP, to industrial controller  17  for communication of control data between the I/O modules  16  of the I/O rack  10  and the industrial controller  17 . 
         [0044]    Generally, the industrial controller  17  will include at least one processor  30  executing a stored control program  32  contained in a communicating memory  34  to provide for communication with the control network  28  to exchange signals with the I/O rack  10  for control of the industrial process or machine  18  according to a stored control program  32 . The stored control program  32  reads signals indicating the state of the industrial process or machine  18  (from sensors or the like) and writes outputs to the I/O rack  10  to control the loads  20  according to those sensed inputs and the logic of the control program  32 . The industrial controller  17  may include a connection to a user interface terminal  35  (for example including a display, computer, keyboard, and mouse or the like) allowing programming and other control of the industrial controller  17  and providing for output of information to an operator. Industrial controllers  17  suitable for use with the present invention include, for example, programmable controllers commercially available from Rockwell Automation, Inc. 
         [0045]    Some of the I/O modules  16  of each I/O rack  10  may include a provision for an isolation barrier  36  (indicated by the like numbered dotted line in  FIG. 2 ) between the electrical power and circuitry of the industrial process or machine  18 , and directly connected portions of the I/O module  16  and the circuitry of the industrial control system including the industrial controller  17 , the network  28 , the racks  10  and remaining portions of the I/O modules  16 . Circuitry on the side of the isolation barrier  36  toward the industrial process or machine  18  will be termed “field-side” circuitry and circuitry on the side of the electrical isolation towards the industrial controller  17  will be termed “controller-side” circuitry. Electrical isolation, as is understood in the art, may be accomplished by an optical isolator or transformer that prevents DC conduction through the isolation barrier  36  to prevent damage to the controller-side circuitry in the event of fault on the field-side circuitry. This isolation prevents simple sharing of electrical power between the circuits on the controller-side in the field-side by conductors spanning the isolation barrier  36 . 
       I. AC Output Module 
       [0046]    Referring now to  FIG. 3 , each I/O rack  10  may provide a backplane  40  providing a set of electrically releasable electrical connectors  42  communicating with a backplane bus  44  providing parallel or serial digital communication and electrical power. Electrical power may be provided by a power supply  43 . The digital communication on the backplane bus  44  may be moderated by a rack controller  46  including network circuit  45  communicating with the industrial control network  28 . The rack controller  46  and network circuit  45  may be, for example, one or more electronic processors executing a stored program stored in memory or the like. 
         [0047]    The electrical connectors  42  forming the backplane bus  44  may connect with corresponding electrical connectors  47  positioned at the rear of each I/O module  16  allowing electrical connection between controller-side circuitry of the I/O rack  10  and the controller-side circuitry of the I/O module  16  for directly communicating data and power therebetween when an I/O module  16  is inserted into the I/O rack  10 . Within each I/O module  16 , a module controller  48  may execute a stored program  50  to coordinate functions of the I/O module  16  by communicating with multiple output circuits  52  and corresponding single shared or multiple zero crossing detector circuits  54 . Each of these output circuits  52  and zero crossing detector circuits  54  may provide internally for isolation barrier  36  between controller-side and field-side circuitry as will be described. Each of these output circuits  52  and zero crossing detector circuits  54  also communicates with a shared set of terminals  24  (only three shown for clarity) that may, for example, be screw type terminals releasably receiving electrical conductors  26 . Generally two of the terminals  24  will connect to line voltage (if needed for zero crossing detection) and a third will provide a switched output. 
         [0048]    As will be understood in the art, the zero crossing detector circuit  54  provides a detection of zero crossing of the waveform applied to the terminals  24  connected to the line voltage to produce a zero crossing signal  55  transmitted to the module controller  48  and indicating the timing of the zero crossing of the AC waveform on the terminals  24 . The zero crossing detector circuits may be any of a wide variety of different circuit types, for example, circuits receiving an AC signal through a full wave rectifier that drives a conventional optocoupler. The optocoupler&#39;s input may be in series with a voltage threshold sensitive device (such as a zener diode or a properly configured transistor) providing a switching near the zero crossings of the AC signal to produce a pulse zero crossing signal  55  near the zero crossing of the AC signal. 
         [0049]    The module controller  48  receiving this zero crossing signal  55  provides a switch state signal  56  for each output circuit  52  indicating a desired control of electrical power at the terminals  24  (based on signal  56 ). In one mode of operation, conduction between terminals  24  (switch state) is changed by output circuit  52  (either by opening or closing the circuit between terminals  24 ) only at the zero crossings of the AC waveform on the terminals  24  to reduce power dissipation in the internal switching elements used to control power conducted through the terminals  24  to accommodate the architecture of the switching element characteristics (for example when a thyristor is used), and/or to reduce the generation of electromagnetic interference. 
         [0050]    In another mode of operation, conduction between the terminals  24  may be changed by the output circuit  52  (again either by opening or closing the circuit between terminals  24 ) at a predetermined offset from the zero crossing. This allows modulation of the AC signal, for example, to control power to a load by switching the AC signal on and off with different duty cycles much in the manner of pulse width modulation. Unlike conventional triac circuitry, the FET transistors  76  and  78  may both switch on and off at any point in the cycle of the AC waveform. 
         [0051]    Referring now to  FIG. 4 , the output circuit  52  may receive a switch state signal  56  from the controller  17 , for example, at the base of the PNP transistor  60 . The PNP transistor  60  may in turn drive an LED  62  in a photovoltaic optocoupler  64 . The emitter of the PNP transistor  60 , for example, may be connected to a controller-side power source  67  being a positive DC voltage and a collector of the PNP transistor  60  may connect to through current-limiting resistor  65  to the anode of the LED  62  whose cathode is connected to ground of the controller-side power source  67 . Light from the LED  62 , when the transistor  60  is on, may generate electrical voltage on the photovoltaic pile  66  comprised of a set of series connected photodiodes. The photovoltaic pile  66  may provide a positive voltage at an anode of the photovoltaic pile  66  connected to circuit point B with reference to a cathode of the photovoltaic pile  66 , the latter connected to field circuit internal ground  68 . Thus, voltage applied at signal  56  results in an increase in voltage at circuit point B. A bypass resistor  81  is placed to shunt the photovoltaic pile  66  to improve the response time of the circuit as will be described below. 
         [0052]    Circuit point B connects to the anode of a diode  70  and to the base  72  of a PNP pull-down transistor  74  whose collector is connected to the field circuit internal ground  68  and whose emitter connects with the cathode of diode  70  at circuit point A. Circuit point A connects in turn to the gates of two back-to-back series connected metal oxide semiconductor field effect (MOSFET) transistors  76  and  78 . Transistor  76  has its drain connected to a first terminal  24  and its source connected to the source of second transistor  78 . Second transistor  78  has its drain connected to the second terminal  24 . The junction between transistors  78  and  76  is connected to the field circuit internal ground  68 . An MOV voltage-limiting device  80  is placed to shunt the terminals  24  to protected transistors  76  and  78  from high-voltage spikes from conductors  26  and load-stored inductive energy. 
         [0053]    Transistors  76  and  78  are connected in series such that when transistor  76  is conducting in first quadrant mode, transistor  78  is conducting in third quadrant mode. If polarity is reversed, transistor  76  will conduct in third quadrant mode and transistor  78  will conduct in first quadrant mode. First quadrant mode is drain positive and source negative, third quadrant is source positive and drain negative with n channel devices. 
         [0054]    Referring now to  FIGS. 4 and 5 , it will be appreciated that when the LED  62  is conducting (before time t 0 ) the voltage  82  at circuit point B rises to a high state turning off pull-down transistor  74  and biasing point A and the gates of transistors  76  and  78  to a similar high state at voltage  84  substantially equal to voltage  82 . This biasing of the transistors  76  and  78  allows bipolar conduction of current between terminals  24  through the transistors  76  and  78 . 
         [0055]    When the LED  62  is deenergized (after time t 0 ), the voltage  82  at point B drops as a result of current passing through a bypass resistor  81  shunting the photovoltaic pile  66 . As the voltage  82  at point B drops, the voltage at point A remains relatively high by virtue of the blocking action of diode  70  and the high impedance of the gates of transistors  76  and  78 , continuing the conduction of the transistors  76  and  78  in a low impedance conduction mode. As long as the declining voltage  82  at point B remains above a transistor turn on voltage  86 , pull-down transistor  74  is back biased to remain off, preserving the voltage level of A at voltage  84 . When the voltage at point B drops below the voltage  86 , pull-down transistor  74  turns on, rapidly pulling the voltage level A near to ground. This rapid turn off reduces power dissipation in the transistors  76  and  78  during a turn off of these transistors. As noted, turn off and turn on of the transistors  76  and  78  may be synchronized to zero crossings of an AC waveform, with the decay of waveform at point B being relatively short by virtue of the bypass resistor  81 . 
         [0056]    Referring now to  FIGS. 6   a - 6   c , the use of MOSFET transistors  76  and  78 , as shown, permit a number of different configurations of the load  20  and field-side power source  22  and connection to the terminals  24  of the I/O module  16 . In a conventional AC control situation, per  FIG. 6   a , an AC field-side power source  22  may be connected between a field-side ground and either of the terminals  24  and the load  20  connected between the remaining terminal  24  and the field-side ground  84 . Similarly, but not shown, the load  20  may be placed in any other location in the circuit path shown including between the field-side power source  22  and the terminal  24 . In this way versatile AC signal control may be obtained. 
         [0057]    It should be emphasized that the grounds associated with the field-side power sources  22  (external field side grounds  84 ) are distinct from the internal field-side ground  68  described above and the two should not be connected. The ability of the internal field side ground  68  to float with respect to the external field side grounds  84  provides the flexibility of the current circuit. 
         [0058]    Referring to  FIG. 6   b , for the control of a DC circuit including DC field-side power source  22 ′, the I/O module  16  may act as a “sinking” module with the load  20  connected between a positive terminal of the field-side power source  22 ′ and one terminal  24  and the remaining terminal  24  connected to ground. Alternatively, as shown in  FIG. 6   c , the positive terminal of the field-side power source  22 ′ may be connected directly to a terminal  24  and the remaining terminal  24  may be connected to the load  20  which then leads to ground in a so-called “sourcing” mode. 
         [0059]    Referring now to  FIG. 7 , in one embodiment, the output circuit  52  may provide for a printed circuit board  90  associated with multiple removable modules  92  and  92 ′ that may be alternatively releasably attached to the printed circuit board  90 , for example, through electrical connector sockets  94  on the printed circuit board  90  and corresponding pins  96  on the modules  92  or  92 ′. The module  92  may hold the FET transistors  76  and  78  having a first voltage or current characteristic (for example, high-voltage operation) and the module  92 ′ may hold FET transistors  76 ′ and  78 ′ having a different voltage or current characteristic (for example, higher current operation but lower voltage operation than transistors  76  and  78 ). The printed circuit board  90  may hold the remaining circuitry of the output circuit  52 , allowing the FET transistors  76  and  78  to be selected between high-voltage FET transistors and lower voltage higher current FET transistors by simply plugging in one or the other of modules  92  or  92 ′. In this way, a single I/O module  16  may provide for a wide range of different interface types, including both AC and DC voltages configured in different ways as discussed above with respect to  FIG. 6  and having different voltage or current requirements, by the replacement of the module  92  or  92 ′ at a distributor or in the field. 
         [0060]    It will be appreciated that many modifications may be made to the circuit including, for example, use of the photovoltaic optocoupler  64  with other solid-state switching element configurations including, for example, a single MOSFET having its drain and source connected between terminals  24 . 
       II. DC Output Module 
       [0061]    Referring now to  FIG. 8 , in an alternative embodiment, electrical connectors  42  forming the backplane bus  44  may connect with corresponding electrical connectors  47  positioned at the rear of a dedicated DC output I/O module  100 . As discussed above, these interconnections may allow electrical communication between controller-side circuitry of the I/O rack  10  and the controller-side circuitry of the I/O module  100  for directly communicating data and power therebetween when an I/O module  100  is inserted into the I/O rack  10 . 
         [0062]    Within each I/O module  100 , a module controller  48  may execute a stored program  50  to coordinate functions of the I/O module  100  by communicating with multiple output circuits  102 . As with the AC I/O module  16  discussed above, these output circuits  102  may provide internally for an isolation barrier  36  between controller-side and field-side circuitry as will be described. Generally, each of these output circuits  102  also communicates with a shared set of terminals  24  (only two shown for clarity) that may, for example, be screw type terminals releasably receiving electrical conductors  26 . 
         [0063]    The module controller  48  provides a switch state signal  56  for each output circuit  102  indicating a desired control of electrical power at the terminals  24 . 
         [0064]    Referring now to  FIG. 9 , each DC output circuit  102  may receive the switch state signal  56  from the controller  48  at a controller-side data input  103  of a power/data magnetic isolator  104  providing the isolation barrier  36 . The switch state signal  56  is relayed in isolated fashion to a field-side data output  110  so as to communicate the information from the state switched signal  56  to the field-side data output  110 . 
         [0065]    Data may also flow in the opposite direction from a field-side data input  112  through the power/data magnetic isolator  104  to an isolated controller-side data output  114 , similarly relaying data received at the field-side data input  112  in isolated fashion to the controller-side data output  114 . 
         [0066]    Generally, the power/data magnetic isolator  104  will also receive a source of controller-side power  106 , for example a DC voltage on the controller-side of the isolation barrier  36  (referenced to controller ground  152  isolated from and floating with respect to ground  150 , and will provide a field-side power output  108  providing the same or a different DC voltage derived from the controller-side power  106  (for example, by DC to DC conversion). 
         [0067]    Referring also to  FIG. 10 , in one embodiment, the power/data magnetic isolator  104  may provide for a first and second isolation transformer  118   a  and  118   b , each having corresponding primary windings  120  ( 120 ′) and secondary windings  122  ( 122 ′) respectively with isolation transformer  118   a  having its primary winding  120  on the controller-side of the isolation barrier  36  and secondary windings  122  on the field-side of the isolation barrier  36  and transformer  118   b  having its primary winding  120 ′ on the field-side of the isolation barrier  36  and secondary winding  122 ′ on the controller-side of the isolation barrier  36 . 
         [0068]    It will be understood that electrical power (and data) communicated between the primary windings  120  ( 120 ′) and secondary windings  122  ( 122 ′) will be isolated (meaning that there is no DC electrical conduction path at the operating voltages between the windings) and that the electrical power at the secondary winding will be “floating” with respect to the voltage at the primary winding (meaning generally that there is no DC connection between the ground references for these two voltages within the output circuit  102 ). 
         [0069]    Electrical power may be communicated through the power/data magnetic isolator  104  by transforming the controller-side power  106  at an oscillator  124  into an alternating current waveform driving the primary winding  120  of the first transformer  118   a . A corresponding oscillating AC waveform will be received at the secondary winding  122  which may be processed by a rectifier  126  and a voltage regulator  130  to provide a DC power output  108 . The particular voltage of the DC power output will depend on the turns ratio of the transformer  118   a , the regulation voltage of the voltage regulator  130  which may include DC to DC conversion, for example, through isolated feedback control of the oscillator  124 . 
         [0070]    Data may be communicated through the power/data magnetic isolator  104 , for example, from controller-side data input  103 , by providing the data to a modulator  132 , for example, converting a binary pulse train into high frequency pulses at the transitions of the pulse train. These high frequency signals may be received by a demodulator  134  connected to the secondary winding  122  to provide data at the field-side data output  110 . Generally the electrical power communicated through the power/data magnetic isolator  104  will provide power to the demodulator  134  and a buffer output so that data on the field-side data output  110  is expressed as an electrical signal similar to that obtained through standard logic gate. 
         [0071]    Similarly, data received on the field-side data input  112  may be received at modulator  136  to be transmitted to the primary winding  120 ′ of isolation transformer  118   b  to be conveyed to a corresponding demodulator  138  connected to the secondary winding  122 ′ of the transformer  118   b  to be provided to the controller-side data output  114 . 
         [0072]    The transformers  118   a  and  118   b  may be implemented, for example, using integrated circuit techniques providing planar coils separated by planar layers of the CMOS circuit or the like. A power/data magnetic isolator  104  suitable for this purpose is commercially available from Analog Devices of Norwood Mass., USA under the trade name of ADuM5201. 
         [0073]    Referring again to  FIG. 9  the signal  56 , as transferred in isolated fashion through the power/data magnetic isolator  104 , may pass through a resistor  140  to the gate of a metal oxide semiconductor field effect (MOSFET) transistor  142 . Transistor  142  has its drain connected to a first terminal  24  and its source connected to a second terminal  24  of the DC I/O module  100  (shown in  FIG. 8 ). A voltage limiting protective element  144 , such as a zener diode or MOV, may be connected across the drain to source to protect the transistor  142  from high voltage spikes applied to the terminals  24  exceeding drain to source output voltage of the transistor  142 . 
         [0074]    In one embodiment, the transistor  142  may be an auto-protected transistor providing one or more of voltage clamping, current limiting, and temperature protection by internal transistor structures. In the event of over temperature, for example, the path through the transistor  142  between the drain and source may open and the path between the gate and source may be shunted with a low resistance by internal protective mechanisms. A MOSFET transistor suitable for use in this application is commercially available from STMicroelectronics of Geneva, Switzerland under the trade name of OmniFet II. 
         [0075]    This and other similar fault condition of the transistor  142  may be detected through the use of a PNP transistor  146  having its base connected through resistor  141  to the junction of resistor  140  and the gate of transistor  142  and having its emitter connected to the junction between resistor  140  and field-side data output  110  of the power/data magnetic isolator  104 . A collector of the transistor  146  is connected through resistor  148  to a ground output  150  (providing the reference for output  108 ) being isolated from a corresponding ground  152  (providing the reference for controller-side power  106 ) on the controller-side of the isolation barrier  36 . A junction between the resistor  148  and a collector of the transistor  146  is connected to the field-side data input  112 . 
         [0076]    During normal operation of the transistor  142 , the emitter and base of transistor  146  will be substantially at equal potential (either both high-voltage or low-voltage) because of the high impedance of the gate of the transistor  142 . In this state, the transistor  146  will be off and the voltage at the field-side data input  112  will be low indicating no fault condition. 
         [0077]    When a fault of the transistor  142  occurs, the shunting of the gate to the source by a low resistance lowers the voltage on the base of transistor  146  with respect to its emitter (when the field-side data output  110  is in its high state) causing a conduction of the transistor  146  raising the voltage at the field-side data input  112 . This voltage at field-side data input  112  is relayed in isolated fashion to controller-side data output  114  to provide an indication to the controller  48  that the connected transistor  142  is in a fault condition. 
         [0078]    Referring now to  FIGS. 11   a  and  11   b , as discussed above, the use of the MOSFET transistor  142  powered by a floating gate voltage, permit a number of different configurations of the load  20  and field-side power source  22  in connection to the terminals  24  of the I/O module  100 . These configurations are possible because the signal at the gate of MOSFET transistor  142  (V gs ) is independent of the field-side power source  22  and thus unaffected by different configurations of the field-side power source  22  with respect to attachment to the terminals  24 . Referring to  FIG. 11   a , for the control of a DC circuit including DC field-side power source  22 ′, the I/O module  100  may act as a “sinking” module with the load  20  connected between a positive terminal of the field-side power source  22 ′ and one terminal  24  (connected to the drain of transistor  142 ) and the remaining terminal  24  connected to external ground  85 (not connected to internal ground  150 ). Alternatively, as shown in  FIG. 11   b , the positive terminal of the field-side power source  22 ′ may be connected directly to a terminal  24  and the remaining terminal  24  may be connected to the load  20  which then leads to ground  85  in a so-called “sourcing” mode. 
         [0079]    Referring now to  FIG. 12  as discussed above, in one embodiment, the output circuit  100  may provide for a printed circuit board  90  associated with multiple removable modules  92  and  92 ′ that may be alternatively releasably attached to the printed circuit board  90 , for example, through electrical connector sockets  94  on the printed circuit board  90  and corresponding pins  96  on the module  92  or  92 ′. The module  92  may hold the FET transistors  142  having a first voltage or current characteristic (for example, high-voltage operation) and the module  92 ′ may hold FET transistors  142 ′ having a different voltage or current characteristic (for example, higher current operation but lower voltage operation than transistors  142 ). The printed circuit board  90  may hold the remaining circuitry of the output circuit  100 , allowing the FET transistors  142  to be selected between high-voltage FET transistors and lower voltage higher current FET transistors by simply plugging in one or the other of modules  92  or  92 ′. In this way, a single I/O module  100  may provide for a wide range of different voltage or current requirements by the replacement of the module  92  or  92 ′ at a distributor or in the field. 
         [0080]    It will be appreciated that many modifications may be made to the circuit including, for example, use of the photovoltaic optocoupler  64  with other solid-state switching element configurations including, for example, a single MOSFET having its drain and source connected between terminals  24 . 
       III. AC and DC Output Module 
       [0081]    While the FET transistors are described in the above embodiments with respect to switching external field-side AC and DC power, it will be appreciated that they may also be used to switch general analog signals (AC or DC) or to provide effectively bounce less contacts mimicking the operation of a relay or the like. 
         [0082]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0083]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0084]    References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
         [0085]    Co-pending patent applications entitled Input Circuit For Industrial Control With Low Heat Dissipation Ser. No. ______ and Zero-Crossing Detector For Industrial Control With Low Heat Dissipation Ser. No. ______ filed on even date herewith and assigned to the same assignee as the present invention are hereby incorporated by reference in their entirety. 
         [0086]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications are hereby incorporated herein by reference in their entireties.