Abstract:
An I/O module for an industrial controller provides single terminal outputs that may either sink or source current. This capability is provided through the use of dedicated sourcing and sinking transistors connected to the terminal and controlled by lockout logic ensuring activation of only the appropriate transistor in the correct phasing for sinking or sourcing operation modes.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to industrial controllers used in controlling industrial machines and processes and, more particularly, to I/O modules being part of the industrial controller and providing an electrical interface between the industrial controller and the machine or process. 
     Industrial controllers are employed in industrial and commercial applications to control the operation of machines and processes. Generally the industrial controller executes a stored control program to control outputs to actuators on the machine or process according to inputs received from sensors on the machine or process. 
     Industrial controllers differ from conventional computers in providing real-time control subject to predetermined maximum delays. In addition, industrial controllers are normally constructed in a highly modular fashion to permit their hardware to be customized according to different control applications. In this latter regard, inputs and outputs to or from the industrial controller are normally handled by input/output (I/O) modules that may be attached to the industrial controller in different combinations to provide for the necessary electrical interface to the controlled machinery. 
     Different types of I/O modules may be used for different control applications. Input I/O modules provide specialized input circuits to receive signals from sensors and the like, and output I/O modules provide specialized output circuits to provide signals to actuators or the like. Two common output circuits are alternating current (AC) output circuits, typically employing an SCR or thyristor to switch an AC signal, and direct current (DC) output circuits typically employing a transistor to switch a DC signal. DC output circuits are often distinguished according to whether they provide “sinking outputs” that is, a switchable connection to ground that may receive current or “sourcing outputs” that provide a switchable connection to a power source that may output current. 
     In certain control applications, it may be desirable to provide both sinking and sourcing DC output circuits in a single I/O module. One method of accomplishing this, to be discussed further below, provides a single “floating” transistor that may be alternately connected to a load (e.g. an actuator) to provide a source of current or with a different connection, a sink of current. 
     Referring now to  FIG. 1 , a prior art universal output circuit  10  provides a first terminal  12  and a second terminal  14  connected across a drain and source terminal (respectively) of a field effect transistor (FET)  18 . 
     The gate of the FET  18  is connected to a floating gate drive circuit providing a constant voltage  20  with respect to an isolated system ground  27  as received from a power supply  22 . The output of the power supply  22  is floating with respect to an input system power source  24  and a system ground  25 . 
     When the prior art universal output circuit  10  is operating in a sinking mode, an actuator  26  will have one terminal connected to terminal  12  and the other terminal connected to a positive terminal of an externally provided DC power source  28 . The negative terminal of the externally provided DC power source  28  is connected to terminal  14 . Activation of an optical isolator  30  by digital control signal  32  causes conduction of transistor  34  of the optical isolator  30  pulling down the source terminal of the FET  18  with respect to the gate terminal at voltage  20  to bias the FET  18  into conduction drawing current into terminal  12 . 
     Referring now to  FIG. 2 , when the prior art universal output circuit  10  is operating in a sourcing mode, the actuator  26  must be disconnected from terminal  12  and connected to terminal  14 . The remaining terminal of the actuator  26  is then reconnected to the negative power supply terminal of power source  28  while the positive power supply terminal of power source  28  is connected to terminal  12 . 
     In this mode, activation of the optical isolator  30  by digital control signal  32  again pulls down the source terminal of the FET  18  with respect to the gate terminal, but this time to cause a sourcing of current through terminal  14 . This dual mode of operation requires power supply  20  to be floating with respect to the power source  28 . In addition, changing the mode of operation requires a rewiring of the actuator  26  with respect to the terminals  12  and  14  and the power source  28 . This latter reconnection of terminals either requires changing the connections to the terminals of the I/O module or the use of an internal, high current capacity, multi-pole switch connected between the FET  18  and the terminals  12  and  14 . 
     While this approach provides great flexibility in using outputs of the I/O module, the need to change connections to the single transistor requires undesirable switching circuitry or confusing change in external wiring procedures of actuators to the I/O module terminals. The control of a floating transistor requires a floating power supply that may be susceptible to damage. 
     SUMMARY OF THE INVENTION 
     The present invention provides an I/O module DC output circuit that may either sink or source current through a given single terminal of the I/O module as determined by a single low current direction signal. This former feature simplifies wiring actuators to the I/O module and the latter feature eliminates the need for rerouting of high current signals through a switch mechanism. The complexity of creating a drive signal for a floating transistor using a floating power supply is also eliminated. 
     These features are provided by using a dedicated sinking and sourcing transistor permanently connected to a single output terminal but driven by lockout circuitry allowing only one transistor to operate at a time. 
     Specifically, the present invention provides an I/O module for an industrial controller, the I/O module providing digital control signals for controlling application of electrical power to actuators on a controlled machine during a true state of the digital control signals. The I/O module includes an electrical connector for releasably attaching the I/O module to the industrial controller for communication of the digital control signals therebetween. For a given digital control signal, the I/O module provides:
         (1) a releasable terminal providing a connection to the actuator for the provision of electrical power to the actuator in accordance with the state of the given digital control signal;   (2) a first transistor device connected between the releasable terminal and a terminal receiving a first DC voltage to provide a current flow from the first DC voltage to the releasable terminal when the first transistor device is turned on;   (3) a second transistor device connected between the releasable terminal and a terminal receiving a second DC voltage to provide a current flow from the releasable terminal to the second DC voltage source when the second transistor is turned on; and   (4) a logic circuit providing transistor control signals to the first and second transistor and receiving the given digital control signal and a direction signal indicating whether the releasable terminal should operate as a sourcing DC output or a sinking DC input.       

     The logic circuit operates so that when the direction signal indicates that the releasable terminal should operate as a sourcing DC output:
         (i) a transistor control signal is provided to the first transistor turning the first transistor on only when the given digital control signal is true and otherwise turning the first transistor off, and   (ii) a transistor control signal is provided to the second transistor turning the second transistor off both when the given digital control signal is true and otherwise.       

     On the other hand, the logic circuit operates so that when the direction signal indicates that the releasable terminal should operate as a sinking DC output:
         (i) a transistor control signal is provided to the second transistor turning the second transistor on only when the given digital control signal is true and otherwise turning the second transistor off, and   (ii) a transistor control signal is provided to the first transistor turning the first transistor off both when the given digital control signal is true and otherwise.       

     It is thus a feature of at least one embodiment of the invention to provide a universal DC output circuit that may either sink or source current depending on a simple setting received by a logic circuit. 
     The first and second transistors are both n-channel MOSFET transistors. 
     It is thus a feature of a least one embodiment of the invention to eliminate the need for p-channel MOSFETs that in the absence of drive signals will conduct possibly creating an undesirable control state. 
     The I/O module may further include an electrical switch manually operable to provide the direction signal. 
     It is thus a feature of at least one embodiment of the invention to provide a simple signal to control the mode of operation of the output signal eliminating the need to switch high current output signals. 
     The I/O module may include a housing fitting within a chassis. When the I/O module is in the chassis mating electrical connectors engage between the chassis and I/O module communicating the digital control signals to the I/O module. An exposed face of the housing holds the releasable terminals and the electrical switch is positioned to be covered by the chassis when the housing is fit within the chassis. 
     It is thus a feature of a least one embodiment of the invention to provide a user controllable mode of operation for the I/O output circuits that is resistant to inadvertent alteration after wiring is complete. 
     The electrical switch may provide a switchable connection to a first electrical voltage and a resistive connection to a second electrical voltage so that when the switchable connection is open the direction signal indicates that the releasable terminal should operate as a sinking DC output. 
     It is thus a feature of a least one embodiment of the invention to provide a switch that fails in the sinking mode reducing the possibility of an undesired control state. 
     The I/O module may further include a releasable terminal receiving the first DC voltage. 
     It is thus a feature of a least one embodiment of the invention to permit the use of user supplied voltage sources for driving actuators. 
     The I/O module may further include a first and second optical isolator receiving at an isolated input the transistor control signals for the first and second transistors and receiving at an isolated output the first DC voltage for generation of the isolated transistor control signals. 
     It is thus a feature of a least one embodiment of the invention to provide electrical isolation between the actuator signals and the signals of the industrial controller. 
     The I/O module may include a third releasable terminal receiving the second DC voltage. 
     It is thus a feature of a least one embodiment of the invention to permit a ground terminal that does not change depending on the operation of the device as sinking or sourcing. 
     The I/O module may include an illuminated indicator associated with the releasable terminal illuminating when the digital control signal is logically true to provide a visual indication of when the digital control signal is logically true. 
     It is thus a feature of a least one embodiment of the invention to provide a consistent and familiar visual indication of the logical state of the output signal regardless of whether it is sinking or sourcing. 
     The I/O module may further include a serial communication circuit exchanging serial data with the industrial controller to provide the digital control signals. 
     It is thus a feature of a least one embodiment of the invention to provide a circuit suitable for use with modern industrial controllers using serial communication data. 
     These particular features 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 
         FIG. 1  is a prior art I/O circuit for providing both sinking and sourcing using a floating transistor and floating drive with the prior art circuit configured in a sinking mode with a first connection between the load and the floating transistor; 
         FIG. 2  is a figure similar to that of  FIG. 1  with the I/O circuit configured in sourcing mode with a second connection between the load and the floating transistor; 
         FIG. 3  is a perspective exploded view of an industrial controller having interchangeable I/O modules; 
         FIG. 4  is a block diagram of a simplified single I/O module of  FIG. 3  providing multiple DC universal outputs circuits per the present invention; and 
         FIG. 5  is a schematic diagram of a single universal output circuit of  FIG. 4  per the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 3 , the present invention provides an I/O module  40  for an industrial controller  39 . The I/O module  40  is depicted in a first embodiment implementing “chassis I/O” where the I/O module  40  has a housing  42  that may be slidably received within a chassis  44  along with other I/O modules (not shown) and other modules including a power supply  54  and a programmable logic controller  56 . 
     Alternatively, the I/O module may also be implemented as “distributed I/O” and the differences in this implementation will be described below. 
     In the chassis I/O implementation, the chassis  44  may have a backplane  48  providing a set of releasable electrical connectors  50  interconnected by a backplane bus  52  extending along a rear wall of the chassis  44 . A corresponding connector  58  on the rear of the housing  42  of the module  40  mates with the corresponding connectors  50  when the module  40  is placed within the chassis  44  providing electrical connection, for example, between the programmable logic controller  56  and the circuitry of the module  40 . The bus  52  is typically a high-speed serial bus providing efficient multi-bit communication. The other modules of the power supply  54  and programmable logic controller  56  may have similar connectors  50  and  58  for this purpose. 
     The I/O module  40  may provide access to electrical switch  90  through an opening in the housing  42  when the I/O module  40  is not fully received within the chassis  44 . When the I/O module  40  is fully received within the chassis  44 , only a front faceplate  46  of the module  40  is exposed, and the remaining portions of the housing  42  as well as the switch  90  are enclosed within the chassis  44 . In a distributed I/O implementation, the switch may remain accessible through an opening in the housing. 
     The front faceplate  46  of the I/O module  40  may open by means of a swinging door  60  to reveal a set of screw terminals  62  to which wires may be connected to connect internal I/O circuits of the I/O module  40  to various actuators  26 . Indicator lights  64  corresponding to each of the different output circuits and hence to particular output terminal  62  are positioned to be visible through a bezel on the front faceplate  46 . 
     In a distributed I/O implementation, multiple connectors  58  may provide for an Ethernet connection, an auxiliary power supply connection, input connection, and expansion board connection and the like. The housing  42  may provide for mountings to a DIN rail or the like. 
     Referring now to  FIG. 4 , the connector  58  on the rear of the housing  42  of the I/O module  40  may connect to a decoder circuit  70  which receives digital control signals encoded in serial fashion from the programmable logic controller  56  to provide a set of separate digital control signals  32  each having a logically true or logically false state. Typically, and in this described embodiment, the logically true state is a positive voltage and the logically false is a nominally zero voltage. 
     The lights  64  are connected to the digital signals  32  by appropriate amplification circuitry to provide a visual indication to the user of the industrial controller  39  of the state of the particular signal  32 . In addition, the digital signals  32  are each provided to an output circuit  72  providing a universal (i.e. sinking or sourcing) output signal through given terminals  62 . In a preferred embodiment, each output circuit  72  provides an output terminal  74  to which an actuator  26  may be attached. To reduce terminal numbers, a single external power terminal  77  for receiving a positive voltage from an externally supplied voltage source  22  and a single external ground terminal  79  for receiving a ground voltage from the externally supplied voltage source  22  are shared among the output terminal  74 . 
     As depicted, a single load  26  may be configured in sinking mode connected to a power supply  122  having its positive terminal connected to power terminal  77 . Alternatively, not depicted, the single load  26  may be configured in a sourcing mode connected to power supply  122  having its negative terminal connected to ground terminal  79 . 
     While only four output circuits  72  are shown, a typical I/O module may provide 10 channel output and thus have ten outputs circuits  72  and employ only 12 total terminals. 
     Referring now to  FIG. 5 , output terminal  74  is connected to a junction between the source of a first n-channel MOSFET  76  and the drain of a second n-channel MOSFET  78 . The drain of the first MOSFET  76  may be attached to the positive voltage obtained from terminal  77  while the source of MOSFET  78  may be attached to a ground obtained from terminal  79 . 
     The gate of MOSFET  76  is attached to an output of optical isolator  80  providing internally a phototransistor (not shown) sourcing power from terminal  77  so as to provide a voltage compatible with particular connection as will be described. Similarly the gate of MOSFET  78  is connected to the output of an optical isolator  82  also receiving power from terminal  77  to source this power to the gate of MOSFET  78  under the control of an internal phototransistor. 
     The inputs of optical isolator  80  and  82  are attached to cathodes of light emitting diodes (not shown) to receive the outputs of the AND gates  84  and  86  respectively. The anode of the light emitting diodes is connected to nonisolated ground  87 . 
     One input of each AND gate  84  and  86  is attached to the digital signal  36  associated with a particular output circuit  72  and driving light  64 . The remaining input of AND gate  84  connects to one terminal  91  of a mechanical switch  90 . This terminal  91  is also connected to a resistance  92  to ground  87 . The other terminal of the switch connects to a nonisolated power source  94  so that the power source  94  is switchably connected to terminal  91 . 
     It will be understood that damage to the switch  90  such as prevents good electrical flow (including possible contact corrosion) will therefore result in terminal  91  being pulled to ground through resistance  92 . 
     Terminal  91  connects through an inverter  96  to the remaining input of AND gate  86 . 
     The position of the switch  90  provides a direction signal at terminal  91 . When this direction signal is high, the output circuit  72  operates in a sourcing mode and when this direction signal is low, the output circuit  72  operates in a sinking mode as will be described. Specifically, when the switch  90  is closed, the output of AND gate  86  will always be low causing MOSFET  78  to be turned off (nonconducting) while the output of AND gate  84  will follow digital signal  36  causing MOSFET  76  to turn on when digital signal  36  is in a true state (high) and off when digital signal  36  is in a false state (low). This will provide a sourcing of current out of terminal  74  to actuator  26  connected between terminals  74  and  79 . 
     Conversely when switch  90  is opened, terminal  91  will be low causing the output of inverter  96  to go high and of the output of AND gate  86  to follow the digital signal  36  turning MOSFET  78  on when digital signal  36  is in a true state and off when digital signal  36  is in the false state. In this mode, AND gate  84  will always have a low output turning MOSFET  76  off (nonconducting). Accordingly, in this state, current is sinked into terminal  74  when digital signal  36  is in a high state to provide a sinking of current through actuator  26  connected between terminals  77  and  74 . 
     In this way, a single terminal  74  may source or sink current from actuators  26  or other loads without the need for a mechanical switch controlling high current flow. In an alternative embodiment, the signal at terminal  91  may be provided from the decoder circuit  70  permitting software selection (for example, using the control program on the programmable logic controller  56 ) of the state of each output terminal  74 . 
     It will be further understood that failure of switch  90  will cause terminal  91  to go to a low state, putting output circuit  72  into the sinking mode which will generally be a safer mode when the particular wiring of the actuator  26  between terminals  77  and  74  or  74  and  79  is unknown. In one embodiment, the signal at terminal  91  produced by the switch  90  may generated by the industrial controller so that the configuration of the I/O module terminals as sinking or sourcing mode may be controlled through software. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from liberal language of the claims. Generally, as will be recognized by those of ordinary skill in the art, the features of the present invention may be implemented in different combinations of hardware and software executing on an electronic computer including just one or the other.