Isolated Output Circuit for I/O Modules with Internal Output Power Support

Abstract

An output module for an industrial controller provides electrical isolation between each of the output terminals in the module. The output module receives control signals from the industrial controller indicating a desired output state for each of the output terminals and selectively connects power from the output of the electrical isolation to the output terminal. During normal operation, a switching device connects the power to the output terminal responsive to the control signal. A current sensor monitors the current conducted at the output terminal. If the current exceeds a predefined threshold, a current limit circuit clamps the current being output at the terminal. A control circuit may allow the output terminal to ride through a temporary spike in current or disable the output terminal if a fault condition is detected.

We claim: 
     
         1 . An output module for an industrial controller, the output module comprising:
 a power supply terminal for receiving power from an external power supply;   a plurality of output terminals for providing an output signal for driving an external device;   a system control circuit to receive a plurality of control signals, wherein each of the plurality of control signals corresponds to one of the plurality of output terminals;   a plurality of current limit circuits, wherein each of the plurality of current limit circuits corresponds to one of the plurality of output terminals;   a plurality of isolation circuits, wherein each of the plurality of isolation circuits:
 corresponds to one of the plurality of output terminal, 
 receives the power from the power supply terminal, and 
 supplies electrically isolated power for the corresponding output terminal; 
   a plurality of terminal control circuits, wherein each of the plurality of control circuits:
 corresponds to one of the plurality of output terminals, 
 receives one of the plurality of control signals for the corresponding output terminal, 
 receives the electrically isolated power from one of the plurality of isolation circuits for the corresponding output terminal, and 
 controls one of the plurality of current limit circuits to provide the electrically isolated power as the output signal from the corresponding output terminal. 
   
     
     
         2 . The output module of  claim 1 , wherein each of the plurality of isolation circuits comprises an isolation transformer having a primary winding and a secondary winding, wherein the primary winding is electrically connected to the power supply terminal and the secondary winding provides the electrically isolated power to the corresponding current limit circuit and the corresponding terminal control circuit. 
     
     
         3 . The output module of  claim 1 , further comprising a plurality of switches, wherein each of the plurality of switches corresponds to one of the plurality of output terminals and is operatively connected between the corresponding current limit circuit and the corresponding output terminal. 
     
     
         4 . The output module of  claim 3 , wherein each of the plurality of switches selectively connects the corresponding isolation circuit to the corresponding output terminal in one of three operating states, and wherein the three operating states include an off operating state, an on operating state, and a current limited operating state. 
     
     
         5 . The output module of  claim 1 , wherein each of the plurality of terminal control circuits generates a first intermediate output signal to enable the output signal for the corresponding output terminal and a second intermediate output signal corresponding to current limited operation of the corresponding output terminal. 
     
     
         6 . The output module of  claim 5 , wherein each of the plurality of current limit circuits receives the first and second intermediate output signals from the corresponding terminal control circuit, and wherein each of the plurality of current limit circuits further comprises:
 a first electronic switching device controlled by the first intermediate output signal to enable an output of the current limit circuit; and   a second electronic switching device controlled by the first and second intermediate output signals to enable a current limited operating state.   
     
     
         7 . The output module of  claim 5 , wherein each of the plurality of terminal control circuits further comprises a processor operative to:
 receive the control signal for the corresponding output terminal from the system control circuit,   generate the first intermediate output signal and the second intermediate output signal responsive to the control signal for the corresponding output terminal,   output the first intermediate output signal to the current limit circuit, and   output the second intermediate output signal to the current limit circuit.   
     
     
         8 . The output module of  claim 7 , further comprising a plurality of current sense circuits, wherein each of the plurality of current sense circuits corresponds to one of the plurality of output terminals and is operative to generate a current feedback signal corresponding to an amount of current output at the corresponding output terminal, and wherein the processor is further operative to:
 receive the current feedback signal for the corresponding output terminal, and   generate the first intermediate output signal and the second intermediate output signal responsive to the control signal and responsive to the current feedback signal for the corresponding output terminal.   
     
     
         9 . The output module of  claim 1 , wherein the system control circuit includes a backplane connector to receive the plurality of control signals over a backplane from a processor module for the industrial controller. 
     
     
         10 . A method of limiting current in an output module for an industrial controller, wherein the output module includes a plurality of output terminals, the method comprising the steps of:
 receiving a plurality of control signals at a system control circuit in the output module, wherein each control signal defines a desired output state of one of the plurality of output terminals;   receiving power from an external power supply at a power supply terminal for the output module;   supplying the power from the external power supply to a plurality of electrical isolation circuits, wherein each of the plurality of electrical isolation circuits corresponds to one of the plurality of output terminals;   transferring electrically isolated power from each of the plurality of electrical isolation circuit to a current limit circuit and a terminal control circuit within the output module for the corresponding output terminal;   selectively supplying the electrically isolated power from an output of the electrical isolation circuit via the current limit circuit to the corresponding output terminal responsive to the corresponding control signal.   
     
     
         11 . The method of  claim 10 , wherein each of the plurality of electrical isolation circuits comprises an isolation transformer having a primary winding and a secondary winding, wherein the primary winding is electrically connected to the power supply terminal and the secondary winding provides the electrically isolated power to the corresponding current limit circuit and the corresponding terminal control circuit. 
     
     
         12 . The method of  claim 10 , further comprising the step of selectively controlling a switch corresponding to each output terminal with an output from the current limit circuit, wherein the switch is electrically connected between an output of the electrical isolation circuit and the corresponding output terminal. 
     
     
         13 . The method of  claim 12 , wherein the switch corresponding to each output terminal is selectively controlled by an output from the current limit circuit in one of three operating states, and wherein the three operating states include an off operating state, an on operating state, and a current limited operating state. 
     
     
         14 . The method of  claim 10 , further comprising the steps of:
 generating a first intermediate output signal from each terminal control circuit to enable an output signal for the corresponding output terminal; and   generating a second intermediate output signal from each terminal control circuit to enable a current limited output signal for the corresponding output terminal.   
     
     
         15 . The method of  claim 14  further comprising the steps of:
 transmitting the first and second intermediate output signals from each terminal control circuit to the corresponding current limit circuit; 
 controlling a first electronic switching device in each current limit circuit by the first intermediate output signal to enable the output signal; and 
 controlling a second electronic switching device in each current limit circuit by the first and second intermediate output signals to enable the current limited output signal. 
 
     
     
         16 . The method of  claim 14 , wherein each terminal control circuit includes a processor, the method further comprising the steps of:
 receiving at the processor the control signal for the corresponding output terminal from the system control circuit;   generating with the processor the first intermediate output signal and the second intermediate output signal responsive to the control signal for the corresponding output terminal;   outputting the first intermediate output signal from the processor to the current limit circuit; and   outputting the second intermediate output signal from the processor to the current limit circuit.   
     
     
         17 . The method of  claim 16  further comprising the steps of:
 receiving a current feedback signal at the processor from a current sense circuit for the corresponding output terminal, and 
 generating the first intermediate output signal and the second intermediate output signal responsive to the control signal and responsive to the current feedback signal for the corresponding output terminal. 
 
     
     
         18 . The method of  claim 10 , further comprising the step of transmitting the plurality of control signals over a backplane from a processor module for the industrial controller to a backplane connector in the system control circuit. 
     
     
         19 . An output circuit for an output module used in an industrial controller for providing an electrically isolated output, the output circuit comprising:
 an output terminal for providing an output signal for driving an external device;   an isolation transformer having a primary winding and a secondary winding, wherein the primary winding receives power from a power supply terminal on the output module, and the secondary winding supplies electrically isolated power for the output terminal;   a terminal control circuit receiving a control signal from a system control circuit on the output module, generating a first intermediate output signal to enable the output signal for the output terminal, and generating a second intermediate output signal corresponding to current limited operation of the output terminal;   a current limit circuit receiving the first and second intermediate output signals from the terminal control circuit, wherein the current limit circuit includes a first electronic switching device controlled by the first intermediate output signal to enable the output signal and a second electronic switching device controlled by the first and second intermediate output signals to enable the current limited operation of the output terminal; and   a switch selectively controlled by the current limit circuit to provide the electrically isolated power as the output signal.   
     
     
         20 . The output module of  claim 19 , wherein the switch selectively connects the secondary winding of the isolation transformer to the output terminal in one of three operating states, and wherein the three operating states include an off operating state, an on operating state, and a current limited operating state.