Abstract:
The circuit of the invention provides protection for both a spur circuit and the network while allowing other spurs on the network to continue operation and is applicable to any type of fieldbus network including both FF and ASi type networks. A control circuit monitors a current limiter coupled to each spur circuit. If the spur circuit goes into an overcurrent condition for a preset period of time, the control circuit turns it off. After a second period of time has elapsed, the control circuit turns the current limiter back on permitting current to flow in the spur once again while continuing to monitor the current limiter.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application is a continuation in part of U.S. patent application Ser. No. 09/886,187 filed May 24, 2001, which is a continuation of U.S. patent application Ser. No. 09/771,152 filed Jan. 26, 2001, which is a continuation of U.S. patent application Ser. No. 09/344,408 filed Jun. 24, 1999.  
         [0002]    Pursuant to 35 U.S.C. 119, applicant claims priority for this application based upon U.S. provisional application serial No. 60/418,908 filed Oct. 14, 2002. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0003]    Not applicable.  
         REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX  
         [0004]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0005]    The present invention relates to a fieldbus network and, more particularly, to a current limiter to protect a fieldbus network from an electrical short in a spur cable or a device attached to a spur cable.  
           [0006]    In a typical industrial plant application, sensors measure position, motion, pressure, temperature, flow, and other parameters related to the operation of process machinery and activities. Actuators, such as valves and motor controllers, control the operation of the machinery and process activities. The sensors and actuators are remotely located from the human and computerized controllers which gather information from the sensors and direct operation of the actuators. A communication network links the controllers with the sensors and actuators located in the field.  
           [0007]    Heretofore, communication between controllers, remote sensors, and actuators in industrial applications has been by means of analog signaling. The prevailing standard for analog networking of field devices and the control room in industrial applications has been the Instrument Society of America standard, ISA S50.1. This ISA standard provides for a two-wire connection between the controller and each field device. One wire of the system carries the analog signal between the remote device and the controller. The analog signal may be converted to a digital signal useful to a computerized controller. The second wire of the circuit supplies DC power for operation of the remote sensor or actuator. Both wires provide return paths for data and power respectively  
           [0008]    Communication utilizing digital signaling reduces the susceptibility of the communication system to noise and provides a capability for conveying a wide range of information over the communication network. Digital communication also permits several different devices to communicate over a single pair of wires. Remote devices used in connection with a digital communication system typically incorporate local “intelligence.” This permits sensors and actuators to perform diagnostic, control, and maintenance functions locally. Further, the local intelligence permits the devices to communicate directly with each other and perform some functions without the necessity of involving a central control facility, thus promoting the development of distributed control systems.  
           [0009]    Fieldbus is a generic term used to describe a digital, bidirectional, multidrop, serial communication network for connecting isolated field devices, such as controllers, actuators and sensors, in industrial applications. One such fieldbus called Foundation Fieldbus or “FF” is defined by the Instrument Society of America standard, ISA SP50.02. This system utilizes a two-wire bus to provide simultaneous digital communication and DC power to remotely located devices.  
           [0010]    A current limiter is disclosed in U.S. Pat. No. 6,366,437 which is assigned to the same assignee, the contents of which are incorporated herein by reference. The protection circuit of the &#39;437 patent minimizes the impact of short circuits on a fieldbus network. It does this by limiting the current that is supplied from the home run cable to the spur cables and connected devices. Any short or overcurrent condition on the spur cable results in the failure of any devices on that spur but the rest of the spurs, with their associated devices and the network, continue to operate normally as long as there is sufficient current for the trip point on the protection circuit.  
           [0011]    Another device network called AS-interface, or ASi, is similar to FF but ASi devices use substantially more current. To raise the current trip point limit on a circuit such as disclosed in U.S. Pat. No. 6,366,437, could cause the circuitry to overheat and self-destruct during a sustained short circuit because of the amount of power available on the network. Moreover, on some FF networks, especially those used for intrinsically safe installations, the available power is limited in order to prevent a spark from causing an explosion in the hazardous area. In such a case, the difference between the power the device on the spur uses and the protection circuit trip point current is more than is available on the segment. A short on the spur in this situation would not prevent failure of the entire segment.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    The circuit of the invention provides protection for both a spur circuit and the network while allowing other spurs on the network to continue operation and is applicable to both FF and ASi type networks as well as any other type of data and power network. A control circuit monitors a current limiter coupled to each spur circuit. If the spur circuit goes into an overcurrent condition for a preset period of time, the control circuit turns it off. After a second period of time has elapsed, the control circuit turns the current limiter back on permitting current to flow in the spur once again while continuing to monitor the current limiter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS  
       [0013]    [0013]FIG. 1 is a block schematic diagram of a protection circuit for a fieldbus network of either FF or ASi type and spur circuit.  
         [0014]    [0014]FIG. 2 is a block schematic diagram of a typical four-port connection block coupling four spur lines to a fieldbus trunk line.  
         [0015]    [0015]FIG. 3 is an expanded block schematic diagram of the power supply and control block of FIG. 2.  
         [0016]    [0016]FIG. 4 is a schematic diagram of the control circuit block of FIG. 1.  
         [0017]    [0017]FIG. 5 is a detailed schematic diagram of a typical current limiter circuit.  
         [0018]    [0018]FIG. 6 is a detailed schematic diagram of a microprocessor circuit as used in the control circuitry block of FIG. 1.  
         [0019]    [0019]FIG. 7 is a flowchart diagram illustrating the operation of the microprocessor circuit of FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    Referring to FIG. 1, a fieldbus network includes a “home run” cable or trunk line  10  which comprises a positive lead  12 , a negative lead  14  and a shield  16 . Connected to the home run network  10  are one or more spur cables  20 . Each spur cable includes a positive line  22 , a negative line  24  and a shield  26 .  
         [0021]    The home run  10  is a trunk line of the conventional type described above, that is, it is either an ASi network or a foundation fieldbus network or some similar network. As such, the positive and negative wires  12  and  14  carry both power and data. Coupled in parallel with the home run  10  and the spur cable  20  is a protection circuit which includes a current limiter  30  coupled to control circuitry  32 . In FIG. 1, the current limiter  30  is coupled in-line with the negative lead  24  of the spur cable  20 . In the event of an overcurrent condition that persists for a first time period, the current limiter  30  turns off current in the spur cable  20  thus serving as a protection circuit for the home run  10  and all of the other spurs and devices connected to it. The current limiter is controlled by control circuitry  32  which both detects the overcurrent condition and turns the spur  20  on and off. Control circuit  32  turns the spur  20  back on after a second predetermined period of time to retest for the short circuit condition.  
         [0022]    The network is shown in more detail in FIG. 2 in which a block of spur connections  34   a  through  34   d  are connected to the fieldbus trunk line  10 . Four connections have been shown but this is for illustration purposes only. Any practical number of spurs may be used in such a block. The home run network  10  extends between a pair of trunk connectors  18  and  28 . The block of FIG. 2 includes an optional terminator circuit  36 . Each of the spur connectors  34   a  through  34   d  have current limiter circuits  30   a  through  30   d  coupled respectively to the negative line  12  of the fieldbus network  10 . All four of the current limiter circuits  30   a  through  30   d  are controlled by control circuitry  32 .  
         [0023]    [0023]FIG. 3 shows in more detail functional aspects of control circuitry  32  and current limiter  30 . A reverse polarity protection block  38  prevents current from flowing in the wrong direction which would otherwise damage internal circuitry connected to the fieldbus network  10 . The polarity protection circuit  38  is coupled to an adaptive current source circuit  40  which provides the necessary power to the internal circuitry of the control circuitry block  32  and the current limiter  30  while at the same time maintaining a high impedance to the fieldbus  10 . The high impedance connection permits the protection circuitry to operate without adversely affecting the data on the rest of the trunk line  10 . A 5.0-volt regulator circuit  42  is coupled to the positive lead  12  of the fieldbus. This regulates the voltage for semiconductor devices in control circuitry  32  and current limiter  30 . The 0.2-volt reference block  44  develops a voltage which is used by the current limiter circuit  30 . The 9.0-volt detector circuit  46  detects the presence of the incoming bus voltage and determines whether it is above 9.0 volts. If the bus voltage is above this value, the microprocessor and reset chip  48  turns on an LED (not shown) indicating that power on the fieldbus trunk line  10  is normal. Should the microprocessor and reset chip  48  fail to power up properly, bad block circuitry  50  is turned on. This circuit drives an LED that indicates that the entire block (shown generally in FIG. 2) is not functioning properly. The microprocessor and reset chip  48  drives a protection circuit  52 . This block may be constructed along the lines of the current limiter circuits  30  such as a set of four current limiter circuits  30   a  through  30   d  as shown in FIG. 2.  
         [0024]    Referring to FIG. 4, which shows the operational circuit details of FIG. 3, reverse voltage protection is provided by diodes D 7 , D 8  and D 10 . Three diodes are necessary instead of one in order to meet certain hazardous location certification requirements which require double fault survival. An adaptive current source comprises Q 2 , R 12  and C 4 . Resistor R 12  and capacitor C 4  form a time constant. The gate of Q 2  charges up to a point where enough current is being fed to the internal circuitry. This time constant is relatively long and the gate voltage does not change when a fieldbus signal reaches the device. This network thus makes this device look like a high impedance at fieldbus frequencies. U 1 , C 1 , C 2  and C 3  regulate a 5.0-volt supply for the internal circuitry. Resistors R 8  and R 10  form a voltage divider from the input bus voltage. This feeds a comparator U 2  whose output indicates whether the bus voltage is above 9.0 volts. This signal is then fed to the microprocessor U 4  (FIG. 6) via the line -PWRGOOD. Resistor R 7  provides an amount of positive feedback for hysteresis. Amplifier U 2  also generates the 0.2-volt reference that is used for this comparison. C 7  is a filter for the 0.2-volt reference. The circuit formed by R 4 , D 29  and D 28  along with LED D 5 , is optional. This optional circuit is provided in applications in which the current limiter circuitry described herein is not used. The purpose of this circuit is to indicate the presence of power on the trunk line by causing LED D 5  to light. When current limiter circuitry is used, bus power is indicated by the combination of R 9  and D 5  and the microprocessor. Diodes D 11  and D 12  are 36-volt TVS diodes which protect the internal circuitry from electrostatic discharges.  
         [0025]    Transistor Q 15 , resistor R 37 , diode D 22  and resistor R 13  form a network that indicates that the microprocessor is working on power-up. The network briefly turns on LED D 22  but after the microprocessor boots up, Q 15  is turned off.  
         [0026]    Referring to FIG. 5, a current limiter network corresponding generally to the current limiter block  30  of FIG. 1 is shown in which a transistor Q 6  limits the amount of current flowing through a spur cable  20 . The negative wire  14  of the trunk cable is connected to a diode D 21  which is a TVS diode to protect the circuit from electronic static discharge. F 1  is a protection fuse that is set relatively high so that it will not fuse open unless there is a catastrophic failure within the apparatus. R 25  is a charging resistor which allows current to bypass the protection circuitry when it has been turned off by the control circuitry  32 . The transistor Q 6  is a MOSFET which varies its impedance in response to an overcurrent condition. When the current is under the current limit however, its impedance is near zero. Resistor R 15  provides voltage to the gate of Q 6  to turn it on. D 9  is an LED that will be on if the current limiter  30  is in current limit or if the microprocessor turns it off. D 20  is a diode that isolates the output of the operational amplifier U 5 A from the microprocessor output. R 23  is the current sense resistor. The voltage developed across this resistor is coupled to the input of operational amplifier U 5 A. Resistor R 29  and transistor Q 7  are used to improve the response time of the current limiter circuit  30 . Capacitor C 9  is a filter capacitor to eliminate circuit oscillation.  
         [0027]    Normally, MOSFET transistor Q 6  is fully on and current within an acceptable range flows to the spur circuit  20 . If a short occurs on the spur however, current rapidly increases to the point at which the voltage across R 23  and R 29  increases, thus turning Q 7  on. When Q 7  turns on, the voltage on the gate of Q 6  decreases which raises its source-drain impedance. As the impedance between the source and drain of Q 6  rises, current in the spur decreases. The voltage across R 23  exceeds the reference voltage VREF but it takes some time for the operational amplifier U 5 A to react to this condition. When it finally reacts, it adjusts the impedance of Q 6  such that the voltage across R 23  is the same as the reference voltage. When this occurs, the voltage at the base of Q 7  is too low to turn it on and it no longer has an effect. The voltage at the gate of Q 6  will be less than 2.0 volts so the diode D 9  turns on. The voltage at IN 1  will be near the bus voltage which is high enough to signal the microprocessor that the current limiter is in current limit. Once the microprocessor has seen the current limiter at its limit for a predetermined “trip time,” it drives OUT 1  low (refer to FIG. 6). This action turns off Q 6  completely. At this point, the only current flowing to the spur circuit  20  comes from R 25 . After another predetermined time, the “reset time,” the microprocessor allows OUT 1  to go “high,” resetting Q 6  and turning it on. If the short circuit still exists however, the process described above will repeat and Q 6  will shut down; otherwise, Q 6  turns fully on again and the spur circuit  20  operates as normal.  
         [0028]    Referring to FIG. 6, U 4  is a microprocessor that operates four current limiter circuits  30   a  through  30   d.  R 22  and C 5  set the frequency for the RC oscillator in the microprocessor. U 3  is a microprocessor supervisor. It insures that the microprocessor is held in reset when there are low voltage conditions. R 16 , R 17 , R 18  and R 24  limit the current that can flow from each of the spurs  34   a  through  34   d  to the microprocessor U 4 . These lines (IN 1  through IN 4 ) indicate if one of the four current limiters  30   a  through  30   d  is in current limit. D 16 , D 19 , D 26  and D 27  provide voltage clamping on the input lines (IN 1  through IN 4 ) to prevent voltages that may be damaging from reaching the microprocessor U 4 . D 24  and D 25  are TVS diodes that protect the microprocessor U 4  from surge voltages on lines OUT 1  through OUT 4 . These lines connect to the respective gates on each of the current limiter MOSFETs (such as Q 6 ).  
         [0029]    Referring now to FIG. 7, the operation of the control circuitry  32  in conjunction with the current limiter  30  is shown by way of a flowchart.  
         [0030]    Upon power up or reset, the microprocessor U 4  is initialized. The microprocessor initializes variables and clears a watchdog timer. Next, the microprocessor U 4  updates the status of each of the LEDs in the circuits of FIG. 4 and FIG. 5. Once power up and updating have been accomplished, the microprocessor U 4  queries each of the spur circuits  34   a  through  34   d  to determine if any of the current limiters  30   a  through  30   d  are in current limit. If not in current limit, the program loops back to clear the watchdog timer but if in current limit starts another clock which runs for a predetermined “trip time” for that current limiter. If the current limiter under consideration is in current limit for the trip time, the microprocessor U 4  turns the particular spur circuit off and loads a reset time counter. The reset time counter defines a second predetermined time period.  
         [0031]    The next time the microprocessor queries the spur, if it has been turned off, it checks to see if the reset time has elapsed. If the reset time has not elapsed, the microprocessor loops back to the watchdog timer and, after a number of clock cycles, queries the current limiter again. If the reset time has elapsed, U 4  turns the particular spur circuit on and reloads the trip time counter.  
         [0032]    Also, upon power up the microprocessor turns on each spur circuit in turn with a preset time interval between each one so as to avoid a current surge through all four spur circuits  34   a  through  34   d  at once.  
         [0033]    In the example given, the current limiter circuits  30   a  through  30   d  are connected to the negative lead of home run  10 . However, the circuit can be configured to couple these circuits to the positive lead. The particular selection of polarity is not critical to the invention.  
         [0034]    A microprocessor has been shown by way of illustration as one way of implementing the control circuitry  32 , however, other hard-wired circuit elements could be used as well. The illustration disclosed in FIG. 2 contains four spur circuits monitored by a single microprocessor but more or fewer circuits could be used, if desired.