Abstract:
A cable drive circuit allows a transmitter in a process control system to drive signals across a capacitive length of cable. The cable drive circuit uses a charge pump to store charge, which is then used in conjunction with open drain outputs to transmit logic signals across a cable to a peripheral at the other end of the cable. A microcontroller or other device specifies the number of communication pulses to send across the cable before allowing the charge pump to recharge. By transmitting communication pulses for a small percentage of the time the average current is minimized, making the circuit suitable for operation under the 4-2OmA standard for two-wire devices.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to a drive circuit for a remote transmitter in a process control loop. More specifically, the invention relates to a drive circuit for driving signals across a cable from a microcontroller to a sensing apparatus located at a remote site. 
     II. Discussion of the Prior Art 
     Measurement transmitters sensing process variable information, such as differential pressure, line pressure, and temperature, are known. The transmitters are typically used in a process control industry installation where the transmitter power requirement is a concern. Measurement transmitters provide a current output representative of the variable they are sensing, where the magnitude of the current varies between 4-20 mA as a function of the sensed process variable. The current needed to operate a measurement transmitter must be less than 4 mA in order for the transmitter to adhere to this process control industry communications standard. 
     Most prior art transmitters are required to be located in the immediate vicinity of the location where the process variable information is being sensed. This is due primarily to the low power constraints imposed on the transmitter by the 4-20mA standard, in which the operating current of the measurement transmitter must be less than 4 mA. One way to place the sensing electronics at a longer distance from the measurement transmitter is to use a transmission cable between the sensing electronics and the measurement transmitter. The transmission cable acts as a capacitive load, requiring substantially more current to drive signals between the measurement transmitter and sensing electronics, while maintaining adequate rise and fall times. The longer the transmission cable is, the greater the capacitive load becomes. As a result, prior art transmitters have been required to be placed very close to the process variable being sensed. Often, the sensor electronics are, at most, a few inches from the transmitter components. 
     To meet certain Intrinsic Safety standards, the sensing electronics must be “infallibly” electrically isolated from the rest of the transmitter. Such standards are set forth by, for example, European CENELEC standards EN50014 and 50020, Factory Mutual Standard FM3610, the Canadian Standard Association, the British Approval Service for Electrical Equipment in Flammable Atmospheres, the Japanese Industrial Standard, and the Standards Association of Australia. The Intrinsic Safety requirements are intended to guarantee that instrument operation or failure cannot cause ignition if the instrument is properly in an environment that contains explosive gases. This is accomplished by limiting the maximum energy stored in the transmitter in a worst case failure situation. Excessive energy discharge may lead to sparking or excessive heat which could ignite an explosive environment in which the transmitter may be operating. 
     The prior art has primarily used two techniques to achieve infallible isolation between the sensor circuitry and the transmitter circuitry. The first technique is to provide sufficient mechanical segregation or spacing in the sensor such that it is impossible for a component failure to cause electrical shorting to another component or ground. the second technique is to design the entire system such that isolation is not required by using components which are rated for large power dissipation such that they themselves are considered infallible. 
     One problem with both of these techniques is that they require a sufficiently large transmitter housing to provide the required spacing between components or the relatively large size of the high power components. Thus, reduction in transmitter size has been limited when complying with Intrinsic Safety requirements using the above two techniques. 
     III. Objects and Advantages 
     Accordingly, it is a principal object of the present invention to provide an improved system for interfacing a measurement transmitter or microcontroller with a peripheral, such as a sensing electronics capsule, at a remote sensing site or location while still adhering to the 4-20 mA industry standard. 
     A further object of the invention is to provide an intrinsically safe cable drive circuit so that operation can be maintained even in hazardous gas environments. 
     SUMMARY OF THE INVENTION 
     The present invention provides a low power drive circuit for driving serial digital bits along a cable. The circuit uses a charge pump to store charge, which is then incrementally discharged (pulsed) to drive signals along the cable. After the capacitor has discharged to a certain level, the signal transmission is stopped and the capacitor is allowed to recharge. A microcontroller can be used to send or receive signal pulses and to control the time the circuit is driving signals along the cable. Directional control is provided so that signals can be transmitted or received along the cable. Voltage and current in the circuit are limited, so the circuit is intrinsically safe and can be used in a hazardous environment. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The foregoing features, objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a preferred embodiment, especially when considered in conjunction with the accompanying drawings in which: 
     FIG. 1 is a simplified block diagram of a transmitter including the present invention. 
     FIG.  2 . is a simplified block diagram showing the connections between the microcontroller and the present invention. 
     FIG. 3 is a schematic diagram showing the preferred embodiment of the invention at a gate level. 
     FIG. 4 is a schematic diagram showing the preferred embodiment of the invention at a transistor level. 
     FIG. 5 is a diagram of part of a process control system having a transmitter incorporating a drive circuit according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a simplified block diagram of part of a process control system  12  incorporating the drive circuit  22  of the present invention. Transmitter  20  is coupled to control loop  10  and to a cable  30  connecting the transmitter  20 , at a first location  26 , to sensor electronics  40 , at a second location  44 . The transmitter  20  includes, for example, microcontroller  24  and drive circuit  22 . Transmitter  20  may include other devices as well. 
     Transmitter  20  is powered through electric current I which is received from control loop  10 . In one embodiment, control loop  10  operates under the fieldbus protocol, described in ISA 50.02-1992 Section 11, and carries digital information. Sensor  42  senses a process variable which is related to a parameter of the process, such as temperature, pressure, or flow. The sensor electronics  40  conditions the sensed process variable information. When a transmit data request is received from the control loop  10  by the transmitter  20 , the microcontroller  24 , at a first location  26 , sends an instruction through the drive circuit  22 , across the cable  30 , and to the sensor electronics  40 , at a second location  44 . Upon receiving an instruction from the microcontroller  24 , the sensor electronics  40  performs the desired operation. If a retrieval of process variable information is called for, then the sensor electronics  40 , at a second location  44 , sends the requested information back across cable  30 , through drive circuit  22 , and to the microcontroller  24 , at a first location  26 , for further processing. 
     FIG. 2 shows microcontroller  24  and drive circuit  22 , along with electrical connections associated with the preferred embodiment. Supply node  28  of drive circuit  22  is connected to a DC supply of an appropriate voltage. In the preferred embodiment shown, node  28  is connected to an externally supplied 3.15+/−0.15 Vdc supply line. Cable  30  is shown connected to drive circuit  22 . Microcontroller node  56  is connected to drive circuit node  58  by connection  60 . Microcontroller node  62  is connected to drive circuit node  64  by connection  66 . Node  54  of microcontroller  24  is connected to terminal  52  of drive circuit  22  by data line  50 . Data line  50  serves as the information path between the microcontroller  24  and the drive circuit  22 . Digital signals can be transmitted along data line  50 . The connections between the microcontroller  24  and the drive circuit  22  are preferably very short, and may be traces on a Printed Wiring Board (PWB). Connections  60  and  66  are used to control the direction of data transmission along data line  50  between the microcontroller  24  and the drive circuit  22 . In the preferred embodiment, when node  56  of the microcontroller is at logic 0 and node  62  of the microcontroller is at logic 1, data line  50  is used to receive information through drive circuit  22  from the sensor electronics (not shown). When node  60  of microcontroller  24  is at logic 1 and node  66  of microcontroller  24  is at logic 0, data line  50  acts as a data line transmitting information from the microcontroller  24 , at a first location, to the drive circuit  22  and sensor electronics (not shown), at a second location. 
     FIG. 3 shows a preferred embodiment of the present invention. Cable  30  is connected at one of its ends  32  to terminal  70  of drive circuit  22 . Another end  34  of cable  30  is connected to sensor electronics  40 . As an example, sensor electronics  40  could be a peripheral, consisting of a remote microcontroller, an A/D converter, sensors, or other devices. The length of cable that can be driven with the drive circuit of the present invention will depend on the cable characteristics. 
     Terminal  70  of drive circuit  22  is connected to end  32  of cable  30  and serves as the open drain signal line. Terminal  70  is also connected to switching circuitry, shown generally at 80. Terminal  70  is also connected to a pull-up supply line  74  through pull-up resistor  72 , which preferably has a value of 1.5 kΩ. 
     A 3.15+/−0.15 Vdc supply line  28  is shown connected through resistor  76  to capacitor  78 . A different supply voltage could be used, which would change the cable drive capabilities. The preferred values for resistor  76  and capacitor  78  are 150Ω and 22 μF, respectively. Resistor  76  governs the time to charge capacitor  78 , which serves as the charge pump or storage device. Zener diodes  82  and  84  limit the voltage on capacitor  78 . As a result, energy stored in capacitor  78  is limited, as is shown by the equation E={fraction ( 1 / 2 )}CV 2 , where E is energy, C is capacitance, and V is the voltage across the capacitor. Zener diodes  82  and  84  preferably are 5.6 V clamps. Two or more zener diodes are used to provide redundancy, in case one should fail. This redundancy is necessary to make the drive circuit intrinsically safe. Node  74  is a reference node and represents the pull-up supply line for terminal  70  and for the open drain outputs of switching circuitry  80 , which will be described in detail shortly. 
     In the preferred embodiment, open drain NAND gates  90 ,  92 ,  94 , and  96  make up switching circuitry  80 , which provides directional control and signal propagation between the microcontroller  24 , at a first location, and the cable  30  that leads to the sensor electronics  40 , at a second location. National Semiconductor Open Drain Quad NAND 74HC03 would be an appropriate choice for implementing switching circuitry  80 . Switching circuitry  80  could also consist of alternative types of logics gates, as well. For example, other types of push-pull outputs could be used. One reason open drain gates have been selected is because they offer higher impedance than conventional gates. 
     Nodes  100 ,  102 , and  104  are open drain outputs that are relatively low capacitive loads. Resistors  106 ,  108 , and  110  are connected between the open drain outputs at nodes  100 ,  102 , and  104 , and the pull-up supply line  74 . Resistors  106 ,  108 , and  110  are used to pull up the low capacitance signal paths on the open drain switching circuits. In the preferred embodiment of switching circuitry  80 , gate inputs  112  and  114  of NAND gates  92  and  94  are connected to the Vdc supply line  28  to ensure that a signal is always replicated based on the status of the directional control lines  60  and  66 . 
     Node  104  is connected to data line  50  which is used for transmitting and receiving data to or from microcontroller  24 . Connections  60  and  66  from microcontroller  24  are inputs to open drain NAND gates  90  and  96  respectively. 
     Connections  60  and  66  may be used by the microcontroller  24  to control the direction of data flow along the data line  50 . A logic 1 level on connection  60  enables a write operation from the microcontroller  24  at a first location through the drive circuit  22  to the sensor electronics  40  at a second location. A logic 1 level on connection  66  enables a read operation from the sensor electronics  40  at a second location through the drive circuit  22  to the microcontroller  24  at a first location. Nodes  60  and  66  are configured to have opposite logic levels at all times. One way to accomplish this is to have a single directional control line coming out of the microcontroller, which splits into two lines, with one line going into one of connections  60  or  66 , and the other line going through an inverter before going into the other of nodes  60  or  66 . 
     FIG. 4 shows the drive circuit  22  of the present invention, with NAND gates  90 ,  92 ,  94 , and  96  of FIG. 3 replaced with open drain MOSFET transistors. Cable  30  and sensor electronics  40  are also shown in FIG.  4 . 
     Once the signal direction has been set by nodes  60  and  66 , communication pulses can be transmitted. To transmit a logic low voltage level switching, transistor  120  is activated (turned on) to pull terminal  70  to ground. A logic high voltage level is realized by deactivating (turning off) transistor  120 . This will allow pull-up resistor  72  and charge pump capacitor  78  to charge the cable capacitance to a valid high. Activation and deactivation of transistor  120  is accomplished automatically by the propagation of logic lows and highs through the switching circuitry since for a logic low the gate of transistor  120  will be high, allowing the discharging of capacitance to ground through the source of transistor  120 . 
     Use of an open drain output  130  at the sensor electronics end of the cable  34  can be used to transmit signals from the sensor electronics  40  to the transmitter. Open drain output  130  can National Semiconductor Open Drain Quad NAND 74HC03, for example. When the open drain output  130  is grounded, cable  30  is pulled to a low logic level, which will also pull terminal  70  low, allowing the logic low voltage level to be received by the microcontroller (not shown). Similarly, open drain output  130  can be used to pull charge off the charge pump capacitor  78  to charge cable  30  and terminal  70  to a logic high voltage level. No open drain output is needed at end  34  of cable  30  if signals are not transmitted from the peripheral at the remote second location to the transmitter at the local first location. Additionally other methods may be used at the second location to send signals across the cable to the microcontroller at the first location. 
     Open drain outputs are used in the preferred embodiment because they allow multiple outputs to be connected together without having logic high contention between the outputs. When one circuit grounds the output node, all other circuits connected to that same node will be at a logic low level as well. Similarly, all circuits must be opened to get a logic high level. The use of pull-up resistors allow the open drain device to achieve a logic high level. 
     Resistor  72  and the capacitance associated with cable  30  make up the RC time constant of discharge that determines what length of cable can be driven while maintaining desired rise times. The relationship is governed by the following equation: 
     
       
           V   c   =V   cc (1− e   RC/   −1 )  
       
     
     where Vc equals the voltage that appears at the distant end of the cable, Vcc is the supply voltage from the charge pump, R is the value of the pull-up resistor ( 72  in FIG.  4 ), C is the capacitance of the cable, and t is the time required for discharge. 
     To achieve certain rise times, for example those mandated by International Institute of Communications (IIC) protocol, the capacitive load of the cable must be charged in a specified time. In FIG. 4, cable  30  is charged through resistor  72 . If ⅔ Vcc is treated as a valid high logic level at the end of the cable containing the sensor electronics, and Vcc is 5 volts, then a voltage of approximately 3.33 volts must be attained. If the value of 72 is 1.5 kΩ, and a rise time of 1000 nS is desired, then insertion of the relevant quantities into the equation  c =V cc (1−e RC / −1 ) leads to a maximum driveable capacitance of 607.9 pF. If a cable having a capacitance of 10 pF/foot is used, then the maximum length of cable that could be driven would be 60.79 feet. 
     However, as charge is pulled off capacitor  78  during communication pulses, Vcc drops, which limits the amount of capacitance that can be driven under the acceptable rise time constraints. Therefore, a slightly shorter cable can be used, which will allow more than one communication pulse to be driven across the cable before the capacitor is required to be recharged. Computer simulations may be used to determine how many communication pulses can be sent or received with a certain load under specified rise time constraints. The pulsing will then be stopped, and the capacitor is allowed to recharge. As an example, a microcontroller in a transmitter might send or receive 10-30 communication pulses under IIC communications protocol, and then wait a period of time to allow recharging of the charge pump before initiating another 10-30 communication pulses. Node  70  in FIGS. 3 and 4 might only be pulsed for one percent of the total communication time, depending on the cable length, the available supply voltage and current, the value of the charge pump capacitor, and other variables. This period of inactivity in communication limits the average current through resistor  76  in FIGS. 3 and 4 to under 70uA, while maintaining the ability to drive a capacitive load requiring 3 mA to meet rise time constraints. 
     Intrinsic safety is realized by incorporating high impedance paths and energy limiting features into the drive circuit. The open drain gates  90 ,  92 ,  94 , and  96  in FIG. 3 offer high impedance. Resistors  72 ,  76 ,  106 ,  108 , and  110  in FIG. 3 limit the current from capacitor  78 . Zener diodes  82  and  84  limit the voltage across capacitor  78 . 
     FIG. 5 is a diagram showing part of one embodiment of a process control system  150  incorporating the drive circuit of the present invention. Transmitter  20  is coupled to control loop  10  and to a cable  30  connecting the transmitter  20 , at a first portion of a tank  152 , to sensor electronics  40 , at a second portion of the tank. The transmitter  20  includes the drive circuit of the present invention. Transmitter  20  may include other devices as well, such as a microcontroller. Transmitter  20  is shown operating with two separate sensors. Sensor  42  at an upper portion of the tank, is connected to sensor electronics  42 , through cable  30  to transmitter  20 . Sensor  142 , at a lower portion of the tank, is connected to sensor electronics  140 , which is connected to transmitter  20 . Sensor electronics  140  may also be inside the housing of the transmitter  20 . Since sensor  142  and sensor electronics  140  are relatively close to the transmitter, no cable is needed, and the drive circuit of the present invention may possibly be omitted for a local device such as this. However, sensor  42  and sensor electronics  40  are relatively far from transmitter  20 . The drive circuit of the present invention therefore allows the transmitter to communicate with the more remote sensor electronics  40  through cable  30 . The transmitter  20  may contain electronics, including a microcontroller, which can process the data retrieved from sensors  42  and  142 , and can cause appropriate action to be taken if the measured data differs from the allowable range for proper control of the particular process being controlled. This action may involve sending information across the 4-20mA control loop  10  to a central control room. 
     Now that the description of cable drive circuit  22  according to the teachings of the present invention has been set forth, many advantages and subtle features can be set forth and appreciated. For example, a key advantage of using the cable driver described by the present invention is the ability to drive signals at an acceptable rise time across a long cable while meeting low power constraints. The length of cable that can be driven is related to the cable characteristics. Data acquisition across a 30-40 foot long cable has been realized using the present invention, and longer length cable could be used as well, depending on the cable capacitance per foot, and other factors. Another advantage of the present invention is its intrinsically safe construction due to the current and voltage limiting characteristics. Another advantage of the present invention is that relatively few components are required. 
     Open drain NAND gates are used in the preferred embodiment because most conventional NAND gates are only able to drive about 10 μA. Output current from an open drain NAND gate is only limited by the supply voltage and the value of the pull-up resistors. Conventional NAND gates would not be able to drive as long a cable as open drain outputs or other push-pull outputs are able to drive. 
     The present invention allows the transmitter to be placed at a location that is isolated from the sensor electronics. Cable lengths of more than 30 feet can be driven with the cable drive circuit described herein. The characteristics of the cable, such as capacitance per linear foot, and the limitations in power availability are two constraints on the length of cable that can be driven. 
     By driving a long cable, the present invention allows the transmitter to be placed at a convenient location instead of requiring the transmitter to be placed close to the process variable being sensed. This may result in easier maintenance, increased safety, and more flexibility. Another advantage is that one transmitter can interface with two or more sensor electronics capsules, which can decrease total system costs. As an example, pressure in a large storage tank can be measured near the top of the tank and near the bottom of the tank using one transmitter. A transmitter could be placed near the pressure sensor at the bottom of the tank, while the cable drive circuit described herein could be used to drive a cable connected between the transmitter and a sensor electronics capsule near the top of the tank. Alternatively, two cable drive circuits could be used if it desired to place the transmitter at a distance from the sensor electronics capsules. 
     Although the present invention has been described with reference to preferred embodiments, one of ordinary skill in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, although the present invention has been described in conjunction with a microcontroller, other devices may be used instead of a microcontroller, including for example, a computer or microprocessor, or simpler means for providing a periodic digital signal. In addition, the preferred embodiment described above refers to sensor electronics being at the end of the driven cable. Other peripherals or devices may be substituted, such as electronically controlled valves or user interface devices or communicators. Another alteration of the preferred embodiment is to drive signals in only one direction by modifying the switching circuitry, such as by removing NAND gates  90 ,  92 ,  94 , and  96  in FIG.  3  and replacing them with one open drain output connected between the microcontroller and node  70 . For example, a switching transistor such as transistor  120  could be connected between a digital signal at data line  50  and node  70 . Microcontroller  24  or some other control means could provide the digital signal and limit the duration or time period of the digital signal according to the teachings of the present invention. Such an embodiment would be useful, for example, to send information to a remote location, such as an electronic valve. Additionally, the transmitter has been described as being at a first location, and the sensor electronics at a second location. The correspondence between the first and second locations and their associated devices is purely arbitrary and may be reversed. The component values described herein are purely suggested values and other values would work as well, depending on the desired characteristics of the drive circuit. 
     Thus since the invention disclosed herein may be embodied in other specific forms without departing from the spirit or general characteristics thereof, some of which forms have been indicated, the embodiments described herein are to be considered in all respects illustrative and not restrictive. The scope of the invention is to be indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.