Abstract:
Messages on controller area net work (CAN) buses are communicated over subsea optical links. An adaptor couples a CAN bus to an optical link. The adaptor detects a direction of transmission, that is, whether a signal began on the CAN bus coupled to the adaptor or on the optical link coupled to the adaptor. Signals from the CAN bus are conditionally transmitted to the optical link depending on the detected direction of transmission. The adaptor can operate at the physical layer without analyzing contents of the CAN bus communications.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of communication buses and, in particular, to systems and methods for adapting controller area network buses for subsea optical communication. 
         [0002]    Subsea systems, such as those used in exploration and production of oil and gas, continue to increase in complexity. A subsea well can include sensors and actuators located at or below the sea floor. The sensors can be, for example, pressure sensors, temperature sensors, and erosion detectors. The actuators can be, for example, valves, pumps, and other flow control devices. Information from the sensors is commonly processed by equipment at a surface facility. Similarly, controls for the actuators commonly originate at a surface facility. Accordingly, communication is needed between the subsea devices and equipment at the surface. 
         [0003]    Controller area network (CAN) buses are used to interconnect sensors, actuators, controllers, and other devices in applications such as automobiles, industrial automation, and medical equipment. Many circuits and devices have been developed for CAN bus communications. However, current CAN bus based subsea systems face several limitations. Network size is restricted due to the impedance drop that results from connecting multiple electrical devices in parallel. Additionally, the driver component of the network is susceptible to data corruption or damage from a short circuit failure if long transmission lines cause time delays. 
       SUMMARY 
       [0004]    Systems and methods for communicating controller area network buses via subsea optical links are provided. In one aspect, the invention provides a device for adapting a controller area network (CAN) bus for optical communication. The device includes: a CAN transceiver coupled to a CAN bus, configured to sense levels on the CAN bus and supply a first signal indicating the sensed levels, and configured to receive a second signal and drive a corresponding level on the CAN bus; an opto-electrical converter configured to convert an optical input to an electrical signal to supply the second signal; an electro-optical converter configured to convert a third electrical signal to an optical input to supply an optical output, a direction detection flip-flop configured to receive the second signal from the opto-electrical converter and the first signal from the CAN transceiver and to produce a direction detection signal by latching the status of the second signal at transitions of the first signal corresponding to transitions of the CAN bus from a recessive state to a dominant level; and a transmit enable buffer coupled to the first signal and configured to produce the third electrical signal by conditionally outputting the first signal based on the direction detection signal. 
         [0005]    In another aspect, the invention provides a device for adapting a controller area network bus for subsea optical communication. The device includes: direction detection circuitry configured to receive a first signal indicative of a status of the controller area network bus and a second signal indicative of a status of an optical signal received by the device, and to supply a transmission-direction signal indicating a transmission direction using the received signals; and transmit-enable circuitry configured to supply a transmit signal to an electro-optical converter for optical transmission, the transmit signal supplied by combining the first signal and the transmission-direction signal. 
         [0006]    In another aspect, the invention provides a subsea device. The subsea device includes: an underwater housing having an optical input and an optical output; a controller area network device coupled to a controller area network bus; a controller area network transceiver coupled to the controller area network bus, configured to sense levels on the controller area network bus and supply a first signal indicating the sensed levels, and configured to receive a second signal and drive a corresponding level on the controller area network bus; an opto-electrical converter coupled to the optical input and configured to convert a signal received on the optical input to an electrical signal to supply the second signal; an electro-optical converter coupled to the optical output and configured to convert a third signal to an optical signal for transmission on the optical output; direction detection circuitry configured to receive the second signal and the first signal and to supply a transmission-direction signal indicating whether a signal is being transmitted from the controller area network bus; and transmit-enable circuitry coupled to the controller area network transceiver, the direction detection circuitry, and the electro-optical converter and configured to supply the third signal equaling the first signal when enabled by the transmission-direction signal indicating transmission from the controller area network bus. 
         [0007]    In another aspect, the invention provides a device for adapting a controller area network (CAN) bus for subsea communication. The device includes: a CAN transceiver coupled to a CAN bus, configured to sense levels on the CAN bus and supply a first signal indicating the sensed levels, and configured to receive a second signal and drive a corresponding level on the CAN bus; an acoustic modem configured to convert an acoustic input to an electrical signal to supply the second signal and to convert a third electrical signal to an acoustic signal to supply an acoustic output; a direction detection flip-flop configured to receive the second signal from the acoustic modem and the first signal from the CAN transceiver and to produce a direction detection signal by latching the status of the second signal at transitions of the first signal corresponding to transitions of the CAN bus from a recessive state to a dominant level; and a transmit enable buffer coupled to the first signal and configured to produce the third electrical signal by conditionally outputting the first signal based on the direction detection signal. 
         [0008]    Other features and advantages of the present invention should be apparent from the following description which illustrates, by way of example, aspects of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The details of the present invention, both as to its structure and operation, may be gleaned in part by study of the accompanying drawings, in which like reference numerals refer to like parts, and in which: 
           [0010]      FIG. 1  is a block diagram of a controller area network in accordance with aspects of the invention; 
           [0011]      FIG. 2  is a functional block diagram of an adaptor for communicating a controller area network bus optically in accordance with aspects of the invention; 
           [0012]      FIG. 3  is a functional block diagram of a subsea device with an adaptor similar to the adaptor of  FIG. 2  installed in a subsea equipment enclosure in accordance with aspects of the invention; and 
           [0013]      FIG. 4  is a schematic diagram of an adaptor for communicating a controller area network bus optically in accordance with aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a block diagram of a controller area network in accordance with aspects of the invention. The controller area network (CAN) includes three bus groups. A first bus group includes a CAN device  15   a  connected to a first CAN bus  10   a . A second bus group includes CAN devices  15   b  connected to a second CAN bus  10   b . A third bus group includes CAN devices  15   c  connected to a third CAN bus  10   c.    
         [0015]    The CAN bus  10  of each bus group serves as a communication medium for the CAN devices  15  of that bus group. For example, one of the CAN devices  15   b  of the second bus group may communicate over the second CAN bus  10   b  with another one of the CAN devices  15   b  of the second bus group. Data are transmitted on the CAN bus  10  as a sequence of binary pulses. The binary pulses are received by all of the CAN devices  15  connected to the CAN bus  10  including the transmitting one of the CAN devices  15 . In the CAN bus protocol, messages are transmitted in standard formats. The messages may include an identifier of the associated device, message data, and various control fields. The messages can vary in length and may include more than 100 bits. 
         [0016]    Each CAN bus  10  commonly uses a differential pair of signal wires. The signal wires are termed a high signal “CANH” and a low signal “CANL.” The CAN protocol designates a logic 0 as a “dominant” signal and a logic 1 as a “recessive” signal. Recessive signals are represented by a lower voltage on the CANH signal and a higher voltage on the CANL signal. Dominant signals are represented by a higher voltage on the CANH signal and a lower voltage on the CANL signal. Other signal representation may be used. The electrical characteristics of transmitters coupled to a CAN bus are such that if a dominant signal is transmitted from any transmitter, a dominant signal appears on the bus. This occurs even if other transmitters are transmitting recessive signals. This may also be viewed as the transmitters only transmitting the dominant signal with the default state of the CAN bus being recessive. 
         [0017]    The CAN devices  15  may include sensors and actuators. The CAN devices  15  also include circuitry to interface to the CAN bus  10 . The interface circuitry includes electrical and protocol functions. When one of the CAN devices  15  can initiate communication with other CAN devices  15 , the initiating device may be considered a master and the other devices considered slaves. In some embodiments, the controller area network has a single master CAN device. 
         [0018]    Pairs of the bus groups are connected via inter-bus links  20 . Each of the inter-bus links  20  may use a pair of optical links with one optical fiber for communication in one direction and a second optical fiber for communication in the opposite direction. Alternatively, wired or wireless links may be used rather than optical links. For example, acoustic modems may be used for communication on one of the inter-bus links  20 . Additionally, a single optical fiber may be used, for example, by using wave division duplexing. 
         [0019]    Adaptors  100  couple the CAN buses  10  to the inter-bus links  20 . The adaptors  100  transmit sequences of binary pulses received from a CAN bus  10  to an inter-bus link  20  and transmit sequences of binary pulses received from the inter-bus link  20  to the CAN bus  10 . A pair of the adaptors  100 , one at each end of one of the inter-bus links  20  can thus couple two of the CAN buses  10 . Each of the adaptors  100  may be the same or similar. 
         [0020]    In the controller area network of  FIG. 1 , a first adaptor  100   a  couples the first CAN bus  10   a  to a first inter-bus link  20   a , and a second adaptor  100   b  couples the second CAN bus  10   b  to the first inter-bus link  20   a . Similarly, a third adaptor  100   c  couples the second CAN bus  10   b  to a second inter-bus link  20   b , and a fourth adaptor  100   d  couples the third CAN bus  10   c  to the second inter-bus link  20   b . Each of the adaptors  100  provides bidirectional communication so that information can be communicated between the bus groups in either direction. Additionally, the CAN devices may communicate through multiple inter-bus links  20 . For example, the CAN device  15   a  in the first bus group may transmit data to one of the CAN devices  15   c  in the third bus group via the first CAN bus  10   a , the first adaptor  100   a , the first inter-bus link  20   a , the second adaptor  100   b , the second CAN bus  10   b , the third adaptor  100   c , the second inter-bus link  20   b , the fourth adaptor  100   d , and the third CAN bus  10   c . The adaptors  100  function in the same manner whether a CAN device  15  on the CAN bus  10  that one of the adaptors  100  is connected to is considered a master or a slave device. 
         [0021]    The inter-bus links  20  and associated adaptors  100  may allow the CAN buses  10  and the associated CAN devices  15  to be separated by large distances, for example, several kilometers. The adaptors  100  can be used to provide electrical isolation between the CAN buses  10 . The adaptors  100  can also be used to couple CAN buses with incompatible electrical characteristics. The use of bus groups interconnected through adaptors may also allow a larger number of CAN devices to communicate, for example, by avoiding low impedances caused by a large number parallel connected devices. 
         [0022]    In an embodiment, one or more of the bus groups is located subsea and the inter-bus links  20  are provided by optical fibers. Accordingly, the controller area network of  FIG. 1  can be considered to provide a subsea optical CAN bus. 
         [0023]    The adaptors  100  break circular traffic that could block communication in the network. The adaptors  100  only transmit dominant signals to the inter-bus link when the dominant signals are first received from the local CAN bus. Dominant signals that are first received from the inter-bus link and then appear on the local CAN bus are not retransmitted to the inter-bus link. 
         [0024]    The adaptors  100  transmit pulses between a CAN bus  10  and an inter-bus link  20  pulse by pulse. That is, each pulse is transmitted when it is received without analyzing subsequent pulses to determine whether to transmit the pulse. 
         [0025]    Prior devices for coupling buses have stored all or part of an entire message before transmitting the first pulses of the message. For example, the identifier of the messages may have been looked up in a table of device identifiers to determine the location of the corresponding device and based on the location determine whether to transmit the message. Storing the messages leads to undesired delays during signal transmission. The circuitry for analyzing and storing the messages increases complexity and cost. Additionally, a table of device identifiers and corresponding locations may be difficult to configure and maintain. In other words, prior devices include functions that operate at higher protocol levels, such as a data link layer. 
         [0026]    In an embodiment, a single master processor can communicate with every CAN device in the network. Additionally, protecting a system with at least one back up processor results in a very reliable and cost effective system in the event of a primary processor failure. 
         [0027]    Although the controller area network is illustrated in  FIG. 1  with a particular number of bus groups and a particular combination of CAN devices, other embodiments may have a different number of bus groups and a different combination of CAN devices. Additionally, to enable a concise description, this disclosure uses the terminology of the CAN bus protocol, for example, as described in the ISO 11898 series of standards. The devices, methods, and techniques described are also applicable to other protocols. 
         [0028]      FIG. 2  is a functional block diagram of an adaptor for communicating a controller area network bus optically in accordance with aspects of the invention. The adaptor transmits signals received from an optical receive link  21  to a CAN bus  10 . The adaptor also transmits signals from the CAN bus  10  to an optical transmit link  22 . 
         [0029]    The adaptor of  FIG. 2  may be used as one of the adaptors  100  of the controller area network described with reference to  FIG. 1 . For example, when the adaptor is used as the first adaptor  100   a  of the controller area network of  FIG. 1 , the CAN bus  10  corresponds to the first CAN bus  10   a  and the optical receive link  21  and the optical transmit link  22  make up the first inter-bus link  20   a.    
         [0030]    The adaptor includes a CAN transceiver  110 . The CAN transceiver  110  is coupled to the CAN bus  10 . The coupling may be via a coupling network, for example, a network of resistors and capacitors for impedance matching. The CAN transceiver  110  may be an integrated circuit, for example, a TJA1054A from NXP Semiconductors. A receive signal RX is supplied by the CAN transceiver  110  that signals the level the CAN transceiver  110  senses on the CAN bus  10 . The receive signal is a logic 0 when a dominant level is sensed and a logic 1 when a recessive level is sensed. The CAN transceiver  110  receives a transmit signal TX and drives a corresponding level on the CAN bus  10 . 
         [0031]    The CAN transceiver  110  may include additional functions, such as fault detection and various power states. For example, the CAN bus transceiver  100  may detect a persistent dominant state and then disable driving the CAN bus. This may be useful for initialization, or for recovering from an erroneous state caused, for example, by a noise transient. In some embodiments, a persistent dominant state is detected by other blocks of the adaptor. 
         [0032]    The adaptor also includes an electro-optical converter  160  and an opto-electrical converter  170 . The electro-optical converter  160  converts an electrical signal from a transmit enable module  140  to an optical signal for transmission on the optical transmit link  22 . The electro-optical converter  160  may include devices such as a light-emitting diode or a laser diode. The opto-electrical converter  170  converts an optical signal received from the optical receive link  21  to an electrical signal for use in the adaptor including as the transmit signal TX supplied to the CAN bus transceiver  110 . The opto-electrical converter  170  may include devices such as a phototransistor or a PIN diode. In an embodiment, a light-on signal is used for the dominant level and a light-off signal is used for the recessive level. In alternative embodiments, the electro-optical converter  160  and the opto-electrical converter  170  may be replaced with converters appropriate to the type of communication used on the corresponding inter-bus link. In an embodiment, the electro-optical converter  160  and the opto-electrical converter  170  are replaced with an acoustic modem that can convert an electrical signal to an acoustic signal for transmission and convert a received acoustic signal to an electrical signal. 
         [0033]    The transmit enable module  140  supplies a signal to the electro-optical converter  160  for transmission from the adaptor. The signal supplied is based on the receive signal RX from the CAN transceiver  110  and a direction detection signal from a direction detection module  120 . 
         [0034]    The direction detection module  120  determines whether a signal is being transmitted from the CAN bus  10  to the optical transmit link  22  or from the optical receive link  21  to the CAN bus  10 . The direction detection module  120  determines the direction using the signal from the opto-electrical converter  170  and the receive signal RX from the CAN transceiver  110 . The direction detection module  120  can determine whether a recessive-to-dominant transition on the receive signal RX from the CAN transceiver  110  occurs before a recessive-to-dominant transition on the signal from the opto-electrical converter  170 . When the transition occurs first on the receive signal RX from the CAN transceiver  110 , the dominant signal originated on the CAN bus  10 . 
         [0035]    When the dominant signal originated on the CAN bus  10 , the transmit enable module  140  passes the receive signal RX from the CAN transceiver  110  to the electro-optical converter  160 . When the dominant signal did not originate on the CAN bus  10 , the transmit enable module  140  supplies a recessive signal to the electro-optical converter  160 . Since a recessive signal is the default state of a CAN bus, the adaptor may be considered to be transmitting when the transmit enable module  140  produces a dominant signal and not transmitting when the transmit enable module  140  produces a recessive signal. Not transmitting may also be viewed as transmitting a recessive signal. 
         [0036]    By only transmitting dominant signals to the optical transmit link  22  when the dominant signal originated on the CAN bus  10 , the adaptor prevents a closed-loop lock that could otherwise occur. If an adaptor simply converted the receive signal RX from the CAN transceiver  110  to an optical signal and transmitted it on the optical transmit link  22 , a dominant signal (originating from the CAN bus or the optical receive link) would persist indefinitely. For example, a dominant signal received from the optical receive link  21  by the opto-electrical converter  170  is supplied to the CAN transceiver  110  and driven on the CAN bus  10 . Since the CAN transceiver  110  reflects the state of the CAN bus  10 , it supplies a dominant signal on the receive signal RX. Without a transmit enable module, the dominant signal is transmitted by the electro-optical converter  160  on the optical transmit link  22 . A like adaptor at the remote end of the optical links would function in the same way and retransmit a dominant signal on its optical transmit link which is received by the opto-electrical converter  170 . Thus, closed-loop lock results unless the adaptors are configured to prevent this problem. 
         [0037]      FIG. 3  is a functional block diagram of a subsea device with an adaptor similar to the adaptor of  FIG. 2  installed in a subsea equipment enclosure in accordance with aspects of the invention. The subsea device includes a CAN device  15  which may include an input device such as a pressure or temperature sensor as well as a CAN bus controller which controls sending and receiving of signals between the input device and a CAN bus  10 . The CAN device  15  is housed in an enclosure  200 . The enclosure  200  may be a sealed subsea equipment housing. The enclosure  200  also houses an adaptor  100 . 
         [0038]    The adaptor  100  includes a CAN transceiver  110 , a direction detection module  120 , a transmit enable module  140 , an electro-optical converter  160 , and an opto-electrical converter  170 . The components of the adaptor  100  function in a like manner to like named components of the adaptor of  FIG. 2 . The electro-optical converter  160  transmits a signal on an optical transmit link  22 . The opto-electrical converter  170  receives a signal from an optical receive link  21 . The subsea device may include a connector  210  for connecting a fiber optic cable associated with optical transmit and receive links  20 ,  21  to subsea optical links or cables. The connector  210  may be provided by a suitable underwater connector or penetrator. The connector  210  may, for example, be used to connect the subsea device to a subsea optical or electro-optical cable. 
         [0039]    In the embodiment illustrated in  FIG. 3 , the CAN bus  10  is not connected external to the subsea device. That is, the CAN bus  10  only connects to the CAN device  15  and the CAN transceiver  110 . Some circuitry may be shared between the CAN device  15  and the CAN transceiver  110  and circuitry used to transmit and receive to and from the CAN bus may be simplified or eliminated. In other embodiments, additional CAN devices may be coupled to the CAN bus  10 . 
         [0040]      FIG. 4  is a schematic diagram of an adaptor for communicating a controller area network bus optically in accordance with aspects of the invention. The adaptor operates in substantially the same manner as the adaptor of  FIG. 2 . The adaptor of  FIG. 4  transmits signals received from an optical input  21  to a CAN bus  10 . The adaptor transmits signals from the CAN bus  10  to an optical output  22 . The adaptor may be used as one of the adaptors  100  of the controller area network described with reference to  FIG. 1 . 
         [0041]    The adaptor includes a CAN transceiver  110 . The CAN transceiver  110  is coupled to the CAN bus  10  via a coupling network  112 . The coupling network  112  may be a network of resistors and capacitors arranged for impedance matching. The CAN transceiver  110  may be the same or similar to the CAN transceiver of the adaptor of  FIG. 1 . The CAN transceiver  110  supplies a receive signal RX that signals the level on the CAN bus  10 . The CAN transceiver  110  receives a transmit signal TX and drives a corresponding level on the CAN bus  10 . In the illustrated embodiment, on the receive and transmit signals, a low level corresponds to a logic 0 or dominant level and a high level corresponds to a logic 1 or recessive level. 
         [0042]    The adaptor includes an electro-optical converter  160  that includes a light-emitting diode  161  to produce the optical output  22 . The light-emitting diode  161  is coupled between a positive supply signal VCC and a ground signal in series with a PNP transistor  162  and a first transmit resistor  163 . The PNP transistor  162  operates to switch the light-emitting diode  161  off and on and thereby control signaling on the optical output  22 . The first transmit resistor  163  limits current through the light-emitting diode  161 . The PNP transistor  162  has its base terminal coupled to the positive supply signal VCC by a second transmit resistor  164  and a third transmit resistor  165  coupled in series. The midpoint between the second transmit resistor  164  and the third transmit resistor  165  is coupled to a transmit enable buffer  149  to receive a signal to be transmitted. The second transmit resistor  164  and the third transmit resistor  165  provide biasing and provide a pullup when the transmit enable buffer  149  is disabled. 
         [0043]    The adaptor includes an opto-electrical converter  170  that include a phototransistor  171  to receive the optical input  21 . The phototransistor  171  has its collector terminal connected to the positive supply signal VCC and its emitter terminal coupled to the ground signal via a first receive resistor  173 . The emitter terminal of the phototransistor  171  is also coupled the base terminal of an NPN transistor  172 . The NPN transistor  172  has its emitter terminal connected to the ground signal and its collector terminal coupled to the positive supply signal VCC via a second receive resistor  174 . The NPN transistor  172 , the first receive resistor  173 , and the second receive resistor  174  combine to amplify the signal from the phototransistor  171 . The amplified signal is used in the adaptor including as the transmit signal TX supplied to the CAN bus transceiver  110 . In the illustrated embodiment, a light-on signal is used for the dominant level and a light-off signal is used for the recessive level on the optical input  21  and the optical output  22 . 
         [0044]    A transmit enable module  140  includes a series of inverters and the transmit enable buffer  149 . The transmit enable buffer  149  has an output signal that supplies the signal to be transmitted as the optical output  22 . The transmit enable buffer  149  is a tristate buffer that is enabled by a direction detection signal from a direction detection flip-flop  121 . The transmit enable buffer  149  receives a data input from a fourth transmit enable inverter  144  that is connected in series with a third transmit enable inverter  143  that is connected in series with a second transmit enable inverter  142  that is connected in series with a first transmit enable inverter  141 . The first transmit enable inverter  141  receives as its input the receive signal RX from the CAN transceiver  110 . 
         [0045]    When disabled, the output of the transmit enable buffer  149  is pulled high by the third transmit resistor  165 . When enabled, the output of the transmit enable buffer  149  matches the receive signal RX from the CAN transceiver  110  (albeit delayed). The delay provided by the transmit enable invertors is used to match circuit delays incurred to produce the direction detection signal. Thus, the delay provided by the transmit enable inverters does need to be adjusted for different data rates. Since the delay is used to match the delay of other digital circuits, the delay can be provided without the use of an analog circuit, which could be difficult to implement. 
         [0046]    A direction detection module  120  in the embodiment of  FIG. 4  includes the direction detection flip-flop  121  that is used in determining whether a signal is being transmitted from the CAN bus  10  to the optical output  22  or from the optical input  21  to the CAN bus  10 . The direction detection flip-flop  121  receives as a data input the amplified signal reflecting the optical input  21 , which is the transmit signal TX supplied to the CAN transceiver  110 . The direction detection flip-flop  121  latches its data input triggered by rising edges of a clock input to the direction detection flip-flop  121 . The clock input is an inverted version of the receive signal RX from the CAN transceiver  110  supplied from a direction detection inverter  122 . The direction detection flip-flop  121  supplies its data output to the transmit enable buffer  149  as the direction detection signal. 
         [0047]    The direction detection signal is high and the transmit enable buffer  149  is enabled when the direction detection flip-flop  121  stores a high signal. The direction detection signal is low and the transmit enable buffer  149  is disabled when the direction detection flip-flop  121  stores a low signal. Since the direction detection flip-flop  121  stores the state of the transmit signal TX at falling edges of the receive signal RX, the transmit enable buffer  149  is enabled when the transmit signal TX is high (recessive) when the receive signal RX falls (recessive to dominant transition). Conversely, the transmit enable buffer  149  is disabled when the transmit signal TX is low (dominant) when the receive signal RX falls (recessive to dominant transition). 
         [0048]    Thus, the direction detection flip-flop  121  and associated circuitry serve to determine whether a recessive-to-dominant transition on the receive signal RX occurs before a recessive-to-dominant transition on the transmit signal TX. When the transition occurs first on the receive signal RX, the dominant signal originated on the CAN bus  10 . Since the adaptor only transmits dominant signals to the optical output  22  when the dominant signal originated on the CAN bus  10 , the adaptor prevents a closed-loop lock that could otherwise occur. 
         [0049]    In an example embodiment, the direction detection flip-flop  121  is provided by a 74HC74 integrated circuit. The direction detection inverter  122 , the first transmit enable inverter  141 , the second transmit enable inverter  142 , the third transmit enable inverter  143 , and the fourth transmit enable inverter  144  are provided by a 74HC540 integrated circuit. The transmit enable buffer  149  is provided by a 74HC126 integrated circuit. The light-emitting diode  161  is an IF-E91A from Industrial Fiber Optics. The PNP transistor  162  is a 2N2907. The first transmit resistor  163  is 220Ω, the second transmit resistor  164  is 3.9 kΩ and the third transmit resistor  165  is 4.7 kΩ. The phototransistor  171  is an IF-D92 from Industrial Fiber Optics. The NPN transistor  172  is a 2N2222A. The first receive resistor  173  is 220Ω and the second receive resistor  174  is 2.2 kΩ. It should be recognized that many variations are also possible. 
         [0050]    Those of skill will appreciate that the various illustrative blocks and modules described in connection with the embodiments disclosed herein can be implemented in various forms. Some blocks and modules have been described above generally in terms of their functionality. How such functionality is implemented depends upon the design constraints imposed on an overall system. Skilled persons can implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the invention. In addition, the grouping of functions within a module, block, or step is for ease of description. Specific functions or steps can be moved from one module or block without departing from the invention. 
         [0051]    The various illustrative blocks and modules described in connection with the embodiments disclosed herein can be implemented in or with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor can be a microprocessor, but in the alternative, the processor can be any processor, controller, microcontroller, or state machine. A processor can also be implemented as a combination of computing devices, for example, a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
         [0052]    The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly limited by nothing other than the appended claims.