Abstract:
Electronic devices require correct power in order to operate correctly. Receiving an incorrect power signal could potentially result in immediate and/or long term harm to the electronic device and/or catastrophic conditions. Certain devices, such as programmable logic controller (“PLC”) modules, may provide variable power to electronic devices, such as field devices comprising sensors and/or actuators. The present inventors have recognized that deciphering between AC and DC input signals before coupling electronic devices to such power sources may advantageously avoid harmful and/or catastrophic conditions. Aspects of the present invention provide an electronic circuit for deciphering between an AC and a DC input signal. Systems and methods are also described.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to electronic circuits, and in particular, to an electronic circuit that deciphers between AC and DC input signals. 
         [0002]    Electronic devices typically use power from alternating current (“AC”) and/or direct current (“DC”) sources. For example, an electronic device may receive an AC power signal from an AC power source for its power, or may receive an AC power signal from an AC power source for internally deriving a DC power signal for its power, or may receive a DC power signal from a DC power source for its power. Some electronic devices could use both AC power signals and DC power signals concurrently for its power. In many industrial applications, AC power signals may be provided by AC power sources between 87 to 265 Volts at 50 to 60 Hertz, and DC power signals may be provided by DC power sources between 18 to 32 Volts. 
         [0003]    Industrial controllers, such as programmable logic controllers (“PLC&#39;s”), are specialized electronic computer systems used for the control of industrial processes or machinery, such as in a factory environment. Industrial controllers differ from conventional computers in a number of ways. Physically, they are constructed to be substantially more robust against shock and damage and to better resist external contaminants and extreme environmental conditions. The processors and operating systems are optimized for real-time control and execute languages allowing ready customization of programs to comport with a variety of different controller applications. Industrial controllers may also have a user interface for accessing, controlling and/or monitoring the industrial controller, such as through a locally connected terminal having a keyboard, mouse and display. 
         [0004]    Industrial controllers often have a modular architecture that allows different numbers and types of network cards or adapters to be used to connect the controllers to electronic devices or field devices, such as sensors and, actuators for controlling and interacting with industrial processes and/or machines. Field devices may be dedicated to a particular function, such as detecting an input signals or running a motor, and may be located at convenient control points near the industrial processes and/or machines. Field devices may receive AC and/or DC power locally, or in preferred applications, may receive AC and/or DC power from the industrial controller. 
         [0005]    Electronic devices or field devices require correct power in order to operate correctly. Receiving an incorrect power signal could potentially result in immediate and/or long term harm to the electronic device and/or catastrophic conditions, including possible fire, personal injury or death. 
       SUMMARY OF THE INVENTION 
       [0006]    The present inventors have recognized that deciphering between AC and DC input signals before coupling electronic devices, such as field devices, to a power source may advantageously avoid harmful and/or catastrophic conditions. As such, the present inventors have invented circuits, applications and methods for deciphering between AC and DC input signals before coupling electronic devices to a power source. 
         [0007]    Aspects of the present invention provide an electronic circuit for deciphering between an AC and a DC input signal comprising: an AC filter receiving an input signal and coupled to a first leg, the AC filter permitting an AC signal to pass to the first leg and preventing a DC signal from passing to the first leg; and a DC alter receiving the input signal and coupled to a second leg, the DC filter permitting a DC signal to pass to the second leg and preventing at least part of an AC signal from passing to the second leg. A DC signal passing to the second leg causes the second leg to produce a second output for indicating a DC input signal, and an AC signal passing to the first leg causes the first leg to produce a first output for indicating an AC input signal and causes a lock out circuit to prevent the second leg from producing the second output. 
         [0008]    The AC filter may comprise a series capacitor, and the DC filter may comprise a series diode. 
         [0009]    The first and second outputs may be coupled together to produce a third output for indicating an AC or a DC input signal. The first and second outputs may be coupled to light emitting diodes, and/or may produce audible tones and/or digital data signals. 
         [0010]    The lock out circuit may comprise a transistor in the second leg controlled by circuitry in the first leg. 
         [0011]    Another embodiment may provide a programmable logic controller module for providing power to a field device comprising: an enclosure; an electrical terminal on the enclosure coupled to a power signal; an AC filter contained in the enclosure receiving the power signal and coupled to a first leg of a circuit, the AC filter permitting an AC signal to pass to the first leg and preventing a DC signal from passing to the first leg; and a DC filter contained in the enclosure receiving the power signal and coupled to a second leg of the circuit, the DC filter permitting a DC signal to pass to the second leg and preventing at least part of an AC signal from passing to the second leg. A DC signal passing to the second leg causes the second leg to produce a second output for indicating a DC power signal, and an AC signal passing to the first leg causes the first leg to produce a first output for indicating an AC power signal and causes a lock out circuit to prevent the second leg from producing the second output. 
         [0012]    Yet another embodiment may provide a method for deciphering between an AC and a DC input signal comprising: (a) receiving an input signal; (b) permitting an AC component of the input signal to pass to a first leg of a circuit while preventing a DC component of the input signal from passing to the first leg; (c) permitting a DC component of the input signal to pass to a second leg of the circuit while preventing at least part of an AC component of the signal from passing to the second leg; and (d) producing a second output for indicating a DC input signal following a DC component of the input signal passing to the second leg, and producing a first output for indicating an AC input signal and preventing the second leg from producing the second output following an AC component of the input signal passing to the first leg. 
         [0013]    These and other objects, advantages and aspects of the invention will become apparent from the following description. The particular objects and advantages described herein may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is an isometric view of a bank of programmable logic controller (“PLC”) modules for an industrial control system in accordance with an embodiment of the present invention; 
           [0015]      FIG. 2  is an isometric view of an I/O module in accordance with the embodiment of  FIG. 1 ; 
           [0016]      FIG. 3  is an electronic circuit that deciphers between AC and DC input signals in accordance with an embodiment of the present invention; 
           [0017]      FIG. 4  is another embodiment of an electronic circuit that deciphers between AC and DC input signals; 
           [0018]      FIG. 5  is a diagram showing an industrial controller module and a field device, each in accordance with an embodiment of the present invention; 
           [0019]      FIG. 6  is an isometric view of a handheld diagnostic probe in accordance with an embodiment of the present invention; and 
           [0020]      FIG. 7  is a flow diagram illustrating a method for deciphering between an AC and a DC input signal in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance, with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.” 
         [0022]    Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1 , the present invention shall be described in the context of a bank of programmable logic controller (“PLC”) modules  10  for an industrial control system in accordance with an embodiment of the present invention. The modules  10  include a series of enclosures arranged from left to right, side by side, with complex shapes that may be generally cubic. The series of enclosures may include a left most module  12 , followed by a first double, vertical terminal I/O module  14 , followed by a second double, vertical terminal I/O module  16  and followed by a right most single, vertical terminal I/O module  18 . Each module, may be held in close proximity to the other by attachment to a DIN rail (not shown) thereby facilitating electrical connections side by side. The electrical connections side by side may provide a common system backplane to form a network ring topology and may pass a module power signal starting from the left most module  12  to each neighboring module. Each module may also include one or more internal bus slots for releasably receiving a function card and/or equivalently embedded logic. 
         [0023]    An alternative embodiment may instead provide a single PLC module, a bank of modules with greater separation and having electrical connections supported by cabling, or any other electronic-based system that provides power for another device. 
         [0024]    The left most module  12  may comprise a plurality of connectors  22 , and visual displays and/or LED&#39;s  20  for system monitoring, located on one or more respective surfaces. The connectors  22  may include connections for networking, such as a plurality of RJ-45 network connectors for network ring connectivity of the bank and for access to the bank, for a locally connected terminal, such as a Universal Serial Bus (“USB”) and/or IEEE 1394 connector, and a power connector  24  for powering the left most module  12 , and in turn, the bank of PLC modules  10 . Any other advantageous connection may also be provided as understood in the art. 
         [0025]    I/O modules  14 ,  16  and  18  may each comprise a plurality of electrical terminals or terminal blocks, or screw terminals)  26 ,  28  and  30 , respectively, on each enclosure for receiving electrical conductors exposed on the surface of the enclosure. The conductors may couple to one or more external field devices (not shown), such as sensors and/or actuators interacting with industrial controlled processes and/or machines. Within each I/O module  14 ,  16  and  18 , the electrical terminals may be routed to one or more internal bus slots, and in turn, to releasably received function cards held within the one or more internal bus slots, or to equivalently embedded logic. 
         [0026]    The electrical terminals  26 ,  28  and  30  on each of the I/O modules  14 ,  16  and  18  may be divided into a first row of electrical terminals  32  for communicating with field devices, a second row of electrical terminals  34  for providing a ground signal to the field devices and a third row of electrical terminals  36  for providing a power signal to the field devices. In a preferred embodiment, each of the electrical terminals in the second row  34  of a particular I/O module may be coupled together, and each of the electrical terminals in the third row  36  of a particular I/O module may be coupled together. However, in an alternative embodiment, some or all of the electrical terminals in the second row  34  and/or some or all of the electrical terminals in the third row  36  may be isolated from others to provide a plurality of power and/or ground signals from the same I/O module, such as AC and DC power signals. 
         [0027]    Power originating from a power source may be provided as alternating current (“AC”), such as in the range of 87 to 265 Volts at 50 to 60 Hertz, and/or as direct current (“DC”), such as in the range of 18 to 32 Volts. The left most module  12  may receive, for example, an AC power signal from an AC power source (or a DC power signal from a DC power source) via power connector  24 . In a daisy chain topology, the left most module  12  may then provide the power signal at electrical terminal  38 , and a ground signal at electrical terminal  40 , with the electrical terminals  38  and  40  each exposed on the surface of the enclosure of the left most module  12 . The power signal may be passed from the left most module  12  to the neighboring I/O module  14  via a jumper connection between the electrical terminal  38  on the left most module  12  and the electrical terminal  39  on the I/O module  14 , and the ground signal may be passed via a jumper connection between the electrical terminal  40  on the left most module  12  and the electrical terminal  41  on the I/O module  14 . The power signal may then be passed from the I/O module  14  to the next neighboring I/O module  16  via a jumper connection between the electrical terminal  42  on the I/O module  14  and the electrical terminal  43  on the I/O module  16 , and the ground signal may be passed via a jumper connection between the electrical terminal  44  on the I/O module  14  and the electrical terminal  45  on the I/O module  16 . The power signal may then be passed from the I/O module  16  to the next neighboring I/O module  18  via a jumper connection between the electrical terminal  46  on the I/O module  16  and the electrical terminal  47  on the I/O module  18 , and the ground signal may be passed via a jumper connection between the electrical terminal  48  on the I/O module  16  and the electrical terminal  49  on the I/O module  18 . 
         [0028]    Each of the I/O modules  14 ,  16  and  18  may include, for example, one or more light emitting diodes (“LED&#39;s”)  52 ,  53 ,  54 ,  55 ,  56  and  57  reflecting the presence and/or nature of the power signal provided. Similarly, the left most module  12  may include, for example, LED&#39;s  50  and  51 . One LED may indicate the presence of an AC power signal, whereas another LED may indicate the presence of a DC power signal. Alternatively, a single bi-colored LED may be used in which red or other color illumination indicates the presence of an AC power signal, and a yellow or different color illumination indicates the presence of a DC power signal. Also, a lack of illumination may indicate a lack of any type of power available. Various LED colors and affixed labeling may also be employed. 
         [0029]    In an alternative embodiment, the left most module  12  may instead provide the power and ground signals in a daisy chain topology to neighboring I/O modules via side by side electrical connections as described with respect to  FIG. 2  below. In another alternative embodiment, one or more of the I/O modules  14 ,  16  and  18  may receive, for example, a power signal from a power source via dedicated local power connectors in a star topology, and may, in turn, provide the power and ground signals at the electrical terminal exposed on the surface of the enclosure of the I/O modules as desired. Of course, any combination of the above techniques may be employed as understood by one of ordinary skill in the art. 
         [0030]    Referring now to  FIG. 2 , an isometric view of the I/O module  14  is shown. I/O module  14  may comprise an enclosure providing a left flanking side  70  and a right flanking side  72 . Each side may further comprise a releasable electrical connector  74  with a mechanical mechanism supported on each flanking side exposed to connect adjacent modules on the left and right sides. I/O module  14  may further comprise a releasable mechanical connector  76  for holding the I/O module  14  in proximity to the modules on the left and right sides. The releasable mechanical connector may be adapted to hold the I/O module  14  by attachment to a DIN rail  78 . The I/O module  14  may include internal bus slots  80  and  82  for releasably receiving function cards. 
         [0031]    Referring now to  FIG. 3 , an electronic circuit  100  that deciphers between AC and DC input signals in accordance with an embodiment of the present invention is shown. The circuit  100  may receive a positive DC input signal  102  or an AC input signal  104 , and a ground signal, for example, at a pair of electrical terminals  106  and  108 . The input signal may then couple to a series inductor  110 , which may be 10 mH, providing AC filtering and choke. The input signal may then be sent in two directions, a first direction having a DC filter, herein comprising a series diode  112 , which may be rated to +600 Volts DC, and a second direction having an AC filter, herein comprising a series capacitor  114 , which may be about 1 μF. The AC filter permits an AC signal to pass to a first leg  116  of the circuit  100  and prevents a DC signal from passing to the first leg  116 . The positive voltage with respect to ground in the first leg  116  during an AC positive excursion is the voltage of Zener diode  140 , wherein the cathode of the Zener diode  140  is coupled to the AC filter and the anode of the Zener diode  140  is coupled to ground; the AC negative excursion is the forward voltage drop of the Zener diode  140 . The DC filter permits a DC signal to pass to the second leg  118  of the circuit  100 , and allows only the positive AC signal to the second leg  118 , thereby preventing at least part of an AC signal from passing to the second leg  118 . 
         [0032]    If the input signal is the positive DC input signal  102 , the signal (output of the DC filter) passes to the second leg  118  to produce an output for indicating a positive DC input signal. This may occur by the signal (output of the DC filter) passing to a constant current source  120  in series with an LED  122  coupled to ground. The constant current source  120  may comprise, for example, a larger resistor  124 , which may be about 2 MΩ, coupled between the gate and drain of an n-channel metal oxide semiconductor field effect transistor (“MOSFET”)  126 , an NPN bipolar junction transistor (“BJT”)  128  with the base of the BJT  128  coupled to the source of the MOSFET  126  and the collector of the BJT  128  coupled to the gate of the MOSFET  126 , and a smaller resistor  130 , which may be about 100Ω, coupled between the base and the emitter of the BJT  128 . In operation, following the DC filter, the signal passes to the second leg  118 , enters the constant current source  120  at the drain of the MOSFET  126 , exits the constant current source  120  at the emitter of the BJT  128  (having a current strength of about 10 mA), and illuminates the LED  122 . 
         [0033]    If, on the other hand, the input signal is the AC input signal  104 , the signal (output of the AC filter) passes to the first leg  116  to produce an output for indicating an AC input signal and causes a lock out circuit to prevent the second leg  118  from producing the output for indicating a DC input signal. This may occur by the signal (output of the AC filter) in the first leg  116  being, rectified and passed in two directions: (i) to a diode  146 , then to a constant current source  148  in series, then to an LED  150  in series coupled to ground, and (ii) to circuitry in the first leg  116  used to control circuitry in the second leg  118 . The signal (output of the AC filter) may be rectified in the first leg  116  by a Zener diode  140  with the cathode of the Zener diode  140  coupled to the signal (output of the AC filter) and the anode of the Zener diode  140  coupled to ground, a diode  142  with the anode of the diode  142  coupled to the signal (output of the AC filter) and the cathode of the diode  142  coupled to a resistor  144  to ground, which may be about 46.4 KΩ, and a capacitor  160 , which may be about 2.2 μF, coupled between the cathode of the diode  142  and ground (in parallel with the resistor  144 ). 
         [0034]    As mentioned above, the signal (output of the AC filter), which has been rectified, may be passed to the diode  146 , then to the constant current source  148  in series, which may comprise, for example, a larger resistor  152 , which may be about 10 KΩ, coupled between the gate and drain of an n-channel MOSFET  154 , an NPN BJT  156  with the base of the BJT  156  coupled to the source of the MOSFET  154  and the collector of the BJT  156  coupled to the gate of the MOSFET  154 , and a smaller resistor  158 , which may be about 49.9Ω, coupled between the base and the emitter of the BJT  128 , then to the LED  150  in series coupled to ground, in operation, following the diode  146 , the signal passes to the constant current source  148  at the drain of the MOSFET  154 , exits the constant current source  148  at the emitter of the BJT  156  (having a current strength of about 10 mA), and illuminates the LED  150 . 
         [0035]    Also as mentioned above, the signal (output of the AC filter), which has been rectified, may be used (indirectly) to control circuitry in the second leg  118 . For example, the cathode of the diode  142  may be coupled to the gate of an n-channel MOSFET  162 . The source of the MOSFET  162  may be coupled to ground, and the drain of the MOSFET  162  may be coupled to the gate of the MOSFET  126  in the second leg  118 . In operation, following rectification of the signal (output of the AC filter), the gate of the MOSFET  162  is held high, causing the MOSFET  162  to pull the gate of the MOSFET  126  low, which thereby inhibits current from flowing through the constant current source  120  to illuminate the LED  122 . 
         [0036]    In an alternative embodiment, the circuit  100  may be adapted to produce an additional output for indicating a negative DC input signal, or may be adapted to produce an output for indicating a negative DC input signal instead of a positive DC input signal. For example, in the circuit  100 , if an additional output for indicating a negative DC input signal is desired, the circuit components  112 ,  124 ,  126 ,  128 ,  130  and  122  may be replicated as components  112 ′,  124 ′,  126 ′,  128 ′,  130 ′ and  122 ′ in an additional leg in which the direction of the diode  112 ′ is reversed, the MOSFET  126 ′ is a p-channel MOSFET, the BJT  128 ′ is a PNP BJT and the direction of the LED  122 ′ is reversed. Similarly, if an output for indicating a negative DC input signal is desired instead of an output for indicating a positive DC input signal, the circuit components  112 ,  124 ,  126 ,  128 ,  130  and  122  may be replaced by the components  112 ′,  124 ′,  126 ′,  128 ′,  130 ′ and  122 ′ in which the direction of the diode  112 ′ is reversed, the MOSFET  126 ′ is a p-channel MOSFET, the BJT  128 ′ is a PNP BJT and the direction of the LED  122 ′ is reversed. 
         [0037]    Referring now to  FIG. 4 , another embodiment of an electronic circuit  200  that deciphers between AC and DC input signals is shown. Similar to the circuit  100  as described above with respect to  FIG. 3 , in the circuit  200 , if an input signal is a DC input signal  202 , the signal passes to a constant current source  220  in a second leg  218 , and if the input signal is an AC input signal  204 , the signal passes to a first leg  216  in which it is rectified and passed in two directions: (i) to a diode  246 , then to a constant current source  248  in series, and (ii) to circuitry in the first leg  216  used to control circuitry in the second leg  218 . However, unlike the circuit  100  as described above, the constant current source  220  in the second leg  218  and the constant current source  248  in the first leg  216  do not each couple to LED&#39;s. Instead, the constant current source  220  in the second leg  218  couples in series to the anode of a diode  222 , and the constant current source  248  in the first leg  216  couples in series to the anode of a diode  250 . The cathode of the diode  222  and the cathode of the diode  250  may couple together and, in turn, couple in series to the anode of LED  280 . In operation, if the input signal is either the DC input signal  202  or the AC input signal  204 , the LED  280  will illuminate. In an alternative embodiment, the diodes  222  and  250  may be replaced with LED&#39;s, providing an electronic circuit that deciphers between AC and DC input signals with three LED&#39;s. 
         [0038]    As described above with respect to  FIG. 1 , Various LED colors and affixed labeling may also be employed. In addition, alternative embodiments may provide other output mechanisms for signaling between AC and DC input signals. For example, instead of or in addition to LED&#39;s, a speaker may be configured in each leg, including with push button activation or silencing, to produce variable audible tones to distinguish between AC and DC input signals present. Also, instead of or in addition to LED&#39;s, the output of each, leg may be digitized and coupled as binary bits to a logic controller to flag the presence of AC or DC input signals. Indeed, a variety of other alert mechanisms may be employed as understood by one of ordinary skill in the art. 
         [0039]    Referring now to  FIG. 5 , a diagram showing an industrial controller I/O module  300  and a field device  400 , each in accordance with an embodiment of the present invention, is shown. The I/O module  300  may comprise a left electrical connector  302  and a right electrical connector  304  on flanking sides exposed to connect to adjacent modules on the left and right sides (not shown). The electrical connector  302  and  304  may provide a common system backplane, including communications, power and ground signals as described. A first communications signal  306  may couple between the left electrical connector  302  and a logic controller  316  in the I/O module  300 , and a second communications signal  307  may couple between the logic controller  316  and the right electrical connector  304  to form a network ring with the adjacent modules on the left and right sides. The logic controller  316  may comprise, for example, a function card releasably received in an internal bus slot and/or embedded logic. A module power signal  308 , which may be a DC power signal from the adjacent module on the left side, may couple between the left electrical connector  302  and the right electrical connector  304  and may provide power to the I/O module  300  as necessary, including to the logic controller  316 . 
         [0040]    The logic controller  316  may couple to a plurality of electrical terminals (or terminal blocks, or screw terminals)  318  on the surface of the enclosure of the I/O module  300 . The electrical terminals  318  may receive electrical conductors which may couple to one or more external field devices. The electrical terminals  318  are divided into a first row for communicating with field devices, a second row for providing a ground signal to the field devices and a third row for providing a power signal to the field devices. In this embodiment, each of the electrical terminals  318  in the second row are coupled together, providing a common ground signal, and each of the electrical terminals  318  in the third row are, coupled together, providing a common power signal. Here, electrical terminal  320  in the first row communicates with a field logic controller  402  in a field device  400  via conductor  322 , electrical terminal  324  in the second row provides a ground signal to the field device  400  via conductor  326 , and electrical terminal  328  in the third row provides a power signal to the field device  400  via conductor  330 . 
         [0041]    In one embodiment, an electrical terminal in the second row of the I/O module may receive a power and/or ground signal from one neighboring I/O module via a jumper connection, and another electrical terminal of, the I/O module may provide the power and/or ground signal to another neighboring I/O module via another jumper connection. For example, I/O module  300  may receive a ground signal at electrical terminal  332  in the second row from the adjacent module on the left side and may provide the ground signal at electrical terminal  334  in the second row to the adjacent module on the right side, and I/O module  300  may receive a power signal at electrical terminal  336  in the third row from the adjacent module on the left side and may provide the power signal at electrical terminal  338  in the third row to the adjacent module on the right side. 
         [0042]    In another embodiment, the I/O module  300  may instead receive and provide power and/or ground signals via the electrical connections side by side. For example, a DC power signal  309  and ground signal  310  provided from the adjacent module on the left side may couple between the left electrical connector  302  and the right electrical connector  304  and may couple to power logic  340 . An AC power signal  312  and ground signal  314  also provided from the adjacent module on the left side may also couple between the left electrical connector  302  and the right electrical connector  304  and may also couple to the power logic  340 . The power logic  340 , which may be controlled by the logic controller  316 , may select which power and/or ground signals to couple to which electrical terminals as desired. For example, the power logic  340  may provide the DC power signal  309  to electrical terminal  338  in the third row via connection  342 , and the power logic  340  may provide the ground signal  310  to electrical terminal  334  in the second row via connection  344 . 
         [0043]    In yet another alternative embodiment, the I/O module  300  may receive, for example, power and/or ground signals from a power source via connection  346 , and may provide the power and/or ground signals to the electrical terminals as desired. Of course, any combination of the above techniques may be employed as understood by one of ordinary skill in the art. As such, the I/O module  300  may provide a detection and/or indication of the nature of power being provided, and/or the field device may provide a detection and/or indication of the nature of power being received. 
         [0044]    Here, the I/O module  300  includes an electronic circuit  350  that deciphers between AC and DC input signals in accordance with an embodiment of the present invention. The circuit  350  may receive a power signal from electrical terminal  336  in the third row via connection  352 , and may receive a ground signal from electrical terminal  332  in the second row via connection  354 . In response, the circuit  350  may illuminate LED  356  for indicating a DC power signal via connection  358 , or may illuminate LED  360  for indicating an AC power signal via connection  362 . The I/O module  300  may also include detection logic  364  coupled between the circuit  350  and the logic controller  316  which may, for example, provide a digitized output to the logic controller  316  to flag the presence of AC or DC input signals, or may adjust one or more other properties of the circuit  350 . 
         [0045]    Similarly, the field device  400  includes an electronic circuit  410  that deciphers between AC and DC input signals in accordance with an embodiment of the present invention. The circuit  410  may receive a power signal from electrical terminal  328  in the third row via conductor  330 , and may receive a ground signal from electrical terminal  324  in the second row via conductor  326 . In response, the circuit  410  may illuminate LED  412  for indicating a DC power signal via connection  414 , or may illuminate LED  416  for indicating an AC power signal via connection  418 . The field device  400  may also include field power logic  420  which may, upon the circuit  410  indicating safe power to the field power logic  420  via connections  414  and  418 , safely provide power to the field logic controller  402  and the field device  400  as necessary. The field device  400  may also include field detection logic  422  coupled between the circuit  410  and the field logic controller  402 , and/or coupled between the circuit  410  and the field power logic  420 , which may, for example, provide a digitized output to the field logic controller  402  and/or the field power logic  420  to flag the presence of AC or DC input signals, or may adjust one or more other properties of the circuit  410 . 
         [0046]    Referring now to  FIG. 6 , an isometric view of a handheld diagnostic probe  460  in accordance with an embodiment of the present invention is shown. The probe  460  includes an electronic circuit that deciphers between AC and DC input signals in accordance with an embodiment of the present invention. Such a probe may be used, for example, by a technician to safely determine the presence of AC and DC input signals while reducing the risk of catastrophic results. A tip  462  on the probe  460  may touch, for example, an electrical terminal exposed on the surface of an I/O module. Accordingly, the tip  462  may input the resulting signal to the circuit. In response, the circuit may illuminate LED  464  for indicating a DC power signal, or may illuminate LED  466 , and/or may illuminate LED  468  for indicating a DC or AC power signal. The probe  460  may also include a speaker  470 , which may produce variable audible tones to distinguish between AC and DC input signals present, and a push button  472  which may silence such tones. The probe  460  may also include cabling  474  for providing power, ground and data communications, including providing a digitized output to a remote device for indicating the presence of an AC or DC input signal, though in an alternative embodiment the probe  460  may include such features wirelessly. 
         [0047]    Referring now to  FIG. 7 , a flog diagram illustrating a method for deciphering between an AC and a DC input signal in accordance with an embodiment of the present invention. In step  500 , an input signal is received. Next, in step  502 , an AC component of the input signal is permitted to pass to a first leg of a circuit while a DC component of the input signal is prevented from passing to the first leg. Next, in step  504 , a DC component of the input signal is permitted to pass to the second leg of the circuit while an AC component of the signal is prevented from passing to the second leg. Next, in decision step  506 , it is determined if an AC component of the input signal passed to the first leg. If an AC component of the input signal passed to the first leg, then, in step  508 , a first output for indicating an AC input signal is produced, then, in step  510 , the second leg is prevented from producing a second output. However, as determined in decision step  506 , if an AC component of the input signal did not pass to the first leg, then, in decision step  512 , it is determined if a DC component of the input signal passed to the second leg. If a DC component of the input signal passed to the second leg, then, in step  514 , a second output for indicating a DC input signal is produced. After the second leg is prevented from producing a second output in step  510 , or after the second output for indicating a DC input signal is produced in step  514 , the method may repeat from step  500 . 
         [0048]    Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper,” “lower,” “above,” and “below” refer to directions in the drawings to which reference is made. Terms such as “front,” “back.” “rear,” “bottom,” “side,” “left” and “right” describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first,” “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
         [0049]    When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a,” “an,” “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising,” “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
         [0050]    References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
         [0051]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as coming within the scope of the following claims. All of the publications described herein including patents and non-patent publications are hereby incorporated herein by reference in their entireties. 
         [0052]    The present invention may be part of a “safety system” used to protect human life and limb in a field, warehouse, factory or industrial environment. Nevertheless, the term “safety,” “safely” or “safe” as used herein is not a representation that the present invention will make the environment safe or that other systems will produce unsafe operation. Safety in an industrial process depends on a wide variety of factors outside of the scope of the present invention including: design of the safety system, installation and maintenance of the components of the safety system, and the cooperation and training of individuals using the safety system. Although the present invention is intended to be highly reliable, all physical systems are susceptible to failure and provision must be made for such failure.