Remote monitor and control system
FIELD OF THE INVENTION 
     The present invention relates to remotely controlled switching systems. More particularly, the present invention relates to improvements in monitoring and control systems for remotely located devices, such as power switches. 
     BACKGROUND OF THE INVENTION 
     The need to monitor and control remote apparatus from a central control point is well known. One such need arises in the art of electric power distribution. Power from one or more generator locations is distributed by a network of distribution lines which extend to substations. Further distribution lines extend from each substation in order to reach all of the electric power customers in the service area and distribute power to them. Switching devices are interposed throughout the distribution system and service area to enable the system to be controlled by removal of one or more selected circuits from service in the event of fault or overload. Detection of the fault or overload at the central control point and commands sent therefrom via a communications path (typically provided by the telephone system) to cause actuation of a remotely actuable switch enable the control point operator to remove the fault area from the distribution network forthwith, thereby preventing further breakdown of the network on account of the fault condition. 
     One known approach for providing central monitoring and control of remote switching devices is set forth in U.S. Pat. No. 4,045,714 for &#34;Remote Switch Control and Status Indicator System&#34;. The invention described in that patent provided a workable way to monitor circuit faults and operate remote power switches from a central location. A further improvement in that system is described in U.S. Pat. No. 4,128,856 entitled &#34;Automatic Resetting Control Circuit for Air Switch Operator&#34;. Some pertinent background information may also be found in U.S. Pat. No. 4,520,234 for &#34;Remote Cable Switching System&#34;. 
     The prior system used telephone audio tone signalling between the central control point and each wire center location. A metallic wire pair, usually provided by the telephone company, extended between each remotely controllable switch and its associated wire center. Constant current in a first flow direction and at a nominal voltage of 48 volts was applied over the wire pair, and the resultant voltage drop across the loop established the actual conditions at the remote site. Various resistance elements were selectably interposed in the loop in accordance with monitored conditions at the remote site and thereby affected the voltage drop across the loop. For example, if a fault condition was present, such as overcurrent, an overcurrent detector would trip, shunting across a resistance in the control loop. Another resistance or shunt was present, depending upon whether the remote switch was open or closed. The voltage drop was measured at the wire center and then converted into an analog signal which was sent to the operator at the central control point. The following table sets forth examples of the conditions at the remote site which were being monitored: 
     
         ______________________________________                                    
Voltage   Condition             Code                                      
______________________________________                                    
0-5       Short circuit in wire pair.                                     
                                0                                         
 8.25-13.25                                                               
          Switch closed, fault condition present.                         
                                2                                         
18.00-23.00                                                               
          Switch closed, no fault.                                        
                                4                                         
28.75-33.75                                                               
          Switch open, fault condition present.                           
                                6                                         
38.50-43.50                                                               
          Switch open, no fault.                                          
                                8                                         
45-50     Wire pair open.       5                                         
______________________________________                                    
 
    
     With the prior system the operator at the central control location could dial up a wire center via telephone and obtain the current status of any one or all of the remote switch sites to which the wire center was connected. In addition, the operator could signal remote actuation of the switch if it were provided with an automatic actuator. 
     In order for remote actuation of the switch to occur, the current loop for monitoring conditions was removed from the line, and a power supply of higher voltage, typically 100 volts or more (coin collect or return) was applied across the wire pair in reverse sense or polarity. This higher voltage of reverse polarity passed through a control circuit to charge an actuation capacitor of high storage capacity. Once the charging voltage was removed from the wire pair, the energy stored in the capacitor was applied to operate an actuation solenoid, and the switch was thereupon automatically moved from its closed position to its open position. This new condition, code 8, was then presented across the control loop, and the operator at the central facility could readily determine this new condition. 
     While the solutions proposed by the referenced prior patents worked well, they were manual insofar as monitoring activity via the wire center units was concerned, and their use resulted in a number of flaws and drawbacks. Since those systems relied upon the telephone network for the communications path, certain evolutionary changes in the telephone system have negatively impacted those prior solutions. For one thing, the original remote control circuits were designed to operate when voltage in excess of approximately 56 volts appeared across the tip and ring wires of the communications pair. As electronic telephone switching systems have become prevalent, the telephone central office battery voltage has gone up in value. When applied to a wire pair connected in reverse sense, this higher nominal line voltage has actually caused the remote switch to be operated, instead of being monitored, thereby inadvertently disconnecting service area customers from the electric power distribution network and resulting in customer complaints. 
     This problem of reversed wire pairs has led directly to another problem. Prior telephone maintenance practice and procedure has been to apply an ohmmeter across the tip and ring wires in order to sense in which direction (actuation polarity) the capacitor is seen to be charging, by deflection of the ohmmeter needle. The problem has been that telephone company ohmmeters have applied approximately 60 volts across the capacitor. The capacitor having charged to 56 volts now discharges and the switch is inadvertently opened, cutting off customers from electric power. 
     Another drawback lay in the fact that unless the central control operator had reason to monitor a particular switching unit, no regular monitoring access was made to each remotely located switching unit. During times of inclement or harsh weather conditions, deterioration of failure of the wire pair might go undetected until it was desired to operate the remote switch controlled by the switching unit. By then, it was too late, and a lineman had to be dispatched to operate the switch manually. 
     SUMMARY OF THE INVENTION WITH OBJECTS 
     A general object of the present invention is to provide improvements in monitoring and control systems for remotely located devices such as power switches in a manner which overcomes limitations and drawbacks of the prior art. 
     A specific object is to provide an improved wire center unit by modularizing it to facilitate manufacturing, installation and maintenance and by providing it with a microcomputer supervisor to facilitate improved, more versatile operations. 
     Another object of the present invention is to program a microcomputer supervisor for a wire center unit so that it periodically scans all switching units attached to the unit to be sure that monitored conditions have not changed between scans, and automatically to signal to the central control point any detected change in condition. 
     A further object of the present invention is to program a microcomputer supervisor for a wire center unit so that it automatically senses the order of connection of a wire pair extending to a switching unit and automatically corrects the order in the event that a reversal is detected. 
     One more object of the present invention is to provide a switching unit with a positive threshold circuit which minimizes the liklihood that reversal of the order of connection of the wire pair extending to the switching unit will inadvertently cause a remote switch to be operated instead of being monitored. 
     One further object of the present invention is to provide an improved detection circuit for detecting the presence of switch operation induced transisents on a wire pair generated by operation of a remote switch in response to a remote switching command, thereby providing an independent confirmation of intended remote switching activity. 
     The present invention is within a remote monitoring and control system of the type typically including an operations console at a central control point, a wire center unit located remote from the operations console at a location at which wire pairs for a geographic area are concentrated, and a plurality of switching units in the area to be remotely monitored and controlled from the central control point. Each switching unit is connected to the wire center by a wire pair, and a communications path extends between the wire center and the central control point. 
     An improved wire center unit comprises a plurality of interconnected plug-in modules including: 
     at least one line select module for selecting a switching unit wire pair from among a plurality of pairs connected to the line select module, 
     a line switch decoder module for sensing conditions present at a selected switching unit and for converting sensed conditions into a binary code, and for providing a control signal for operating the selected switching unit, 
     a communications module connected to the communications path for decoding commands received from the operations console and for encoding responses from the wire center unit sent to the operations console, and 
     a programmable digital microcomputer for controlling all operations of the wire center unit in accordance with a series of instructions stored in a read only program memory associated with the digital microcomputer means. 
     In one aspect of the present invention, the digital microcomputer is programmed so that the wire center unit automatically scans periodically all switching units attached thereto in order to determine present status thereof and automatically signals the operations console when any change in status of any switching unit is detected. 
     In another aspect of the present invention, the digital microcomputer is programmed automatically to test the tip and ring order of connection of each wire pair extending to each switching unit from the wire center unit before passing on any operational commands received from the operations console. In this aspect of the invention the wire center unit includes sense reversal circuitry for sensing and reversing the order of connection of a wire pair in the event that improper order of connection of a wire pair is detected. 
     In a further aspect of the present invention, the sense reversal circuitry automatically detects a response from the switching unit characteristic of pair reversal and thereupon automatically reverses the order of connection. 
     In one more aspect of the present invention the line switch decoder module includes a switch operation detection circuit responsive to switching transients induced on a selected wire pair in response to remote switching activity. The circuit generates a binary value readable by said microcomputer means indicative of detection of a switching transient on the selected wire pair. The improved detection circuit results from a special algorithm in the microcomputer program to distinguish the desired switch transient from other transients that may be produced from line noises. 
     In a distinct aspect of the present invention, an improved switching unit is provided for use within a remote monitoring and control system of the type including a wire center unit at a location at which wire pairs for a geographic area are concentrated, and a plurality of switching units in the area to be remotely monitored and controlled from a central control point through the wire unit. In this aspect of the invention each switching unit is connected to the wire center by a wire pair and includes a monitoring circuit which indicates conditions at the switching unit in response to current flow in a forward direction and further includes a control circuit which responds to current flow in an opposite direction by operating e.g. a switch. In this aspect the improved switching unit includes a threshold circuit in series with the control circuit which passes current flow in opposite direction only when the potential difference across the threshold circuit exceeds a predetermined voltage substantially in excess of the potential difference which develops across the threshold circuit from current flow in the forward direction. 
     These and other objects, advantages and features of the present invention will be better understood and appreciated by consideration of the following detailed description of a preferred embodiment present in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the Drawings: 
     FIG. 1 is an overall system block diagram for a remote monitoring and control system incorporating the improvements of the present invention. 
     FIG. 2 is a schematic circuit diagram of a switching unit of the system depicted in FIG. 1 which incorporates improvements in accordance with the present invention. 
     FIG. 3 is a block and circuit diagram of a modular wire center unit of the system depicted in FIG. 1 which incorporates improvements in accordance with the present invention. 
     FIG. 4 is a circuit diagram of a line select module (LCn) of the wire center unit depicted in FIG. 3. 
     FIG. 5 is a circuit diagram of a line switch decoder module (LSD) of the wire center unit depicted in FIG. 3. 
     FIG. 6 is a circuit diagram of a communications module (COM) of the wire center unit depicted in FIG. 3. 
     FIGS. 7a and 7b together provide a circuit diagram of a microcomputer supervisor module (CPU) of the wire center unit depicted in FIG. 3, FIG. 7a providing the left half of the diagram and FIG. 7b providing the right half of the diagram. 
     FIG. 8 is a backplane wiring diagram of the wire center unit depicted in FIG. 3 showing the interconnection of the various modules making up the wire center unit. FIG. 8 should be read in conjunction with FIGS. 3 through 7 in order to obtain an understanding of the interconnection of the modules and the connection of the wire center unit to one hundred switching units and to the operator console at the central operations center. Backplane connectors LC 1 through LC 8 are identically connected, and they have been combined into a single connector in order to save drawing room. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     A remote monitor and control system 10 incorporating principles of the present invention is depicted in overview in FIG. 1. Therein, the system 10 includes an operator console 12 located at a power company operations center 14 at which the generation and distribution of electrical power is controlled. 
     A wire center unit 16 is located in a telephone company central office (wire concentration center) location geographically removed from the operations center 14 and connected to the operations center 14 by a voice grade dedicated private line 18. All communications between the operator console 12 and the wire center unit 16 pass over the private line 18, typically in the form of dual tone modulated frequency (touch tone tm) code sequences. 
     A substation unit 20, similar to the wire center unit 16, may be located at a remotely controlled substation located in the path of a power distribution network 22. The substation unit 20 is connected to the operations center 14 by a second voice grade dedicated line 24. While only one wire center 16 and one substation 20 are illustrated in FIG. 1, it is to be understood that all of the many wire centers and substations of the power distribution network controlled from the center 14 would be connected to the operations center for monitoring and control at the operator console 12. 
     Up to one hundred switching units 26 located in the geographical area serviced by the particular central office are serviced by the wire center 16 located in that central office. Two conductor (tip and ring wires) metallic pairs 28 extend from the central office wire center 16 to each switching unit 26, and the nominal line resistance of each pair 28 ranges from zero to approximately 2000 ohms. 
     Switching Unit 26 
     Each switching unit 26 performs two functions. It monitors conditions at its location, typically a remotely actuable power switch on a line pole or in an underground cable vault, and it controls actuation of the power switch. The basic architecture of the switching unit 26 is shown in FIG. 2, and is described in detail in the reference U.S. Pat. Nos. 4,045,714 and 4,128,856. In normal operation, a nominal -48 volt, 2.5 milliampere constant current is sent through the ring wire of the pair 28 to the selected switching unit 26. The diode D1 of the switching unit conducts, and current flows through the series resistors R1, R2 and R3 and then returns to the wire center 16 via the tip wire which is at ground. In the event that a fault such as overcurrent tripped a fault detector 30, a contact pair of the detector 30 shorts out the resistor R2, increasing the current flow. Moreover, if a line switch 32 associated with the switching unit 26 is open, a sense switch senses this fact and shorts out the resistor R3. The resistors R2 and R3 differ substantially in value. A voltage V1 measured across the tip and ring wires of the pair 28 at the wire center unit 16 provides the status at the switching unit, as already explained in connection with the condition table set forth in the Background of the Invention section of this patent, above. 
     To actuate the line switch, current is reversed on the tip and ring wires, and current passes through the diode D2 and begins to charge a large storage capacitor C3. Charging typically takes from two to five seconds. After the capacitor has charged to 56 volts the SCR Q3 fires, discharging the capacitor into the solenoid S1. The increasing charging voltage is sensed across the series resistor network R7 and R8, and the common node thereof passes through a trigger diode Q2 to trigger the SCR Q3 which enables all of the charge stored in the capacitor C3 to be passed through the winding of the solenoid S1, thereby causing it to operate. 
     As previously mentioned, if the tip and ring wires of a switching unit were inadvertently reversed, attempts to monitor conditions at the switching unit led to unwanted charging of the capacitor C3 and inadvertent operation of the solenoid S1. In accordance with one aspect of the present invention, a protection threshold circuit 34 is included in series with the D2 diode, the storage capacitor C3 and the solenoid S1. The circuit 34 includes an SCR Q1 and a series control network including a resistor R5, a 75 volt zener diode D3, and a resistor R6. This network causes the series SCR Q1 to fire only when a 75 volt potential is applied across the tip and ring wires of the pair 28 leading to the switching unit 26. Thus, the threshold circuit 34 prevents unwanted charging of the capacitor C3 in the event of reversal of the tip and ring wires of the wire pair 28. 
     Wire Center Unit 16 
     The wire center unit 16 is depicted in overview in FIG. 3. In accordance with an aspect of the present invention, the wire center unit 16 is modular, with each module plugging into a cabinet having a backplane. Backplane interconnection of the modules about to be described is shown in FIG. 8, and reference to FIG. 8 should be made in tracing out the circuits comprising the wire center unit. It will be appreciated by those skilled in the art that FIGS. 4 and 5 employ a type of notation for switch contacts referred to as &#34;detached contact&#34; notation in which an &#34;X&#34; shown intersecting a conductor represents a normally open &#34;make&#34; contact of a relay, and a &#34;bar&#34; shown intersecting a conductor at right angles represents a normally closed &#34;break&#34; contact of a relay. 
     There are five types of modules which plug into the cabinet forming the wire center unit 16. A power supply module 36 converts -48 volts and +130 volts supplied from the central office battery into supply voltages of -53 volts, +5 volts and +100 volts which are distributed to the other modules via power buses on the backplane. 
     Up to ten line select modules (LSU) 38 are provided. Each LSU module 38 is capable of selecting one of ten wire pairs 28 extending to ten service units 26. One of these modules is depicted electrically in FIG. 4. 
     A line switch decoder module (LSD) 40 is connected to the line select modules 38 by a common line 42, also referred to as LSR and LST. The line switch decoder module 40 actually measures the voltage V1 across a loop to a selected service unit 26, and also supplies charging current to the capacitor C3 to a selected unit 26 to operate its solenoid S1 and associated power switch 32. The circuitry of the line switch decoder module 40 is depicted in FIG. 5. 
     A communications module (COM) 44 provides DTMF encode and decode operations and thereby interfaces the wire center unit 16 and the operator console 12, via the dedicated private line 18. The circuitry of the communications module 44 is depicted in FIG. 6. 
     A microcomputer supervisor module (CPU) 46 provides all of the necessary control signals to the other modules of the unit 16, in accordance with a control program stored in a program read only memory (ROM). The circuitry of the microcomputer module 46 is depicted in FIG. 7, and a listing of a control program is set forth at the end of this specification. 
     Line Select Module 38 
     Each line select module 38, of which the wire center unit 16 may include up to ten, includes a series of two-input NAND gates U1 through U3 as shown in FIG. 4. One input of each gate is connected in parallel to a backplane line TO-9 to provide a select control for this particular module (the tenth module of the unit 16). Each of the other inputs is connected to an individual line U0 through U9 of the backplane (FIG. 8). The lines U0 through U9 and TO-9 extend to the communications module 44 which is responsible for actually selecting a particular module 38, and also for actually selecting one of the wire pairs 28 serviced by the module 38. Each NAND gate operates a driver transistor Q1 through Q9, and each transistor in turn causes current to pass through the winding of a line select relay L00 through L09. Operation of a particular line select relay causes the contacts of that relay to switch with the effect that a shunt is removed from the selected wire pair 28 and it is connected onto the common bus 42 leading to the line switch decoder 40 (FIG. 5). A shunt to earth ground is applied across each wire pair 28, except when selected, so that the effects of any transient currents which may be induced in the pair 28 will thereby be minimized, and unwanted operation of a switching unit 26 thereby avoided. 
     Line Switch Decoder Module 40 
     The line switch decoder module is connected to the common LSR, LST bus 42. A reversing relay REV is provided on the module 40 in order to reverse the sense of the tip and ring wires of a particular wire pair 28. This reversing relay REV operates under the control of the microcomputer 46 in response to a signal put out on backplane line L from port 1 output, bit position 8. As will be explained in conjunction with the discussion of the microcomputer 46 hereinafter, the sense of each selected pair 28 is first tested, and if a reversal is detected, the microcomputer operates the reversing relay REV and thereby reverses the tip and ring wire orientation of the particular wire pair 28 which has been selected and connected to the common bus 42. 
     A constant current providing circuit 50 applies a constant current of approximately 2.5 milliamperes to the selected wire pair 28 extending to the selected switching unit 16. This circuit comprises the transistors Q9, Q10 and Q11 and related circuitry shown on FIG. 5. A variable resistor P2 is provided to enable a nominal 2.5 milliamperes to be put out by the circuit 50. The power source for the constant current circuit 50 is -53 volts supplied by the power supply module 36. 
     As already explained, the selected switching module will return a particular voltage to the line switch decoder module, in accordance with the conditions actually present at the switching unit. The particular voltage is detected and is converted into a binary code by an analog to digital converter circuit 52 as shown on FIG. 5. This circuit comprises a voltage regulator VR1 which puts out a -27.5 voltage in combination with +5 volts to power a series of voltage comparators U2, U3, U4, U5 and U6, also as shown in FIG. 5. The actual voltage returned to the module 40 on the selected wire pair 28 is sensed by transistors Q7 and Q8 and put out on a line 54 which connects to one input of each of the comparators U2 through U6. A series of reference voltages are derived from the -53 volt supply line by a resistor series comprising the resistors R20, P1, R13, R14, R15, R16 and the R17 and R35 combination. Nodes providing -22.5 volts, -17.5 volts, -12.5 volts, -7.5 volts and -2.5 volts are provided to the other input of the comparators U2, U3, U4, U5 and U6, respectively. 
     If the selected pair 28 is open (substantially infinite impedance), the voltage appearing on the line 54 will be e.g. 25 volts, and this condition will cause a binary word of 11111 to be put out by the comparators U2-U6 on lines 21, 16, 15, 20 and 17 of the backplane. Other voltage combinations in accordance with the condition table set forth in the background of the invention will result in unique binary codes to be present on these lines. The particular binary code is sent to the microcomputer 46 where it is processed in accordance with the control program. 
     The line switch decoder module 40 also provides a reverse polarity charging current via a circuit 56. This circuit 56, including transistors Q3 and Q5 and related circuit elements as depicted in FIG. 5, applies a +130 volts to the selected wire pair 28 via the bus 42 when a charging relay CHG is operated. The microcomputer 46 commands operation of the charging relay by putting out a control signal from port 1 out, bit position 7 via the backplane line M between the CPU module 46 and the LSD module 40. At the line switch decoder module 40, the charging command is received by a transistor Q1 and thence applied through the winding of the charging relay CHG. 
     A switch operation detector circuit 58 is provided to sense the remote operation of the selected switch. When the switch operates, there is a sudden increase in line current which causes a short duration pulse to appear across the tip and ring wires of the selected pair 28. This pulse is passed through the network of R11 and C4 and is applied to the base of a transistor Q6 of the detector circuit. The pulse is amplified and shaped by transistors Q6 and Q4, and is applied to opto isolator U1. The isolator U1 shapes the pulse to be read by the microcomputer. A binary low signal is present at an &#34;SO&#34; output of the isolator U1, and this signal is sent via line U of the backplane to the microcomputer 46 which may read it as bit 6 of port 1 input to confirm that switching has occurred, independently of the condition sensing circuitry 52. 
     Communications Module 44 
     As shown in FIG. 6, the communications module 44 includes two separate circuits, a binary to decimal converter circuit including decoders U3 and U4, and a touch tone decoder/encoder circuit comprising the balance of circuit elements appearing in FIG. 6. 
     Binary codes put out by the microcomputer 46 are used to address the binary to decimal converter circuit. The LSB input to U3 and the MSB input to U4 come directly from port 2 output from the microcomputer 46. The LSB and MSB outputs of the decoders U3 and U4 are applied to select a particular line select unit 38 and a line within that unit. 
     Other binary codes put out by the microcomputer 46 may be converted by the communications module 44 into DTMF code tones to be returned via the dedicated private line 18 to the operator console 12. These codes are put out by the microcomputer 46 at its port 0 output, bits 1, 2, 3 and 4 and enter a decoder U10 as D0, D1, D2 and D3 via backplane lines 50, 48, 52 and 54. The decoded binary codes are converted into one of sixteen available DTMF tone combinations by operation of the U1 tone encoder. The resultant DTMF tone is level set by adjustment of the P1 resistor, amplified by a driven amplifier U2, passed through a DC isolation capacitor C3 and an impedance matching transformer T2. The tone may then be passed through normally open contacts of a direction relay DIR and the windings of a talk circuit transformer T1 and thence via the private line 18 to the operator console. The direction relay DIR is operated by the microcomputer 46 via a signal DIR put out from port 0 output, bit position 7, and line 51 of the backplane leading to the communications module 44. The DIR signal is amplified by U9 and applied to Q1. 
     When the direction relay is not operated, the communications module 44 is conditioned to received DTMF tone commands sent from the operator console. These tones enter the talk circuit transformer T1 via the private line 18. From the transformer T1, the tones enter a touch tone decoder provided by the integrated circuits U6 and U7. These circuits operate to decode the touch tones and put them out as binary values on lines RD0, RD1, RD2, RD3 and RDV which are directly read by the microcomputer at its port 0 input. 
     A hardware reset command (DTMF &#34;#&#34; symbol) may be sent from the operator console by a DTMF tone combination sensed by a NAND gate U5 as shown in FIG. 6. When the particular combination required by the FIG. 6 connections is present, a reset condition is generated by U5 and applied to the microcomputer at its reset input. 
     Another automatic reset command loop is provided from the DIR signal amplifier U9 as shown in FIG. 6. During normal program operation the DIR signal is at a logic low for only about 50 milliseconds at a time when a tone is applied to the line. In the rare event that the CPU 46 should fail to execute its program, the DIR signal will be at a logic low in excess of 50 milliseconds. This situation is detected by the U9 amplifier chain, and U9, pins 5-6, pins 12- and pins 3-4 generate a reset pulse which passes through a blocking diode D2. This reset pulse also causes the microcomputer 46 to restart execution of its program. 
     A light emitting diode LED 1 on the communications module 44 is operated by the microcomputer 46 under program control. With the control program in an idle loop, an alternating high and low logic level on an LED signal line causes the LED 1 to blink to inform an observer at the wire center unit that the unit 16 is properly functioning under program control. 
     Microcomputer 46 
     A conventionally structured microcomputer 46 is depicted in FIGS. 7a and 7b. Therein, a central processor, such as the Intel 8085 microprocessor is connected by address and address/data buses to address decoders U2, and U3. An address/data selector U6 enables both data and the lower 8 address bits to use the same pins of the microcomputer as a known characteristic of this particular microprocessor. Read only memories, U10, U111, U12 and U13 are selected by the address decoder U3. Random access memories U8 and U9 are provided for storing values developed during program execution. Tristate buffers U14, U15, U16, U17 and U18 are selected by address decoder U2 and buffer incoming and outgoing binary signals which pass via the backplane to the other modules of the line center unit 16. 
     BRIEF DESCRIPTION OF WIRE CENTER PROGRAM 
     The wire center control program is written in 29 parts as indicated by the numbers in the margins. The functions of each part appear from the remarks provided therein. Part 1 is a declaration table equating mnemonic phrases to memory locations. 
     At power up to reset, the program jumps to the vectors and code tables (lines 128-155) and to the initialization code in Part 2 (lines 121-190). Part 2 also checks the data in the 100 byte line memory area of the random access memory U8, U9, at line 105, to see if it is valid. If the memory area does not contain valid data, the memory area is refreshed by scanning all 100 control lines and saving the current status of the line switches (lines 191-219). 
     Part 3, the main loop, controls the flashing circuit assurance light emitting diode (LED) located on the communications module 44 (line 236), checks for incoming tones (line 375), and checks the field switches for changes in status every time the LED flashes the number of times defined at line 77. Part 4 is the test for incoming tones. Part 5 gets the tone and checks it to see if it is a command tone (line 383) or a numeric tone. 
     Commands are processed at Parts 6, 13 and 14. A numeric tone, which is used to select a line is processed in part 7. Once two numeric tones have been received, a control line is selected (line 423). 
     Part 8 reads the status of the field switch. The heart of this operation is found in the subroutine located in Part 15. Here, code in Parts 16 and 17 are used to read the condition of the line and determine if the TIP and RING polarity is correct. The results of these operations are returned to Part 7 and transmitted via tone at Part 10. This process continues until the wire center is reset, or until an Execute command tone is received (Part 11) between the outgoing status tones. 
     Part 12 handles the execute operation and contains the algorithm for detecting the line pulse indicating the field switch has operated. On receiving the pulse or at time out (line 65), the wire center resumes the status checking operation on the selected line. 
     When the wire center is reset, the selected line is deselected and the program resumes checking the field switches for changes of status at Part 3. 
     Here follows a computer control program for use with the microcomputer 46. This program is written in accordance with the CP/M Macro Assembler Version 2.0. ##SPC1## 
     Having thus described one preferred embodiment of the present invention, it will be appreciated by those skilled in the art that the objects of this invention have been achieved in realizing a remote monitoring and control system for e.g. power switching and distribution apparatus. To those skilled in the art to which this invention pertains, many changes in construction and widely varying embodiments will be readily apparent without departing from the spirit or scope of this invention. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting.