Abstract:
An improved step-by-step switch assembly includes an electronic controller conversion module. A programmed microprocessor responds to incoming pulse trains of widely variant make and break ratios and generates precisely timed and shaped pulses to operate vertical, rotary and release magnets of the switch assembly. Other features include increased loop sensitivity, selective digit absorption and automatic periodic contact wiping to clean all of the contacts of the assembly.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the art of step by step switching apparatus as used for example in telephone switching systems. More particularly, the present invention relates to a step-by step switch which has been improved and updated by inclusion therein of an electronic programmed digital microcontroller conversion module. 
     Almon B. Strowger invented and patented the first two motion or step-by-step switch in 1981. Since that time Strowger switches have been refined and are still used in a widespread basis in telephone automatic switching systems in this country and throughout the world. 
     Conventional two-motion stepping switches comprise three basic parts. These parts are a pair of contacts banks, the stepping mechanism and the control relays. Typically each bank of contacts contains 100 positions with two metal contacts (for tip and ring wires on one bank and corresponding control wires on the other) at each position. The stepping mechanism uses three electromagnets, a vertical magnet, a rotary magnet, and a release magnet. In conventional telephone parlance the control relays are identified with letters of the alphabet, with selector switch control relays being identified as &#34;A&#34; through &#34;E&#34; and with connector switch control relays being identified as &#34;A&#34; through &#34;J&#34;. 
     A typical 10,000 subscriber telephone exchange implemented with step-by-step switches includes one or more switching trains comprising a &#34;line finder&#34;, a &#34;first selector&#34;, a &#34;second selector&#34; and a &#34;connector&#34;. When a telephone is taken &#34;off hook&#34; by a party about to make a call, the line finder &#34;finds&#34; the line to which the calling party&#39;s telephone is connected and electrically connects to that line. Dial tone is then returned to the calling party&#39;s instrument. As the calling party dials each number the rotary dial generates a pulse train for each digit dialed. These pulse trains are applied in turn to operate the first selector, the second selector and the connector switches. 
     For example, in the case of the first selector, the pulses of the first digit dialed are applied to operate the vertical magnet, and each pulse steps the switch shaft up one vertical position, corresponding to a row of ten contacts. The rotary magnet is then operated by pulses generated at the switch in order to find an available path to a subsequent switch. Each such pulse causes the shaft to rotate one step to each contact position of the selected row. Thus, step pulses are effective to lift the wiper of the shaft to the desired row of contacts and then to rotate the wiper to the desired contact position. 
     At the completion of the call, when the calling party hangs up, ground is removed from the commonly connected control lead, causing the release magnet of each switch in the train to be momentarily energized. This release magnet then releases the shaft from the position reached by operation of the vertical and rotary magnets. Springs apply bias force to the shaft, so that when the release magnet is energized and the shaft is released, it first rotates out of the row of contacts, and then it drops to its original starting location. The switch is then ready to handle the next call assigned to it. 
     Over the years such step-by-step switches have worked very well and have proven to be quite reliable. Nevertheless, they have certain weak points and drawbacks. For example, the control relays have typically required more attention, maintenance, adjustment and replacement than have the other elements of these switches. Adjustment of the control relays has required the provision of special tools and highly trained technicians. 
     Another drawback of the control relays as used in these switches has been their inherent limitation as to sensitivity, measured by the resistance of the overall loop length. Typically, connector switches have only been effective to detect when a called party has answered when the overall loop resistance of the communications path did not exceed 1,500 Ohms. 
     One more drawback of conventional step-by-step switches was that the actual electrical pulses applied through and by the control relays to the vertical, rotary and release magnet varied both in amplitude and in duration, depending upon such factors as the loop resistance to a particular calling party&#39;s instrument, the characteristics of the rotary dial of that instrument, and so forth. With deregulation of the telephone interconnect industry, many rotary dial instruments are now being offered for service, and their low cost dial switches require far greater operating margins than heretofore. 
     Yet another drawback of conventional step-by-step switches was the need for constant cleaning of the contacts to assure that the switches did not become corroded and consequently fail in service. Prior practice called for a maintenance person to wipe all of the contacts of these switches at regular intervals to keep them clean. 
     SUMMARY OF THE INVENTION WITH OBJECTS 
     One general object of the present invention is to overcome limitations and drawbacks of the prior art step-by-step switches. 
     Another object of the present invention is to replace all conventional electromechanical control relays of a step-by-step switch with a small electronic controller module which operates under the control of a programmed digital microprocessor in order to generate precise control pulses for operating the vertical, rotary and release magnets of the switch. 
     Yet another object of the present invention is to reduce maintenance of step-by-step switches by as much as ninety percent (90%). 
     A further object of the present invention is to increase the sensitivity of step-by-step switching apparatus to accomodate greater loop circuit resistance and thereby extend the effective distance that a subscriber may be located from a central office switching facility, thereby reducing the number of such facilities as more and more subscribers are added to the system. 
     One more object of the present invention is to extend the useful life of step-by-step switching installations while reducing the amount of time that such installations require skilled operator attention. 
     Still one more object of the present invention is to provide an automatic mechanism for operating a step-by-step switch to wipe over every contact in order to clean it on a regular interval basis. 
     Yet one more object of the present invention is to provide a programmed digital microprocessor to process incoming dial pulses of a step-by-step switch in order to correct for errors in rate and duration, to eliminate extraneous noise impulses and to shape such pulses into control signals precisely adapted for proper actuation of the vertical, rotary and release magnets of the step-by-step switch. 
     Still another object of the present invention is to convert conventional step-by-step switches to universal switches by replacement of the electromechanical control relays with an electronic controller conversion module which includes a digital microcontroller for operating the switch in accordance with a preprogrammed series of instructions in response to incoming conventional control signals. 
     Still a further object of the present invention is to provide an electronic controller conversion module for converting step-by-step switches to digital control wherein the module is compact, easy to manufacture and program at relatively low cost, far more reliable and maintenance free than the components being replaced thereby, and simple to maintain, should maintenance or reprogramming ever be required. 
     These objects are achieved in a step-by-step switch which includes an electronic controller conversion module having an input connection circuit for connecting the module to a source of control signals such as conventional telephone dialing pulses, an output electronic driver circuit connected to the vertical, rotary and release magnets of the switch, and a digital microcontroller operating under the control of a stored control program of instructions, the microcontroller being connected to receive the control signals, process the control signals and apply them to the output driver in order to operate the vertical, rotary and release magnets. Preferably, the module is packaged as a small printed circuit board for mounting within the housing of a conventional step-by-step switch in place of the electromechanical control relays which are thereby replaced, both physically and functionally. The module may be programmed to act as a universal selector switch controller, or a connector switch controller, and as a selector switch controller the module may include digital absorption circuitry for absorbing up to three predetermined initial digits in a dial up sequence such as seven digits. The module preferably includes timing circuitry and programming to carry out automatic wiping of all contacts of the switch on a timed interval basis to achieve automatic cleaning of the contacts. The module preferably includes pulse processing circuitry to extend the useful sensitivity of the switch to circuit loops having as much as 4000 Ohms of loop resistance, or more; and to correct incoming pulse series for errors in pulse rate and/or duration caused by defective or non-standard rotary dial mechanisms. 
     The method of the present invention provides electronic digital control of a step-by-step switch assembly by performing the steps of: 
     receiving control signals including dial up pulse trains, 
     digitally processing such signals to remove errors and to retime such signals, and 
     applying processed signals to operate the vertical, rotary and release magnets of the switch assembly under the control of a programmed digital microprocessor. 
     Another aspect of the method of the present invention is the conversion of a conventional step-by-step switch assembly to one operating under the control of a programmed digital microcomputer conversion module, comprising the steps of: 
     removing all electromechanical control relays from the assembly other than the vertical, rotary and release magnets thereof, 
     installing and connecting the conversion module in lieu of the removed control relays, 
     receiving and processing incoming control signals in accordance with the program controlling the conversion module, and 
     controlling the vertical, rotary and release magnets by signals processed and put out by the module. 
    
    
     These objects, advantages and features of the present invention will be further understood and appreciated by considering the following detailed description of preferred embodiments, presented in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the Drawings: 
     FIG. 1 is a somewhat diagrammatic mechanical view in perspective illustrating the principles of the present invention in converting a step-by-step Strowger switch to electronic control by replacing the conventional control relays with an electronic micro-controller conversion module of the present invention. 
     FIG. 2 is an electrical block diagram showing the connection of the conversion module to the step-by-step switch. 
     FIG. 2A diagrams jack connections for connecting certain signal lines to the conversion module of FIG. 1. 
     FIG. 3 is a plan of the layout of the FIG. 3A and FIG. 3B drawings to enable them to be read and understood as a single circuit diagram. 
     FIGS. 3A and 3B comprise together a more detailed electrical schematic diagram of a conversion module of the present invention configured for service as a &#34;selector&#34; switch. 
     FIG. 4 is a plan of the layout of the FIG. 4A and FIG. 4B drawings to enable them to be read and understood as a single circuit diagram. 
     FIGS. 4A and 4B comprise together a more detailed electrical schematic diagram of a conversion module of the present invention configured for service as a &#34;connector&#34; switch. 
     FIG. 5 is a graphic comparison of step switch dial pulse compatibility achieved with the present invention as compared to the much more restrictive compatibility of conventional switches as superimposed thereon and enclosed by dashed lines. 
     FIG. 6 is a graph of waveforms illustrating uniform electronic dial pulse processing achieved with the present invention in comparison to two widely variant incoming analog pulse trains. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Step-by-Step Switch 10 
     A conventional step-by-step or &#34;Strowger&#34; switch 10 is shown in perspective in FIG. 1. Therein, the switch includes a vertical wiper shaft 12, a control contact bank 14 having rows of contacts C and EC, and a line contact bank 16 having corresponding rows of contacts T and R (&#34;tip&#34; and &#34;ring&#34;). Two double contact wipers affixed to the shaft 12 are moved from an index or vertically downward, nonengaged normal position to a particular row of contacts and are then rotated into that row of contacts. The vertical step motion, caused by an incoming first train of dial pulses, is brought about by operation of a vertical magnet 18. The vertical magnet 18 moves a pawl 20 which engages a series of bevelled skirts 22 affixed to the shaft 12. As each skirt 22 is engaged by the pawl 20, the shaft moves up one step position. 
     After the desired contact row is reached, a rotary magnet 24 steps the shaft in a rotary direction, one step at a time. When the switch 10 is configured as a &#34;selector&#34; switch in a step-by-step dial system, the rotary magnet is operated by electrical pulses which are generated by control circuitry to cause the wiper to land on contacts corresponding to the first available trunk to a following selector switch or to a connector switch. When the switch 10 is configured as a &#34;connector&#34; switch, the rotary magnet is operated by the last dial pulse train of the number being dialed, which moves the wiper contacts to the contact pair for the subscriber being called. 
     At the completion of a call, a release magnet 26 releases latches set by the vertical and rotary magnets 18 and 24, and the shaft 12 rotates the wipers to the disengaged position and then drops the shaft 12 to the index position. 
     These general principles of step-by-step switches are well understood by those skilled in the art, and are explained in much greater detail in David Tally&#39;s book entitled Basic Telephone Switching Systems, published by Hayden Book Co., Rochelle Park, N.J., (1979) particularly in Chapter 7 entitled &#34;Step-by-Step Dial System&#34;. 
     Each conventional step-by-step switch 10 includes a frame 30 to which the electromechanical elements thereof are secured. One region of the frame 30 is for electromagnetic control relays 32 which are used to operate the switch 10 in accordance with incoming control and dial pulse signals and further in accordance with the state of the switch 10 from time to time. 
     One significant drawback of the relays 32 is that they require almost constant maintenance attention and adjustment. For example, an adjustment screw 34 is provided on each relay 32 for adjusting the amount of throw of the armature during activation. The contacts of the relays 32 are exposed to the air and corrode, requiring periodic cleaning. 
     In accordance with the present invention, the conventional relays 32 are removed from the frame 30 and are replaced by a small printed circuit module 40 containing a programmed microprocessor 42 with associated driver electronics and sealed micro relays 44 and 46. Electronic switches 48 are used to operate the vertical, rotary and release magnets 18, 24 and 26. Contact pins 50 enable the module 40 to be connected with the switch 10, and facilitate configuration of the digit absorption feature. 
     Electronic Selector Conversion Module 60 
     An electronic selector switch conversion module 60 is depicted structurally in FIGS. 1 and 2 and by the electronic block and schematic drawing of FIGS. 3A and 3B. Referring to FIG. 2, signals appearing on R, T and C lines from a preceding switch enter the R, T and C lines of the module 60. Test jacks and a busy key are also provided on the frame 30 in accordance with conventional practice. 
     Certain control lines are provided for connection of the module 60 to routiner equipment including rotary magnet RM, disable digit absorption DA, and loop circuit BH lines which enable the switch 10 to be operated manually for test and adjustment purposes. 
     Further connecting lines are provided to the release magnet battery supply RLS BAT, to ground GND, to supervisory SUPY, and to the -48 volt system supply -48 (MB). A restricted service RS and RSI loop is provided which is interrupted by left normal post springs. A cam springs line CS is normally grounded but when operated applies busy tone if the eleventh contact position of any row is reached during rotary stepping of the switch 10. Dial tone is applied through the dial tone line DT. A vertical off normal VON switch is operated any time the shaft 12 is not in its idle, resting position. 
     A hang up HU line is connected through the VON switch contact to ground whenever the shaft is &#34;off normal&#34;. This signal is important in the sense of telling the module 60 if the switch 10 has become hung up or stuck in an off normal position, even though the release magnet 26 has been actuated. 
     Connections are also provided to the vertical magnet 18 over a vertical magnet coil VMC line, to the rotary magnet 24 over a rotary magnet coil RMC line, and to the release magnet 26 over a release line RLS. 
     Wiper lines ECO and CO extend to the wiper contacts of the control bank 14, and tip and ring lines T and R extend to the wiper contacts of the line bank 16. 
     A switching power supply 62 is provided to convert the minus 48 volt system supply potential to a minus 5 volts for operating the microprocessor in the module 60. The power supply 62 is turned off when the switch 10 is in an idle condition, and the supply 62 is turned on if either a loop condition is established on tip T and ring R, or if the control C lead is grounded which indicates that the switch 10 is already connected to another switch. 
     Referring now to the electrical schematic for the module 60 as depicted in FIGS. 3A and 3B, an input impedance of 200 Ohms per leg on tip and ring is maintained with low wattage resistors R18 and R19. A 20 Ohm positive temperature coefficient PTC device in series with one side of a P relay limits current in case someone inadvertently grounds either tip or ring in the central office where resistances are low. 
     The P relay functions to seize the switch 10. When the P relay operates in response to an incoming pulse on tip and ring, a -5 volt pulse is applied at the input pin 1 of a Schmidt trigger buffer array U3. The output pin 2 thereof is connected to a data bit input port pin of an 8748 Microprocessor as made by Intel Corporation, or equivalent. 
     A switch attempting to connect to the switch 10 must first see a condition on the control C lead other than ground in order to seize the switch 10. A -48 volt signal is preferred to an open condition. If the switch 10 is not previously seized, the -48 volts is applied to the C lead by passing through a conducting transistor Q10. The microprocessor 42 constantly monitors the data status at pin 39 and responds to an incoming pulse by putting out a control pulse at pin 38, which passes through a buffer of the U3 array, pins 11 and 10 and which activates a transistor Q1, causing it to conduct. When Q1 conducts, the base of transistor Q10 is brought low, which turns Q10 off, removing the -48 volts. The C lead is thereupon grounded through a diode D3, and the switch 10 is now seized by the incoming switch. 
     Upon switch seizure, the microprocessor 42 initiates a timer. If dial pulse trains do not arrive within e.g. 30 seconds, the rotary magnet 24 is commanded to rotate to the 11th position to tell the customer that the switch is busy and to hang up. Dial pulses operate the P relay, and the pulses are put into the microprocessor 42 at the data bit input provided at pin 39. The microprocessor 42 analyzes each pulse. If it is greater than 10 msec. in duration, the microprocessor generates a precisely timed step pulse and actuates the vertical magnet 18 with it. The dial pulse train thus causes the shaft 12 to step vertically to the row of contacts corresponding to the number of pulses in the train. The rotary magnet is then operated by the microprocessor 42. 
     The availability of the next following switch is tested by operation of an optoisolator U4. This isolator has its LED input normally connected to the CO line which looks at the control contact of each following selector switch in the switch train. As a ground is encountered in step rotation among the contacts corresponding to next selectors, the LED emits and the microprocessor senses this emission by a pulse at its pin 1. The microprocessor 42 then commands the rotary magnet to step to the next contact position, until a control C contact is encountered which is not grounded. 
     When an available subsequent switch is encountered, the microprocessor 42 operates an F relay by putting out a control signal at its pin 34. The F relay cuts the transmission path, tip and ring, through to the now seized next switch. In addition, it cuts the C lead through to the next switch, so that the ground from the previous switch is now passed to the following switch on the C-CO line. The optoisolator U4 continues to monitor the status of the C-CO line. In order to test whether  the next following switch has properly responded to the ground condition on the CO line to which it is now attached, the microprocessor 42 commands the F relay to operate momentarily. If the ground condition is lost during this momentary test, indicating that the next following switch has not actually been seized, the microprocessor commands the rotary magnet to step to another available switch. 
     Once the microprocessor 42 is satisfied that the next following switch has been effectively seized, it operates the F relay for the duration of the call. In addition, a peg count PC single pulse signal is put out to a jack on the switch 10 through two buffer stages of U3 (pins 5-6 and 3-4) and a transistor Q3 and is available to indicate that a calling path has been successfully established through the switch 10. The peg count is used to monitor traffic to be sure that the central office is connected in the most efficient switching configuration, as is well understood in the art. 
     The microprocessor 42 energizes the vertical magnet coil by putting out a control signal at pin 33 which is buffered through an output buffer (pins 14-15) of an array U2 and ultimately turns on a driver transistor Q6. The driver Q6 operates a high current switch transistor Q9 in series with the vertical magnet coil VMC and the -48 volt supply. Similarly, the rotary magnet 24 is energized when the microprocessor 42 puts out a control signal at its pin 32, the signal being buffered through a buffer (pins 3-2) of the array U2 before operating a driver transistor Q5. The transistor Q5 operates a high current switch transistor Q8 in series with the rotary magnet coil RMC and the-48 volt supply. The release magnet 26 is actuated when a signal is put out at pin 31 of the microprocessor 42, is buffered through a buffer (pins 11-12) of the array U2, and operates a switch transistor Q7 to ground momentarily the coil of the release magnet which is normally connected to -48 volt supply through diode D7 and zener D25. Operation of the vertical, rotary and release magnets is monitored by the microprocessor 42 by a circuit path through diodes D30, D31 and D32 and a buffer (pins 9-10) of the array U2 and another diode D 29 to an input pin 4 of the microprocessor 42. 
     A bank of digit absorbing control pins 50 is provided on the module 60. If a digit is programmed to be absorbed, by appropriate strapping of the panel 50, dial tone must be removed when the absorbed digit is dialed. In order to remove dial tone, a dial tone relay C is actuated by the microprocessor 42 by putting out a control signal at its pin 30. The signal is buffered through a buffer (pins 7-6) of the array U2 and passed on to operate a current switch Q4 in series with the coil of the C relay. 
     In a non-digit-absorbing switch, the C relay operates somewhat differently. The C relay released puts dial tone on tip and ring, whereas when it is operated, busy tone is placed on the loop. 
     Different control signals may be strapped to cause digits to be absorbed. The absorb control A when strapped to a digit will cause that digit to be absorbed (disregarded) if it is the first digit dialed. The absorb repeatedly AR control causes a digit to which it is strapped to be absorbed repeatedly ad infinitum. The block level control causes the wiper to move to the eleventh or busy position of the row corresponding to the digit(s) to which the B signal is strapped. The cut in CI/A, CI/AR and absorb repeatedly/absorb AR/A signals operate in a known manner to provide further flexibility and programmability to the switch 10 than has been heretofore achieved by its electromechanical predecessor. It is important to note that the program conditions established by the strapping appearing at pin bank 50 are constantly monitored by the microprocessor 42 which operates the switch 10 in accordance with the conditions set by the straps and the control program. Strapping of multiple control signals to a single digit position may require use of one-way diodes to isolate each control signal from the other(s). 
     The microprocessor clock is held to a particular frequency by a crystal X1. The 8748 microprocessor contains an internal read only memory into which a suitable control program may be loaded. 
     Connector Module 80 
     A switch conversion connector module 80 for use with a connector switch is shown in FIGS. 4A and 4B. Therein, many circuit elements are the same as for the selector module 60 and will not be discussed again. Tip and ring lines enter the module 80 and are met with a solid impedance termination provided by a one-to-one isolation transformer SPT-101 and a series capacitor C2. 
     When the connector module 80 is seized by closure of the loop between tip and ring, the A relay operates, causing a pulse to pass through a Schmidt trigger buffer (pins 13-12) of an array U4 to an input bit port (pin 39) of the microprocessor. This pulse causes the microprocessor to put out a pulse at pin 38 which passes through a buffer (pins 11-10) of the array U4 to operate a driver transistor Q1 which in turn grounds the C line back to the previous selector switches in the switch train. 
     If a digit is not received within e.g. 30 seconds the microprocessor automatically operates a &#34;busy tone&#34; relay BT, and the caller is forced to hang up and replace the call, thereby freeing the system from callers who tie up switching equipment without completing the dialing sequence (e.g. children playing with the telephone). If a digit pulse train is received, the microprocessor processes the digit and puts out precise pulses to command the vertical magnet 18 to move the shaft 12 up to the appropriate level where it stops, waiting for a second digit. If a second digit is not received in e.g. 30 seconds, the microprocessor times out and rotates the wiper to the busy position. If the second digit pulse train is received, the microprocessor processes the incoming pulses and puts out precisely timed pulses to the rotary magnet 24 to cause the wiper to step to the selected final contact position corresponding to the value of the second (final) digit. This contact position corresponds to and connects with the called party&#39;s telephone line. 
     If the called party is using the telephone, the CW line will be grounded, and this condition will be sensed by the microprocessor by virtue of a signal passing through an optoisolator U6. If the called party&#39;s line is not busy, the microprocessor then operates the ring generator relay RG/RB by putting out a signal at a pin 28, buffering the signal by a buffer (pins 9-10) of the array U2 and operating a relay switch transistor Q2. The ring generator relay grounds the tip line and puts a 90 volt, 20 Hz ringing signal on the ring line. An optoisolator U5 senses the ringing current on the called party&#39;s loop and passes it on to the microprocessor (through a buffer (pins 5-6) of the array U4) which monitors the ringing waveshape. When the called party answers, the waveshape changes by becoming wider, and this change is used by the microprocessor to detect the answer. If the called party goes &#34;off hook&#34; (i.e. answers) during the non-ringing interval, a change in called party loop voltage is sensed by the optoisolator U5 and this condition is provided to the microprocessor. 
     Whenever a change in pulse width or amplitude is detected, the microprocessor closes an F relay for 20 msec, and the called party&#39;s line is tested to see if the called party&#39;s instrument really came off hook by virtue of operation of the D relay. If the customer has not really come off hook, ringing is resumed after the 20 msec test interval. This brief test interval goes unnoticed by the telephone system users. 
     If the D relay has operated by virtue of a closed loop to the called party, the microprocessor removes the ringing signal by deenergizing the RG/RB relay, and continues to operate the F relay. The F relay reverses the tip and ring connections on the incoming side of the connector 80. This reversal is used conventionally to signal preceding equipment that the call path has been completed and to start charging the customer if it is a toll call. Use of more sensitive D relays, together with use of the optoisolator U5 to monitor ringing conditions to detect answering by the called party increases the called party loop sensitivity from about 1500 Ohms to as much as 4000 Ohms or more, the limiting factor becoming the attenuation of the talk path signal strength. Of course, to increase overall loop sensitivity in a system, equivalent measures must be taken with line finder equipment so that distant subscribers will be able to place calls as well as receive them. 
     Trunk hunting is accomodated by virtue of monitoring of the ECW line, which marks the first line of a multiple line trunk. If the first line is busy (as established by the presence of a ground) the microprocessor senses this condition through an optoisolator U7 and steps rotary magnet to the next contact position, and so forth until an available line in the trunk is found. If an available position is not found in the group, the wiper is stepped to the eleventh or busy position and a busy signal is returned to the caller. 
     For trunk hunting in line groups in excess of ten, the connector module 80 operates by actuating the release magnet RLS and at the same time operating the vertical magnet VMC with a reduced current, so that the vertical position is maintained while the rotary position is returned to normal. A switch transistor Q13 is commanded by the microprocessor to apply a reduced current through the coil of the vertical magnet to maintain the current row position after the rotary magnet is released. Then, the vertical switch transistor Q14 is operated and steps the shaft up one vertical step to the next row, where line hunting resumes until an available line is found or until the supply of lines is exhausted, whereupon a busy signal is sent out. 
     Pulse Processing 
     Advantageous operation of the microprocessor in processing dial pulses may be appreciated by considering FIGS. 5 and 6. Therein, an ideal dial pulse is identified as having a 40 msec make time and a 60 msec break time (with a ten pulse per second dial pulse rate, 60 percent is break and 40 percent is make). According to telephone system standards, dial pulses may not vary much from this ideal pulse configuration. The telephone standard margin for error is indicated as lying within the timing region bounded by dashed lines in FIG. 5. 
     The much greater area bounded by solid lines in FIG. 5 depicts the much wider latitude for dial pulses which may be accomodated by employing the principles of the present invention. This wide margin is further illustrated in FIG. 6 wherein waveform A depicts a very severe dial pulse condition wherein the make period is but 10 msec, and the break period is 90 msec. This narrow pulse is shaped by the Schmidt trigger into the squared waveform depicted in waveform B, and the rising edge of waveform B is used by the microprocessor to time the start of the output pulse train, waveform C, which is restored to an ideal 40 msec make, 60 msec break condition. 
     Waveform D is much more conventional, and represents a condition wherein a longer make pulse is processed, waveform E, and results in a constant 30 msec make pulse, waveform F. 
     In both cases, the microprocessor has converted analog pulses of widely variant duration into precise driving pulses for the vertical and rotary magnets, thereby minimizing improper operation of the step-by-step switch 10. 
     Control Programs 
     Here follows two listings in assembly language of computer control programs. The first program is adapted to operate the selector module 60 and the second program is adapted to operate the connector module 80, both in accordance with the principles of the present invention. Each program is prestored in the read only memory portion of the 8748 controller microprocessor. ##SPC1## ##SPC2## ##SPC3## ##SPC4## 
     Having thus described preferred embodiments of the present invention, it will be appreciated by those skilled in the art to which the present invention pertains that many different and widely varying embodiments will be suggested without varying from the spirit and scope of the present invention. The disclosures presented herein are by way of preferred examples and are not in any sense to be considered as limiting of the scope and application of the present invention.