Abstract:
In a computer-based motor drive, a microcomputer is responsive to a plurality of input signals drived from the motor AC lines whenever contacts, or pushbuttons, commanding specific control operations to be performed, are closed. In such case, opto-couplers convert such AC voltage into a square pulse train which conveys a succession of digital high and low levels. A reference signal is derived, through an opto-coupler form the AC line, which in its high and low logic form is delayed in relation to the input signal trains. By sampling the input signal upon the occurrence fo a falling edge of the reference signal, then at least after the delay existing between the signals, the validity of the input signal is tested by comparing the two successive samples, thereby insuring a true command addressed to the microcomputer.

Description:
The present invention relates to computer-based control systems in general, and in particular to an input system for such control system wherein the signal inputs to the computer are guarded against invalid readings. 
     The control system according to the present invention involves electro-optical devices each upon a computer input for coupling an external command, or status signal, to a logic input for the solid state devices handling data within the computer. Typically, an external signal, in the form of a 120 volt, 60 Hertz voltage, is applied to the electro-optical device whenever the command, or status, logic is translated into a closed position for a relay, pushbutton, or similar electromechanical device. When appears, within the control system, a signal indicating a particular command, or status, such as YES, ON, GO, START, RUN, STOP, having a positive meaning for the corresponding input within the computer and understood as a closed connection established onto the opti-optical coupler, a 120 volt, 60 Hertz voltage is applied to the coupler Bit becomes at the output thereof a square pulse train of of equal width at the same frequency and exhibiting +5 volts as the successive polarity thereof. Normally, the computer will detect such 60 Hertz recurrent polarity as an indication that a positive command, or status, is present at the external input of the computer. When no external signal is present, there is no squarewave to be read by the computer, only a constant low voltage. 
     With such a system, however, there is a risk of false reading in two situations: first, a noise might occur at the instant of reading falsely appearing as if it belonged to a High of a squarewave when there is none; secondly, a failure of the opto-coupler places a constant High at its output, again interpreted by the computer as a signal present on the external input line. In order to reliably distinguish between a genuine signal and such false readings, it has been proposed to use a sampling technique wherein three measurements of the input signal are made at 120 degrees in relation to six successive line cycles. See U.S. Pat. No. 4,456,867 of George Mallick et al. 
     The prior art has also considered, in a different context, error detection in the presence of a train of pulses. For instance, U.S. Pat. No. 3,646,453 shows detecting an error whenever two successive pulses of the same polarity are received where a bipolar signal was expected. In U.S. Pat. No. 3,961,203 the aim is at the correlation of positive and negative pulse pairs. U.S. Pat. No. 3,942,037 shows edge sensing to detect a positive, or a negative, transition on the inputted signal. In U.S. Pat. No. 4,179,625 a digital input is analyzed by using delayed leading and trailing edges and a comparator determines whether a noise pulse has been detected. U.S. Pat. No. 4,525,635 involves two successive latchings of the received input signal to eliminate a transient signal. U.S. Pat. No. 4,353,032 relates to glitch detection in a sampled data acquisition system. U.S. Pat. No. 4,409,635 shows the use of two voltage measurements to detect failures in the interface circuit of a microcomputer-based control system. 
     SUMMARY OF THE INVENTION 
     In order to test the validity of the AC inputs of a computer-based control system in which opto-couplers are used to transmit a squarewave at a constant frequency as an indication of a positive request for control at the input of the computer, a synchronized reference square wave signal of the same frequency is used for comparison purpose with the input signal. The reference signal is delayed just enough to create a slight overlap in time between the respective notches, and the instantaneous value of the received input signal is sampled and stored in digital form by latching upon two successive instants, first upon a trailing (or rising) edge occurrence in said reference signal, secondly upon the occurrence of the subsequent rising (or trailing) edge in said reference signal, for deriving two successive digital states. Only one combination of different such successive states as latched is indicative of a valid command, or status, signal at the input. The other combination of different digital states, or any successive occurrence of the same digital state twice, would be indicative of a faulty condition, either in the original signal, or in the intervening opto-coupler. 
     This approach to input validity testing requires only two samples closely related in time and, therefore, uses a minimal processing time, thereby minimizing the number of RAM bytes to be affected to input treatment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an AC motor drive as illustration of a control system embodying the present invention; 
     FIG. 2 shows an automatic transfer switch as another illustration of a control system embodying the present invention; 
     FIG. 3 shows the implementation with auto-couplers of the reliability test according to the present invention as part of the control unit found in FIG. 1 and the computer associated with the motor drive; 
     FIGS. 4A and 4B show the opto-couplers and the latches involved in the reliability test according to the present invention which is part of the control unit found in FIG. 2 for the automatic transfer switch thereof; FIG. 5 shows the hardware interface between the digital inputs to be validated and the computer embodying the RAM&#39;s and effecting the reliability test; 
     FIG. 6 is a block diagram illustrating the key elements of the reliability test circuit according to the present invention; 
     FIG. 7 provides signals among which one is a valid input signal and the three others correspond to faulty situations to be guarded against; 
     FIG. 8 illustrates with a reference signal the reliability test effected upon a received input signal in accordance with the present invention; 
     FIG. 9 is a flow chart illustrating the operation of the reliability test circuit of FIG. 6; 
     FIGS. 10A, 10B constitute FIG. 10 which shows a hardware interface between the computer and the validated inputs to the motor drive in the example of FIG. 1. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention is hereinafter illustrated successively as part of a control system pertaining to an AC motor drive and as part of an automatic transfer switch. 
     The AC motor drive has been more specifically described in U.S. Pat. No. 4,304,243, or in U.S. Pat. No. 4,456,867. For the purpose of describing a computer-based AC motor drive, the two patents are hereby incorporated by reference. 
     FIG. 1, taken from U.S. Pat. No. 4,456,867, shows a motor controller with its control unit 26 responding to a series of contacts and pushbuttons at the input. The motor is supplied with AC current from the mains of an AC power supply at 120 volts and 60 Hertz (L1, L2, L3). Main switches having line contacts M1, M2, M3, will be closed when a coil C2 is energized (contacts MK) in accordance with an operator command closing contacts Ma at the input of the controller unit. Considering such a command to the motor drive, it is important for motor control that the fact whether contacts Ma are open, or closed, be interpreted without error. It is the object of the present invention to ascertain whether an external input to a computer-based control system, like such contacts Ma to a motor drive, is valid for the control unit to perform the corresponding command. FIG. 1 also shows contacts and pushbuttons installed at the input of the control unit, such as E-STOP; ST/FWD; REV; FAST; SLOW; PRE-STP; . . . &#34;all well known command, or status, external input signals for an AC motor drive&#34; which are to be handled by the control unit at various stages of motor operation, as shown in the incorporated-by-reference U.S. patents. 
     It is also observed that upon each contact, or pushbutton, is applied from the mains the 120 v, 60 Hz voltage. Therefore, whenever such contacts, or pushbuttons, are closed, the AC voltage from the mains, at the primary 36 of the transformer shown in FIG. 1, is applied at a corresponding input of the control unit. In addition, the afore-stated transformer (primary 36) applies directly, via lines 1 and 2, the 120 v, 60 Hz voltage to the control unit. It will be shown hereinafter how this voltage will be converted into a reference voltage to be used, in accordance with the present invention, in order to ascertain the validity of an input to control unit 26. It is understood that the control unit 26 embodies a microcomputer which routinely interrogates and handles all the inputs. 
     FIG. 2 shows a microprocessor-based automatic transfer switch (ATS) providing automatic, or manual, transfer by switching from one power source to a second power source in case of an emergency. This has been disclosed in U.S. Pat. No. 4,747,061 entitled &#34;Automatic Transfer Switch For a Wide Range of Source Voltage&#34;. U.S. Pat. No. 4,747,061 is hereby incorporated by reference. 
     Referring to FIG. 2, in an automatic transfer switch, the control unit involves a plurality of contacts, pushbuttons, which determine the operation of a load LD1 either under a SOURCE 1, or a SOURCE 2, here represented as an auxiliary generator G, to be started by a start-circuit 62, depending upon the closing of switches MAIN 1 and MAIN 2. Electrical energy for the control unit 12 may be derived from SOURCE 1, or SOURCE 2, as shown by transformer T1 from phases A and C of SOURCE 1 with inputs 4 and 5 into terminal board TB1C of the control unit 12, and by transformer T2 from phases A and C of SOURCE 2 with inputs 14 and 15 into terminal board TB1C of control unit 12. 
     Externally, a portion of this power is supplied by way of terminals 7 and 12 of terminal board TB1C as the AC LINE and NEUTRAL AC field power for the various external elements interconnected with main controller 12 or the option board 22. The electrical power derived from SOURCE 1 and SOURCE 2, as well as the field power, is 120 volt alternating current. Terminal 6 on terminal board TB2C is entitled MAIN 2 REPORT BACK (RB2) and terminal 5 thereof is entitled MAIN 1 REPORT BACK (RB1). Both of these are externally fed from auxiliary contacts on the circuit breakers MAIN 2 and MAIN 1, respectively. When the circuit breaker MAIN 2 is closed, normally open contacts RB2 are closed, thereby applying AC voltage on terminal 6 of terminal board TB2C. When the circuit breaker MAIN 1 is closed, contacts RB1 are closed, providing AC voltage on terminal 5 of terminal board TB2C. Terminals 7, 8 and 9 of terminal board TB2C are externally connected to the OFF, AUTO, and MANUAL output terminals of a mode selector switch. Correspondingly, the input terminals 7, 8 and 9 of terminal board TB2C are identified as OFF, AUTO and MANUAL, respectively. When the mode selector switch is in the OFF state, controller 12 does not operate to control the circuit breakers MAIN 1 or MAIN 2. If on the other hand, the mode selector switch is in the MANUAL state, the control 12 may be manually manipulated to place the circuit breakers MAIN 1, MAIN 2 in conduction, or non-conduction. Finally, when the mode selector switch is in the AUTO mode, controller 12 will operate to automatically cause the circuit breakers MAIN 1, MAIN 2, to open, or close, in accordance with a predetermined set of conditions which are programmed into the controller 12. Input terminals 10 and 11 of terminal board TB2C are entitled AUXILIARY TRANSFER 1-2 (AUX1) and AUXILIARY TRANSFER 2-1 (AUX2) respectively. These terminals are connected to pushbutton switches also connected to the AC LINE lead. When the mode selector switch is in the MANUAL mode, actuation of a pushbutton will cause a transfer of power from SOURCE 1 to SOURCE 2 by appropriate operation of the circuit breakers MAIN 1, MAIN 2. On the other hand, if the operator depresses another pushbutton, the controller will cause transfer of power from SOURCE 2 to SOURCE 1 by corresponding operation of the circuit breakers MAIN 1, MAIN 2. Terminal 14 of terminal board TB2C is designated GF LOCK-OUT (GFLO) and it is connected to a contact device designated GFI which may be part of a separate ground fault sensing system. The contact GFI is powered by the AC LINE lead. Terminal 12 of terminal board TB2C is connected by way of a pushbutton to the AC LINE lead. Terminal 12 is designated as LOCK-OUT RESET (LOR). In the event that a ground fault lock-out has occured by actuation of the GFI relay, the controller 12 will prevent closure of all circuit breakers MAIN 1, MAIN 2. In order to return to normal operation, a pushbutton is actuated which in turn will reset the circuit breakers MAIN 1, MAIN 2 if they tripped, regardless of what status the mode selector switch is in, and provided the ground fault has cleared. Terminals 13 and 15 of terminal board TB2C are designated GENERATOR STOP and GENERATOR START, respectively. These cooperate internally with the output terminals 1 and 2 of terminal board TB1C which are collectively designated GENERATOR OUTPUT START. An auxiliary generator starting system may be externally interconnected with the terminals 1 and 2 of terminal board TB1C for starting the auxiliary generator at an appropriate time for supplying power to the system. For example, SOURCE 2 may be the auxiliary generator. The actuation of the generator will take place regardless of whether the mode selector switch is in the MANUAL, or AUTO mode. Terminals 10, 11 and 12 of terminal board TB1C represent the common NEUTRAL for SOURCE 1, SOURCE 2 and the output control or field power lead AC LINE. Terminals 8 and 9 of terminal board TB1C cooperate with terminal 7 thereof to provide the aforementioned output field power. The closing coils for circuit breakers MAIN 1 and MAIN 2 are interconnected externally to the input terminals 1 and 2 of the terminal boards TB2C. These outputs are designated CLOSE 1 and CLOSE 2 respectively. Energization of terminals 1 and 2 of terminal board TB2C will actuate the closing coils CC for the circuit breakers MAIN 1 and MAIN 2 and close those circuit breakers. 
     SOURCE 2 in FIG. 1 is illustrated as an auxiliary generator G. There may be provided a transfer motor TM interconnected by way of relay contacts with terminals 1 and 2 on terminal board TB2C. Furthermore, there is interconnected with the terminals 1 and 2 of terminal board TB1C a starting circuit 62 for the generator G. Also, interconnected with terminal 15 of terminal board TB2C is a start pushbutton 58 for the generator start circuit 52. There is connected with terminal 13 of terminal board TB2C a stop pushbutton 60. Manual actuation of the start pushbutton 58 will cause the internally connected GENERATOR START OUTPUT contact to close, thus energizing the start circuit 62 of the generator G and bringing generator G to provide electrical power on phase lines A, B and C. Actuation of the pushbutton 60 will stop the generator G in a similar manner. 
     Referring to FIG. 3, the inputs stated earlier by reference to FIG. 1 which are the signals to be tested) are here shown to be inputted into respective opto-couplers BF1, . . . BF14 (as shown in the aforementioned U.S. Pat. No. 4,456,867) which are inputting into the reliability test circuit RTS according to the present invention. Circuit RTS also receives, from lines 1 and 2 the 120 v, 60 Hz voltage through an opto-coupler BF14 from line 1, and 2 to generate therein a digital reference signal, as explained hereinafter. At the output, circuit RTS provides the computer MP with validated inputs for handling upon request. Each coupler includes a luminescent diode LED1 and a photo-sensitive transistor PTX. As generally known, when the diode is energized (namely when associated external contacts, or pushbutton, are closed and the 120 v, 60 Hz is applied onto the antiparallel pair of diodes D1 and LED1, the base electrode of the transistor is energized and a 5 v potential is applied (after inversion by device A1) representing a High, or a One, digitally to the input of circuit RTS. An opto-coupler may consist in a device sold on the open market under the name of Siemens, Litronix Division Product, as shown in Litronix Division Optoelectronics Catalog 1982/1983. It is observed that for the control unit of the motor drive, what is important is to know as seen from the computer address, whether, at an interrogated input, the signal received externally to the opto-coupler is ON, namely whether the command is to START, STOP, ACCELERATE, RUN, GO, REVERSE, FORWARD. Accordingly, a valid signal is a signal which is truly ON. 
     Referring to FIGS. 4A and 4B, like for the motor drive of FIG. 1, opto-couplers are shown in relation to input signals operating with a control unit between an automatic transfer switch like in FIG. 2 and the associated computer. 
     FIGS. 4A and 4B show the conversion by opto-couplers SC7 to SC14 (FIG. 4A) and SC15 to SC18 (FIG. 4B) into digital form of eleven of signals to be tested as stated by reference to FIG. 3. These signals are upon terminal board TB2C as follows: 
     MAIN 1 REPORT BACK on terminal 5 for SC7; 
     MAIN 2 REPORT BACK on terminal 6 for SC8; 
     OFF on terminal 7 for SC9; 
     AUTO on terminal 8 for SC10. 
     These signals are associated with data inputs D1, D2, D3 and D4 of a latch LX1 outputting respective outputs RB1, RB2, OFF and AUTO for a group X of four signals. 
     Similarly: 
     MANUAL on terminal 9 for SC11; 
     AUXILIARY TRANSFER on terminal 10 for SC12; 
     AUXILIARY TRANSFER on terminal 11 for SC13; 
     LOCK OUT RESET on terminal 12 for SC14. 
     All these on respective data inputs of a latch device LX2 outputting signals MANUAL, AUX1, AUX2, LOR pertaining to a group Y. 
     Then, FIG. 4B shows three more signals: 
     GENERATOR START on terminal 15 for SC17; 
     GROUND-FAULT LOCK-OUT on terminal 14 for SC16; 
     GENERATOR STOP INPUT on terminal 13 for SC15. 
     These signals are distributed upon the respective data inputs of a third latch device LX3 outputting in a group Z signals: GEN-START-IN; GGLO; GEN-STOP-IN. 
     The 120 v, 60 Hz line voltage is also brought to provide a reference signal in the form of square pulse at 5 v and 60 Hz, on line 7. 
     FIG. 5 shows the microcomputer MD receiving on four input lines A the group X, Y, or Z after multiplexing by a multiplexer DMP. The microcomputer also receives the reference signal at 60 Hertz on line 7. Within the microcomputer are RAM devices handling the logic of the received digital signals so as to make the comparison with a standard logic in order to establish the validity of each inputted signal. FIG. 6 shows in block diagram how the validity test is conducted between an input signal and the reference signal of line 7 received from the opto-coupler ladder OCL. How a distinction is made between a valid and a faulty indication, based on what has been received from the outside of the opto-couplers, will be shown hereinafter by reference to FIGS. 7 and 8. 
     In FIG. 6, the reliability testing circuit of FIG. 3 is shown to respond to the outputs of opto-couplers symbolized by an opto-coupler ladder OCL receiving its inputs according to the states of contacts and push-buttons like in FIG. 1, or FIG. 2. Direct lines 1, 2 are also shown carrying the 120 v, 60 Hz line voltage. Lines 4 are the many outputs interrogated, each carrying an external signal representation in logic form to be tested. These outputs are inputted into a latch LAT which is controlled by computer MP to toggle twice, upon instants t1 and t2 (as shown in FIG. 8), the value read from line 4 being thereby stored before being read by the computer MP from lines 5. The test is commanded by the computer on line 8 (command POL) and toggling of the polarity to the latch is effected as shown in FIG. 5 and as generally known. It will be shown by reference to FIG. 8, that this occurs twice successively, upon the occurrence illustratively of the falling and the rising edge of a reference signal, consecutively. Such reference signal is derived from the opto-coupler ladder OCL with a delay established at DEL. The computer then reads the occurrence of the edges from line 7 at the output of delaying device DEL. 
     In FIG. 2 have been shown eleven signals which are now to be tested for validity. These signals also appear in FIGS. 4A, 4B and in FIG. 5 where they are grouped under X, Y and Z. The computer toggles a corresponding latch (latch LX1 for group X; latch LX2 for group Y and latch LX3 for group Z). Thus, testing is effected with respect to the interrogated input (pin D1, D2, D3, D4) thereby outputting the valid signal (RB1, RB2, OFF, AUTO for latch LX1, and similarly for the two other latches of FIGS. 4A and 4B). 
     Typically, the opto-couplers may be devices such as shown in Siemens, Litronics Division, Optoelectronics Catalog 1982/1983, page 193, and called 7400 Series T2L Compatible. The latches LX1, LX2 and LX3 may be devices such as shown in: &#34;RCA Solid State Databook&#34;/CMOS Integrated Circuits--page 172--called CMOS Quad Clocked &#34;D&#34; Latch (type CD4042B). 
     The outputs on pins Q1 to Q4 from latch LX1 of FIG. 4A (group X), the outputs on pins Q1 to Q4 from latch LX2 of FIG. 4A (group Y) and the three outputs on pins Q1 to Q3 from latch LX3 of FIG. 4B (group Z) are received by a multiplexer DMP (FIG. 5) which outputs for interrogation into four pins of the computer MP (P10, P11, P12, P13, P14). The reference signal is received by the computer on line 7 (pin INTR of FIG. 5). RAM&#39;s are contained within the microcomputer (as well known) which are used in the comparison required in accordance with the steps of FIG. 8. 
     Referring to FIG. 7, under (a) is shown the signal at the output of the opto-coupler (like BF1 in FIG. 2) when the interrogated external signal is rightly the 120 v, 60 Hz line voltage due to the switch, or contacts, being in a closed position. The curve shows a train of square pulses occurring at the frequency of the inputted main voltage. Therefore, the computer would, normally, detect a succession of One&#39;s and Zero&#39;s. If the command does not exist, the contacts are open and there is no voltage applied to the opto-coupler. Accordingly, as shown by curve (b), there is a continuous Zero logic at the output of the opto-coupler. Should, however, at the moment of sampling a noise appear on the line, as shown by (c) the reading will be as if there would be a High, thereby falsely leading to the conclusion that the external signal is ON. Another situation is illustrated by (d): if the opto-coupler is failing, there could be a constant voltage on its output line, again translating falsely the presence of an ON status from the external signal. 
     FIG. 8 illustrates with curves validity testing according to the present invention. Under (a) is shown the reference signal illustratively obtained as output from an opto-coupler using at its input the 120 v, 60 Hz main voltage (corresponding to lines 1 and 2 of FIG. 3) used to provide directly the reference signal, instead of applying an external signal for that purpose. Under (b) is shown a tested input signal. Relying upon the pulse train derived from the opto-coupler, it is now proposed to compare to the reference signal (curve (a)) the signal of the interrogated input (curve (b)) as derived from its own opto-coupler. In order to be sure to be upon the edge when sampling, a small delay τ is caused to occur between the reference signal of curve (a) and the tested signal of curve (b). Upon the falling edge of the reference signal, testing is effected a first time thereby insuring that at such instant t1 the tested signal exhibit a ONE (Zone #1). After a time interval of a half a cycle, or somewhat less, another test is performed (time t2) and ZERO as observed in Zone #1 the polarity b since after a half-cycle such second testing is done upon the succeeding rising edge of the reference signal such lack of polarity indicates a valid conclusion. Should however, as under curves (c) or (d) of FIG. 7, the interrogated external signal appear as a continuous ONE (Zone #2 of FIG. 8), at instants t1 and t2 the detection will show twice a High, thus, a parity of polarities indicating an invalid reading of a ON signal. Should, the situation be like under curve (b) of FIG. 7, FIG. 8 shows that two successive Low are now being detected. Again, a parity of polarities indicates an invalid reading. In the two latter situations, the computer will know that the conclusion is failure, or an OFF external signal. 
     FIG. 9 is a flowchart illustrating the system operation with the computer of the reliability testing circuit RCT with the following: 
     At 100 the system provides the input sample interrupt subroutine. Then, by 101 it goes to 102 where the command is to toggle polarity at instant t1 for the first sample (sample #1 of FIG. 8). This is done by line 8 of FIG. 6. Thereafter, by 103 the system insures, at 104, that this is done for all inputs and that the sampling result is thereby stored. Thereupon, by line 105, the system goes to 108 where the second sampling (sample #2 of FIG. 8) is effected by toggling at instant t1 (again by line 8) and the second sample is stored. Then, by line 109 the system goes to 110 where the two successive samples are compared. By line 111 the question becomes at 112 whether there is a High followed by a Low? If it is YES (line 113) the input is valid and the required control can be performed. If it is NO (line 114) the external signal cannot be seen as ON. The contacts are OFF or there is a failure, along the interrogated input channel. 
     Referring to FIGS. 10A and 10B a hardware representation is provided in the illustrative embodiment of the invention of FIG. 1, namely of a motor drive. The various external signals inputs 16 to 26 of terminal board TB1 are shown connected via their opto-couplers OPT1 to OPT6, by groups of four, to three respective latches U14 and U15 in FIG. 10A, and U16 in FIG. 10B. They are similar to the earlier considered groups of signals X, Y, Z of FIG. 4A, 4B and 5. The associated computer (like MP in FIG. 5) toggles by line 8 the corresponding latch for the signal under interrogation (U14, U15 or U16) and testing results with regard to such interrogated signal (pin B1, B2, B3, B4), thereby outputting the validated signal (on pin 2, 10, 11, or 1) stating whether the motor drive should operate under control signal RB1, RB2, OFF, AUTO (for latch U14 in FIG. 10A), MANUAL, AUX1, AUX2, LOR (for latch U15 in FIG. 10A), or GEN-START-IN, GFLO, or SPARE (for latch U16 in FIG. 10B). In FIG. 10B, lines 1, 2 are shown applied to opto-coupler OPT12 which outputs on line 6 the reference signal which becomes delayed on line 7 after a delay τ due to delay circuit DEL shown to include a combination of capacitor and resistor, as generally known. 
     Referring to Appendix A hereinafter, a listing is provided for performing validity testing according to an automatic transfer switch like shown in FIG. 2 and performed in accordance with FIGS. 4A, 4B, 5, 8 and 9. In the Appendix, the steps are referred to from typical lines of the listing to the corresponding blocks of FIG. 9, for illustration. Thus,: 
     1. First sampling test (instant t1) according to block 102: on lines 281-286; 
     2. Sampling inputs according to block 104: on lines 287-292; 
     3. Second sampling test (t2) according to block 108: on lines 330 to 469; 
     4. Effecting a comparison according to block 110: on pages A-2, A-3, A-4; 
     5. Concluding as to validity according to block 112: indicated by the AND functions such as for instance on line 457. 
     In order to explain the peculiarity of using an AND function in recognizing a valid digital combination, it is observed that of the four logic combinations possible according to FIG. 8, the valid one is (1, 0), whereas the three other ones are (11), (00) and (01). As a preferred way of reading the correct combination, an inversion is provided of the second digit (second sampling result at t2) so that the valid combination will appear as (11) and the other ones will read (10), (01) and (00). Therefore the valid one is a combination of ONES which are ADDED by an AND function, immediately recognized. 
     Appendix B provides in the same context of an automatic transfer switch a listing of the software of the RAM assignments and locations for bits and bytes in the operation of the microcomputer (typically MP of FIG. 5), each group of eight lines of the listing corresponding to one signal under test. ##SPC1## ##SPC2##