Previous chapters of this book have been concerned with the application of static devices to logic control in which the decision section of the control system was completely static in nature. There is another ever-growing field of application-the use of static MEMORY-type elements for counting and sorting of mate rial or operations.
A complete analysis of this area of control and the compo nents used is subject matter enough for a complete book . This chapter will give you an introduction to some of the basic concepts.
15 · 1 COUNTERS
In any industrial control system, it is often necessary to count up to a certain number and then have an action occur. Such a case might be a palletizer in which it is desired to load x products into the pallet, move out the pallet, reset the counter, and repeat the cycle with a new pallet in place.
Various counting methods have been developed to handle almost any application, and it becomes necessary to choose the type of counter best suited for the job to be performed. There are basically three types of counters used today in such appli cations, the decade counter, the binary counter, and the ring counter. Pure “counting” is usually done with a decade or binary counter; the ring counter is most often applied for sequencing and is the solid-state equivalent of a stepping switch. For pure counting, decade counters are usually chosen because they are easy to understand and apply, easy to decode, easy to read out (visually), and by definition easy to adapt to the decimal system. They also make clean subtract or add-subtract counters. Binary counters, in comparison, use fewer stepping ( transfer) elements but take more logic circuitry to decode. They also are harder to read out and do not lend themselves easily to downcounting.
Any kind of counter may be easily constructed by making use of one basic element, the step MEMORY (refer to Sec. 11·5, “Special Functions,” for information on operation of this ele ment). (Add-subtract counters are made with the hi-step MEMORY.) The off-return step MEMORY is shown in Fig. 15·1.
Step MEMORYS when connected in chains can store as well as transfer information when the chain is properly inputed. They may also be manually set and reset.
Following are descriptions of the decade, binary, and ring counters. In all such applications, it will be noted that the stepping signal (if originating from a contact-making device) should first be brought through a signal amplifier to filter out contact bounce. T e system of numbers or counting used in this country is called the decimal system. A counter used to count in the decimal system is called a decade counter. Since there are nine numbers plus 0 in this system, one would think that an indicator or counter would require at least ten MEMORYs to count and indicate numbers from 0 to 10. This is not necessarily so, as an examination of Fig. 15·2 will show.
Figure 15 ·2a shows five MEMORY elements, each equipped with an indicating light to show when it is on. Define all lights off as zero. An ON pulse to the first MEMORY will turn on its light and indicate the count of one. An ON pulse to the second MEMORY will turn on its light and indicate a count of two. This process will continue through the count of five, which will be indicated by all lights on.
At this point we can apply an OFF pulse to the first MEMORY, which will turn its light off and leave the other four on. This
indicates a count of six. Successive OFF pulses applied to MEMORYs 2, 3, and 4 will turn their lights out in sequence to indicate counts of seven, eight, and nine. An OFF pulse applied to MEMORY 5 would turn its light off and provide a 10 count, but it would look the same as zero. The decimal system of count ing requires that when we count to nine and add one more count, we register a zero and carry a one. The same thing can be accomplished by applying an OFF pulse to MEMORY 5 and at the same time applying an ON pulse to MEMORY 6 (Fig. 15·2b). The indication of count 10, then, is MEMORYs 1 to 5 OFF and MEMORY 6 ON. MEMORYS 1 to 5 have the ability to count to 10 and therefore are referred to as a decade, even though in themselves they cannot indicate a 10 count as differen tiated from a zero count. This follows the decimal system of numbers in that the highest digit which can be indicated in any single column is nine.
Ten MEMORYs, as shown in Fig. 15·2a and b, will allow a count to 100 in the following manner. MEMORY 6 is left with its light on, and ON pulses are applied in sequence to MEMORYS 1 to 5 for the indication of counts of 11 to 15. OFF pulses are then applied in sequence to MEMORYs 1 to 4 to indicate counts of 16 to 19. The OFF pulse applied to MEMORY 5 must also pulse MEMORY 7 on to indicate the number 20, that is, MEMORYS 6 and 7 both on. The next pulse turns on MEMORY 1 again, and the process repeats itself until the OFF pulse applied to MEMORY 5 has turned on MEMORYS 8, 9, and 10 in sequence, indicating counts of 30, 40, and 50. The next nine pulses will turn on MEMORYS 1 to 5 and then turn off MEMORYS 1 to 4 to indicate counts of 51 through 59. The next pulse will be number 60 and will turn off MEMORYs 5 and 6 to indicate a 60 count. This process is continued until MEMORY 10 is turned off by the 100 pulse. When higher counts are desired, additional decades are added; the fifth MEMORY in each decade transfers its OFF pulse to the next decade, just as you carry numbers in adding written figures.
Practical decade counters might have the indicators separately mounted. They might be arranged horizontally or vertically, or a readout tube or device might be used to indicate the count.
The MEMORY elements used in this type of counter are some what special and carry different names when made by different companies. The step MEMORY discussed in Chap. 11 is one such component. The essential requirement is that the MEMORY ele ment have the ability to receive a signal and store that informa tion until it receives another signal, and then transfer that infor mation to another MEMORY in series.
15 ·2 THE GENERAL ELECTRIC COMPANY DECADE COUNTER
In the decade counter, five step MEMORYs are used to count from 0 to 9 and are coded as shown in Fig. 15·3.
To count to 99, two banks of five step MEMORYs are used (the second bank is coded to represent 10, 20, 30, up to 90); to count to 999, three banks are used; and so on.
The circuit for the decade counter is shown in Fig. 15·4. Five step MEMORYs are used to count to 9. The units are connected in conventional chain fashion (each unit steering on or off the next unit) except that the outputs of the last unit are reversed when they are fed into the steer inputs of the first module. Thus the standard output is fed into the steer-off terminal, and the NOT output is fed into the steer-on terminal. The set terminals (pin I) are all tied to 0 volts. With this connection, the circuit counts by the step M E MORYs, turning on in sequence and then turning off in the same sequence. First count A turns on; second count B turns on ; third count C turns on; fourth count D turns on; fifth count E turns on ; sixth count A turns off; seventh count B turns off ; eighth count C turns off; ninth count D turns off; tenth count E turns off. The counter is now back in the start condition.
With the tenth count, the NOT output of E unit may be used to step the second decade row (if it is desired to count above 9). Each time the top row counts to 10 the second row is then stepped , and therefore the second row counts 1Os. More decade rows may be added for hundreds, thousands, etc., as required. ( Note that the stepping of subsequent rows does not take place when panel power is first applied because of the unit reset, which locks out all step MEMORYs for 15 msec.)
To read out coded information from the decade counter, a decode circuit must be used for each number to be read out from the counter. The decode circuit for 0 to 9 decade counter consists of one two-input AND for each number to be read out. For a 0 to 99 decade counter, the decode circuit is one four input A ND for each number (two inputs from each row); and so on . Figure 15·5 shows which outputs need to be fed into the AN D for each number 0 to 9. For example, to decode the number 6 a two-input AND would be required with the connections as shown in Fig. 15·4. If a momentary readout only is desired and if the step signal is momentary, the step signal may be ANDed with the inputs to the decode ANDs.
Adjustable preset readout. If the readout of the counter is to be adjustable (by means of a selector or thumbwheel switch) , each switch contact signal may be brought through an original input and ANDed with the inputs to the proper decode AND. If many selectable points are desired, a more economical method utilizing the 1200A interface amplifier is shown in Fig. 15·6. A two-deck thumbwheel switch and two-input AN D are used for a 0 to 9 counter ; a four-deck switch and four-input AND would be
used for 0 to 99; and so on. Of course, a source of 24 volts d-e is required.
Down counter. Figure 15·7 shows the connection for a down (subtract) decade counter. Here each step MEMORY steers the one just before it, and so when stepped the counter counts back ward. The first count registers 99, the second count 98, and so on continuously to 0.
Down counter with adjustable preset input. Sometimes when it is required to count a preset number of events and then initiate an action, it may also be required that the preset point be adjustable. In such cases, rather than have a large number of selectable decode circuits, it is more €conomical to set in the required count, count down, and initiate the action when the counter reaches 0. Such a circuit is shown in Fig. 15 ·8. Here, setting in the proper number is made easy by use of decade coded thumbwheel switches, and only one decode AND is needed (to read out the count of 0). The beauty of this approach is
that the decade counter is based on the decimal system (it counts in units, tens, hundreds, etc.) and therefore large numbers may be easily set in with the appropriate number of thumbwheel switches. In Fig. 15·8, for example, the number 82 may be set in by turning the tens switch to 8 (sets the lower decade row to 80) and the units switch to 2 (sets the upper decade row to 2) . When the counter reaches 0 and the action is initiated, the thumbwheel switches are permitted (for 100 p,sec) to set the counter again.
Up-down counter. Figure 15·9 shows the connection for an up-down (add-subtract) counter. The hi-step MEMORY, having two independent steer-step inputs, is employed. The pin 5s (steer on) and pin 2s (step) are interconnected with the pin 8s in the standard manner for an up counter. The pin 6s (steer on) and pin 3s (step) are interconnected with the pin 8s in the standard manner for a down counter. The only exception is that in the . hi-step MEMORY the steer-off inputs are the built-in NOTS of the steer-on inputs, and so it is not necessary to make any connec tions to pin 7 of the step MEMORYs .
The two step MEMORYS on the lower right side of Fig. 15·9 serve to determine whether the counter is counting up or count ing down at the moment the second decade bank is to be stepped .
15 · 3 RING COUNTERS
The ring counter is not practical for pure counting but is a powerful tool when used for sequencing of events . As such it is the solid-state equivalent of a stepping switch and inherently has many additional advantages over conventional stepping switches. In addition to long life, the ring counter may be positively inter locked for foolproof sequencing (see Fig. 15·10), may return to home without retracing any steps, and may go forward or back ward if desired.
The ring counter, shown in Fig. 15·11, may be made by chain ing up step MEMORYs in conventional fashion (one step MEMORY for each step). Note that the standard output of the last step MEMORY is fed back into the steer-on input of the first step · MEMORY and the NOT output of the last step MEMORY is fed back into the steer-off input of the first unit. Thus each step MEMORY is connected in the same manner (thus forming a ring).
With this connection, one step MEMORY must be turned on before any other step MEMORY will be steered on. The chosen step MEMORY may be turned on as shown in Fig. 15 ·11 or may be set on automatically by interchanging its pin 1 and pin 3 connections. Now with the first unit on, it is steering the second
unit on while the last unit is steering the first one off. When the step limit switch is closed, the second step MEMORY turns on, and the first one turns off. Now the third unit is being steered on, and the second one is being steered off. When the step limit closes again, the third one turns on, and the second one turns off. With each step, the output signal will move automatically to the next unit, and when it gets to the last unit, it will ring around to the first one and continue.
A common ring-counter application is one in which the ringcounter automatically steps itself to the next operation upon completion of the current operation. Figure 15·10 shows this circuit. Positive operation of the system is guaranteed at all times by the two-input ANDs, which will not permit stepping unless the right operation is being performed and that operation is then completed .