The two previous systems covered in earlier chapters use English logic. The NORPAK system is built around NOR logic and re quires somewhat different diagrams. This chapter will take up the details of the NORPAK system. The material for this chapter was furnished by the SquareD Company. 1
13 ·1 THEORY OF OPERATION
The heart of the NOR logic element is the transistor (Fig. 13·1) , which is ideally suited for a logic unit because of its reasonable cost, small size, low power consumption, speed of operation, and excellent performance as a switch. A transistor is a crystal line material that exhibits properties of an insulator in one state and a conductor in another state. Therefore, it behaves as an
‘SOURCE: SquareD Co. bulletins M-292-1, M-295C, M-212-1, M-275A , M-276, M-277, M-278-B, M-286-1, M-285, M288-2
open contact in the first state and as a closed contact in the second state. Transistor operation is similar to a vacuum tube in that the base controls the current flow between the emitter and collector in much the same manner as the grid controls the current between the anode and cathode. In a PNP-type tran sistor, because of the inherent properties of the material at the junction of the emitter and the base, a negative voltage on the base allows emitter-base current to flow, while a positive voltage on the base prevents emitter-base current from flowing. Current in the plate circuit of the vacuum tube is controlled by the grid cathode voltage; similarly, current in the collector circuit of the transistor is controlled by the presence of current flow in the emitter-base circuit. These conditions result in static switching because no movin g parts are required to open or close a circuit. Reliability and exceptionally long life are assured by the use of transistors because they are not subject to wear or deterioration.
Figure 13·2 is the circuit of the basic NOR . With no negative input voltage at A, at B, or at C, the base is held to a positive bias by 20 volts through R 3. In this condition there is no emitter-base current ; therefore , the -20 volts is across the load in series with R 2, and current I L flows. This is called NOR logic since we get an output if neither input A , B, nor C is present. The word NOR actually has its derivation from the contraction of a function OR-NOT. That is, if there is an input at A or B or C, the function does not have an output. When a negative input exists at one or more of the input terminals A , B, or C, the base becomes negative with respect to the emitter, and current flows from the emitter to the base and out through R 1. Under these conditions, current flows in the collector circuit; the collector becomes, in effect , grounded (acts as a short circuit across the load) , and no load current passes .
Figure 13·3 shows the logic-symbol equivalent of Fig. 13·2. If we represent the presence of a signal at inputs A , B, or C by 1 and the lack of an input signal by 0, then a 1 at any one or more of the inputs A, B, or C would give a 0 output.
NOR logic elements are quite versatile in that the majority of all logic functions can be accomplished by various combinations of this unit device. In the following illustrations of relay and logic circuitry , a normally open contact is represented by a letter, and a normally closed contact is represented by a letter with a bar
above it (A = normally open; A = normally closed).
In the AND function (Fig. 13·4) an output is obtained when all of a given number of input signals are applied. In the basic relay circuit both A and B must be closed to provide an output at E. The equivalent NOR logic circuit operates similarly; a signal must be present at both inputs A and B of their respective NORS
to provide an output at E. With NOR logic, signals at A and B cause 0 outputs at their respective NORS. These 0 outputs, used as inputs to the third NOR, result in a 1 output at E.
The OR function (Fig. 13·5) produces an output when any one of a number of inputs is present. In the basic relay circuit, closure of either A or B will provide an output at E. With NOR logic, a 1 input at A or B or at A and B will provide a 0 output which, when used as an input to a second NOR, gives an output of 1 at E. An alternate method of OR logic is provided by diodes. A diode or rectifier is a semiconductor which allows current flow from positive to negative. With reverse polarity the resistance is extremely high, preventing current flow. The above NOR logic circuit can be duplicated by two diodes as shown in Fig. 13·6. This is the Square D company two-input OR unit, usually used in place of two NOR units. The equivalent NEMA logic OR symbol is used to indicate the use of diodes.
The NOT function (Fig. 13·7) provides an output when no input is present. In the basic relay circuit, an output will be . present at E so long as contact A is not closed. With NOR logic, a
0 input at A results in 1 output at Ē.
The off-return MEMORY function is identical to an electrically held relay. It provides undervoltage (low-voltage) protection. In the relay circuit (Fig. 13·8), momentary closure of push button A will energize coil CR which sets up its own holding circuit by closing the normally open CR 1 contact, enabling the operator to release push button A. Energy is thus maintained to coil CR. Simultaneously, contact CR2 closes and CR3 opens. Depressing the normally closed push button 
(or power failure) deenergizes
coil CR, allowing CR 1 and CR2 to open and CR3 
to close. If we use NOR logic, momentary closure of push button A provides a 1 input to NOR X, thus giving a 0 output. The 0 output is used as one of the inputs to NOR Y. With the off-return signal absent push button B also delivering a 0 signal, a 1 output is assured from NOR Y.This 1 output signal provides a continuous output at E and is also used as an interlocking feedback signal to NOR X s that push button A can be released . Depressing push button B changes the MEMORY to the opposite state through contact B. When power fails and is restored , an off-return pulse is provided by a power supply to assure return to the off state (i.e., a short 1 pulse is applied to NOR Y).
The retentive MEMORY function is identical to a mechanically latched relay (Fig. 13·9). Momentary energization of magnet coil CRL by closure of contact A closes normally open CR and opens normally closed CR. This condition is maintained by means of a mechanical latch. Momentarily energizing a second coil CRU by closure of contact B releases the mechanical latch,
and gravity or spring action opens normally open CR and closes normally closed 
. A NOR MEMORY used in conjunction with a magnetic core circuit provides the logic equivalent. The entire function is symbolized by the rectangle. A momentary input of 1 at A assures ON output E. Similarly, a 1 input at B results in OFF output Ē. The retentive MEMORY retains the condition of the output corresponding to the input last energized. In case of simul taneous inputs to A and B, the MEMORY delivers an overriding OFF signal (i.e., a 1 will appear at Ē).
Quite often it is necessary to have an output (or the removal of an output) some time after an input signal is applied. Con ventional timers consist of two types-time delay after energiza tion (on DELAY) and time delay after deenergization (off DELAY). TDE timers may have normally open contacts timing closed (NOTC) or normally closed contacts timmg open (NCTO). TDD timers may have normally open contacts timing open (NOTO) or normally closed contacts timing closed (NCTC ) . The NOR logic equivalent of the TDE version (Fig. 13·10) is such that output E is obtained some time after a 1 input signal is applied at A. To accomplish the TDD timing function, a NOR is merely inserted between the input
and the time-delay unit. Complementary signals on both timers are provided by the addition of a single NOR to the output of the basic delay unit.
Figure 13·11 illustrates how basic relay circuits can be duplicated by logic symbols. A relay is designated by a letter, and that letter represents each contact of the relay. A normally open con tact is represented by a letter, and a normally closed contact is represented by a letter with a bar above it. (A = normally open ;
= normally closed.) E represents the output and Ē is its complement signal.