Logic Properties of Transistor Circuits:Basic Properties of Connecting Relays.

Basic Properties of Connecting Relays

Relays are probably the oldest means to realize logic operations. Relays, which are electromechanical devices, and their solid-state equivalents (i.e., transistors) are extensively used in many industrial prod- ucts, such as computers. Relays are conceptually simple and appropriate for introducing physical realization of logic operations. More importantly, the connection configuration of a relay contact network is the same as that of transistors inside a logic gate realized with transistors, in particular MOSFETs (which stands for metal-oxide semiconductor field effect transistors).

A relay consists of an armature, a magnet, and a metal contact. An armature is a metal spring made of magnetic material with a metal contact on it. There are two different types of relays: a make-contact relay and a break-contact relay.

A make-contact relay is a relay such that, when there is no current through the magnet winding, the contact is open. When a direct current is supplied through the magnet winding, the armature is attracted to the magnet and, after a short time delay, the contact is closed. This type of relay contact is called a “make-contact” and is usually denoted by a lower-case x. The current through the magnet is denoted by a capital letter X, as shown in Figure 33.1.

A break-contact relay is a relay such that when there is no current through the magnet winding, the contact closes. When a direct current is supplied, the armature is attracted to the magnet and, after a short time delay, the contact opens. This type of relay contact is called a “break-contact” and is usually denoted by x. . The current through the magnet is again denoted by X, as shown in Figure 33.2.

In either case of a make-contact relay or a break-contact relay, no current in a magnet is represented by X = 0, and the flow of a current is represented by X = 1. Then x = X, no matter whether X = 0 or 1. But the contact of a make-contact relay is open or closed according as X = 0 or 1, because the contact is expressed by x, whereas the contact of a break-contact relay is closed or open according as X = 0 or 1, because the contact is expressed by x .

Let us connect two make-contacts x and y in series, as shown in Figure 33.3. Since X and x assume identical values at any time, the magnet, along with its symbol X, will henceforth be omitted in figures unless it is needed for some reason. Then we have the combinations of states shown in Table 33.1(a), which has only two states, “open” and “closed.” Let f denote the state of the entire path between terminals a and b, where f is called the transmission of the network. Since “open” and “closed” of a make-contact are represented by x = 0 and x = 1, respectively, Table 33.1(a) may be rewritten as shown in Table 33.1(b). This table shows the AND of x and y, defined in Chapter 26 and denoted by f = xy. Thus the network of a series connection of make-contacts realizes the AND operation of x and y.

Let us connect two make-contacts x and y in parallel as shown in Figure 33.4. Then we have the combinations of states shown in Table 33.2(a). Replacing “open” and “closed” by 0 and 1, respectively, we may rewrite Table 33.2(a) as shown in Table 33.2(b). This table shows the OR of x and y, defined in Chapter 27 and denoted by f = x ∨ y.