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through the intersection as if there were no other cars present at all (that's not exactly true according to my drivers manual, but please accept the analogy). When the light changes, traffic stops in one direction and starts in another. In the electronic intersections of the computer, signals may be output by just one device when it receives the proper signal.

To carry the traffic analogy further, recall that most traffic signals have an amber warning light. In real-life situations, cars slow down (or speed up to get through the light before it turns red). Cars on the other side inch forward upon seeing a hint of that amber signal. This is analogous to the rise and fall time of data signals running through the computer, that safe peri od during which signals may change logic state from 1 to 0 or from 0 to 1 with no conflict.

One final analogy: Those of you who have driven in New York City know that the traffic signals are timed so well that a driver can start in the financial district and drive clear through to Columbus Circle. Under these conditions the car maintains a constant speed akin to that of the changing traffic signals; all other vehicles can keep the same speed (or at least out of your way); and no emergencies—-police cars, accidents, or breakdowns—impede the car's progress. In the computer, this is known as "access time." All electronic parts must be able to complete their responses throughout the system in at least as fast as the master traffic signal, the "system clock." Any part whose access time is too slow (like some RAMs) will have an adverse effect on the operation of the computer.

Narrow Streets and Garages

Resistors and capacitors are abundant in the TRS-80, but I'll keep the theory at a low level for this section. Resistors limit the voltage or current received by some part of a circuit. They can be found providing limited current to light-emitting diode displays (LEDs), holding an IC input to a one or zero level, or as part of the audio circuitry in the video output. Variable resistors serve as adjustments to the video display or to the power supply voltage being sent to the computer as a whole. They act like narrow city cross-streets, where only a limited number of cars can move across town from the large potential auto-streams flowing along main avenues.

Capacitors store an electrical charge, releasing it as needed. Capacitors inside the TRS-80 store power supply irregularities, effectively smoothing them out; they delay the transfer of current from one logic IC to the next (to adjust the horizontal or vertical video image, for example); or they store and release energy at regular intervals to provide clocking signals (what keeps your disk drive on for an extra few seconds, or times the characters being output by your printer).

A collection of resistors is shown in Photo 2. At the bottom right is a group of simple resistors, varying in size according to the amount of current they are capable of handling without overheating. Small resistors are most commonly available as carbon composition (inexpensive, packed carbon) and carbon film (resistive film plated on a non-conducting base). Small resistors make up the bulk of those in the TRS-80.

Fig. 4. 7473 Flip-Flop


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