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The earliest external devices attached to computers consisted of independent units
that operated under control of the CPU. That is, an external device usually occupied a
separate physical cabinet, had an independent source of electrical power, and contained
internal circuitry that was separate from the computer. The small set of wires that connected
the computer to the external device, only carried control signals (i.e., signals
from the digital logic in the computer to the digital logic in the device). Circuitry in the
device monitored the control signals, and changed the device accordingly.
For example, we said that many early computers provided a set of lights that
displayed values. Typically, the display contained one light for each bit in the
computer’s accumulator — the light was on when the bit was set to one, and off when
the bit was zero. However, it is not possible to connect a light bulb directly to an accumulator
because even a small light bulb requires more power than a digital circuit can
deliver. Therefore, a display unit needed a set of parallel circuits that each received a
digital logic signal and controlled a light bulb accordingly. Figure 13.1 illustrates the
connection.
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