Hardware Reference
In-Depth Information
Modern Computers
FromUNIVACtothelatestdesktopPCs,computerevolutionhasmovedveryrapidly.The
first-generation computers were known for using vacuum tubes in their construction. The
generation to follow would use the much smaller and more efficient transistor.
From Tubes to Transistors
Anymoderndigitalcomputerislargelyacollectionofelectronicswitches.Theseswitches
areusedtorepresentandcontroltheroutingofdataelementscalled
binary digits
(or
bits
).
Because of the on-or-off nature of the binary information and signal routing the computer
uses, an efficient electronic switch was required. The first electronic computers used va-
cuum tubes as switches, and although the tubes worked, they had many problems.
The type of tube used in early computers was called a
triode
and was invented by Lee
De Forest in 1906 (see
Figure 1.1
). It consists of a cathode and a plate, separated by a
control grid, suspended in a glass vacuum tube. The cathode is heated by a red-hot elec-
tric filament, which causes it to emit electrons that are attracted to the plate. The control
grid in the middle can control this flow of electrons. By making it negative, you cause the
electrons to be repelled back to the cathode; by making it positive, you cause them to be
attractedtowardtheplate.Thus,bycontrollingthegridcurrent,youcancontroltheon/off
output of the plate.
Figure 1.1
The three main components of a basic triode vacuum tube.
Unfortunately, the tube was inefficient as a switch. It consumed a great deal of electrical
powerandgaveoffenormousheat—asignificantproblemintheearliersystems.Primarily
because of the heat they generated, tubes were notoriously unreliable—in larger systems,
one failed every couple of hours or so.
