TRANSISTORS and GATESSetting the StageIn the Boolean Logic article, we looked at 7 fundamental gates. These gates are the buildingblocks of all digital devices. We also saw how to combine these gates together intohigher-level functions like full adders. If you would like to experiment with these gates so youcan try things out yourself, the easiest way to do it is to purchase something called TTLchips and quickly wire circuits together on a device called a solderless breadboard. Let'stalk a little bit about the technology and the process so you can actually try it out! If you look back at the history of computer technology, you find that all computers aredesigned around Boolean gates. The technologies used to implement those gates, however,have changed dramatically over the years. The very first electronic gates were created using electromagnetic relays as discussed in the Boolean Logic page. These gates were slow and bulky. Vacuum tubes replaced relays. Tubes were much faster but they were just as bulky and they werealso plagued by the problem that tubes burn out (like light bulbs). Once transistors were perfected (transistors were invented in 1947), computers started using gates made from discrete transistors. Transistors had many advantages: high reliability, low powerconsumption and small size compared to tubes or relays. These transistors were "discretedevices", meaning that each transistor was a separate device. Each one came in a littlemetal can about the size of a pea with three wires attached to it. It might take 3 or 4transistors and several resistors and diodes to create a gate. In the early 1960's, Integrated Circuits (ICs) were invented. Transistors, resistors and diodescould be manufactured together on silicon "chips". This discovery gave rise to SSI (SmallScale Integration) ICs (Integrated Circuits). An SSI IC typically consists of a 3mm-square chipof silicon on which perhaps 20 transistors and various other components have been etched.A typical chip might contain 4 or 6 individual gates. These chips shrank the size of computersby perhaps a factor of 100 and made them much easier to build. As chip manufacturing techniques improved, more and more transistors could be etched ontoa single chip. This led to MSI (Medium Scale Integration) chips containing simple componentslike full adders made up of multiple gates. Then LSI (Large Scale Integration) alloweddesigners to fit all of the components of a simple microprocessor onto a single chip. The8080 processor, released by Intel in 1974, was the first commercially successful single-chipmicroprocessor. It was an LSI chip that contained 4,800 transistors. VLSI (Very Large ScaleIntegration) has increased the number of transistors steadily ever since. The first Pentiumprocessor was released in 1993 with 3.2 million transistors, and current chips can contain upto 20 million transistors.In order to experiment with gates, we are going to go back in time a bit and use SSI ICs.These chips are still widely available and are extremely reliable and inexpensive. With themyou can build anything you want one gate at a time. The specific ICs we will use are of afamily called TTL (Transistor Transistor Logic, named for the specific wiring of gates on theIC). The chips we will use are from the most common TTL series, called the "7400 (seventyfour hundred) series". There are perhaps 100 different SSI and MSI chips in the series,ranging from simple AND gates up to complete ALUs (arithmetic logic units). 7400 series chips are housed in DIPs (Dual InlinePackages). As pictured on the right, a DIP is a smallplastic package with 14, 16, 20 or 24 little metal leadsprotruding from it to provide connections to the gatesinside. The easiest way to construct something fromthese gates is to place the chips on a solderlessbreadboard. The breadboard lets you wire things togethersimply by plugging pieces of wire into connection holeson the board. Nothing could be simpler.All electronic gates need a source of electrical power. TTL gates use 5 volts for theiroperation. The chips are fairly particular about this voltage, so we will want to use a clean,regulated 5 volt power supply whenever working with TTL chips. Certain other chip families,such as the 4000 series of CMOS chips, are far less particular about the voltages they use.CMOS chips have the additional advantage that they use much less power. However, theyare very sensitive to static electricity and that makes them less reliable unless you have astatic-free environment to work in. Therefore we will stick with TTL