"Apple 1 Replica Creation" by Tom Owad

• Digital signals are either on or off, high or low, zero or one.
• Touch the point of a logic probe to a wire and it will respond with either “high” or “low.”
• This [logic probe] is an invaluable tool for building circuits and one you’ll use frequently.
• A breadboard is almost essential for exercises and is also ideal for making quick prototypes.
• A digital multimeter is a tool everyone should own.
• Wire wrapping is useful if you have unique designs that you want to build, play with for a few months, and then take apart.
• Breadboards are much more convenient for quick, temporary designs, whereas circuit boards are more reliable for long-term setups.
• Soldering components to a circuit board is the most reliable and permanent method for building your Apple 1 replica.
• A good soldering job on a printed circuit board will result in a product indistinguishable from a professionally made board.
• Temperature controlled irons allow you to precisely set the temperature of your iron and ensure that the temperature stays within a narrow range.
• AT [power] supplies are “switching power supplies” and should require a load to operate properly.
• Transistor-Transistor logic (TTL) is a method for creating the logic gates in integrated circuits using two transistors at the output circuit. The primary alternative to TTL is complementary metal oxide semiconductor (CMOS). CMOS circuits use less power than ttl but are also more prone to static damage.
• The original Apple 1 used an ASCII keyboard.
• Voltage (V) is the potential difference between two points, measured in volts.
• Current (I) is used to measure the rate of flow of electrical charge and can be thought of in terms of a water current. Current is measured in amperes (A). Whereas voltage is measured between two points, current measures the rate of flow at a particular point.
• Power (P) is voltage multiplied by current. Power is measured in watts.
• P = V * I
• A diode is a device that allows current to flow in only one direction.
• A resistor is a partial conductor, usually made of carbon.
• A capacitor stores energy in an electric field.
• In building our circuits, we’ll use capacitors as de-spiking capacitors to filter the power supply.
• The 7400 series of ICs serves as the basis for digital logic and is very easy to understand.
• Pin numbering always starts in the upper-left corner, goes down the left side, and follows up the the right side. Pin 1 is marked either by a circle in the upper-left corner or by a notch in the top center.
• When a chip’s inputs are changed, the output does not change instantaneously. The new signals take time to propagate through the chip. This propagation varies depending on the speed and complexity of the design, but TTL gates tend to take about 10 nanoseconds (ns) to complete propagation.
• An LED typically has two leads projecting from its base. The longer lead is the positive lead (aka the anode), and the shorter lead is the negative lead (aka the cathode). The cathode is also denoted by a flat edge on the plastic LED housing.
• Basic Logic Functions and Gates
• AND = 7408
• NOT (Inverter/Buffer) = 7404
• NAND (Not AND) = 7400
• OR = 7432
• NOR (Not OR) = 7402
• XOR (eXclusive OR) = 7486
• Using logic expressions, DeMorgan’s Laws, and Boolean algebra, you can quickly sketch out basic circuits on paper or even in a simple text editor.
• Logic expressions will allow you to show gates and lines with symbols and letters.
• DeMorgan’s Laws will allow you to swap gates to get more efficient circuits.
• Boolean algebra will allow you to simplify your circuits.
• algebraic symbols for digital logic
• ‘ = NOT
• ⦁ = AND
• + = OR
• ⊕ = XOR
• DeMorgan’s Laws
• (A + B)’ = A’ ⦁ B’
• (A ⦁ B)’ = A’ + B’
• There’s an easy way to remember DeMorgan’s Laws in practice. To find the equivalent for any gate, first swap the symbol (OR to AND, AND to OR). Then look at each input and output. Everywhere there’s an inverter, remove it, and everywhere there isn’t, add one.
• A complete set is a collection of chips that can be used to produce and logical statement. One example of a complete set is the 7408 (AND), the 7432 (OR), and the 7404 (NOT).
• A set-reset (SR) latch allows us to set a bit to 1 or reset it to 0. We can do this using ordinary logic gates. The trick is to loop the output back into the input. What’s most important to realize her is that all it takes is a momentary high on the set or reset lines to permanently set or reset the output Q.
• Digital logic has only two states, high or low; so our numbers need to be expressed in a binary (base 2) format. A binary digit, like a digital signal, is always either 1 or 0.
• To distinguish binary numbers from decimal numbers, appane d ab to the end. Hexadecimal numbers are prefixed with a dollar sign ($) or sometimes with 0x.
• Each digit in binary is called a bit.
• Counting on your fingers, with each finger corresponding to a bit, is a good way to get used to the binary system.
• There’s not much that can be expressed with a single bit; so, computers examine a collection of bits at one time. This collection of bits is called a word.
• 8-bits are called a byte.
• Eight bits can be used to represent any letter in the alphabet, a computer instruction, a number, the color of a pixel, or data in countless other formats.
• An 8-bit flip-flop (called a register) is just eight flip-flops connected to the same clock.
• A selection of lines treated as a group is called a bus.
• A bus can be either parallel or serial. In a parallel bus every bit is on its own line and arrives at the same time. This bus is the simplest to use and to understand. The serial bus uses a single data line. All data arrives over this single line, one bit after the next.
• A nibble is 4-bits -- half a byte.
• ASCII uses 7 bits to produce 128 characters.
• A shift register is a collection of flip-flops hooked up in a row such that the output of one is the input to the next. This allows us to store a series of data, as it comes in -- 1 bit for every clock pulse.
• A TTL gate can only drive 10 other gates. To alleviate this problem, we have the buffer.
• A buffer is like an inverter that doesn’t invert -- it just replicates the signal.
• The tri-state buffer has a third state -- off. In this third state, “floating,” no output at all comes from the chip and it will not interfere with other signals on the line.
• The first time you use a new soldering iron, you need to tin the tip, which is done by heating the iron and applying a thin coating of solder to the tip.
• If your [solder] joint is dull and dirty, you have a cold solder joint, possible because you bumped the joint or because the joint is not getting hot enough.
• Nonvolatile RAM (NVRAM) can be used in lieu of static RAM in your replica. NVRAM preserves the contents of memory, even when the computer is turned off. This means that programs you enter by hand will still be accessible the next time you use your replica.
• Beginner’s All Purpose Symbolic Instruction Code (BASIC) is a great high-level programming language for beginners.
• Apple Basic commands
• print -- print strings
• run -- run stored programs
• tab -- indent line
• end -- end of program
• goto -- jump execution to a label
• input -- get user input
• dim -- declare string variables
• for i to x step y -- for loop increment
• next -- next iteration of for loop