| Brandeis University | Physics 29a |
| Spring 2020 | Kevan Hashemi |
Logic Chips: The data sheet for the SN7400N quad two-input NAND gate is on the 29A web page. The "SN" means "Texas Instruments". Any other manufacturer code can precede the 7400 and the functionality is the same. The 74S00 and 74LS00 are faster and lower-power versions respectively. You can use them too. The VCC power supply voltage should be +5V.
TTL Inputs Float HI: The 7400 is a Transistor-Transistor Logic (TTL) device. All TTL inputs are HI when connected to no external signal.
Wire as Switch: A "logic switch" provides a LO or HI according to its mechanical state. Because TTL inputs float HI, we can make a logic switch for a TTL input with a single wire, which we connect and disconnect from 0V.
Logic Indicators: We can use LEDs to indicate if a logic output is HI or LO. We include a resistor to limit the current, and so avoid damaging our logic chip. A HI output can sometimes provide enough current to illuminate a red LED, but not always. A LO output can always provide enough current to illuminate a white LED.
Construction Principles: Plan the arrangement of your logic chips to reduce the number of crossing wires. Use black for 0 V and red for +5 V. Use the column contacts for power.
Part 1: Construct your own two-input NOR gate using diodes, bipolar transistors, and resistors. Connect its two inputs to switches and its one output to one of the logic level indicator lamps on your breadboard. These indicator lamps are LEDs in series with resistors. Demonstrate your NOR gate to an instructor.
Part 2: Consider the following truth table. Write down the boolean equations that express the truth table logic. Manipulate these equations until you arrive at an expression that can be implemented with two-input NAND gates alone. Note that you may construct an inverter out of a NAND gate by connecting its two inputs together.
| Inputs | Output | ||
|---|---|---|---|
| A | B | C | X |
| 0 | 0 | 0 | 1 |
| 0 | 0 | 1 | 0 |
| 0 | 1 | 0 | 1 |
| 0 | 1 | 1 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 0 |
| 1 | 1 | 1 | 1 |
You will find it easier to build the circuit if you try to reduce the number of NAND gates it requires. In this case, the minimum number of gates required to implement the function is five. But you may build the circuit with as many gates as you like, so long as it implements the truth table above. Your finished circuit should have the inputs A, B, and C driven by three logic switches, and the state of X should be indicated by an LED.