While the simulation is running we can change with the computer’s mouse the positions of the switches. This simulation with only 2 bits at the input can be used to verify the truth table just flipping the switches. Note the red LED indicating a HIGH level at the output. Probe1 shows a voltage of 5 volts at the output. The virtual LED shows the threshold voltage of 2.5 VDC. Here, we can see some important details: The inputs are both LOW. We then run the simulation with the Run icon and we can observe the result shown on Fig. Besides it we can see that the threshold voltage for turning it on and off is 2.5 volts. It is available at the Indicators library as Probe.
The output is connected to a display that evokes a test LED. When one or both of the switches is closed (a point on the component shows the ON position) there are 5 volts present at the input. The switch is wired so the digital ground is going to the gate’s input via a pull-down resistor, basically grounding it while the switches are open. This component is a 2 bit DIP switch (DIP stands for Double In-Line Package, that’s the type of components that can fit into a breadboard for prototyping, their typical width is 0.3 inches and the separation between pins is 0.1 in). Power is provided by VCC and it is fixed at 5 VDC. 6.2 we can see some aspects that we are going to discuss here. X Y X NAND Y 0 0 1 0 1 1 1 0 1 1 1 0 We can see that the output will be in a low state when both inputs are at a high level. The circuit is shown in Table 6.1: Truth table for a NAND gate. We can build a circuit using a 74LS00 HEX NAND in order to verify this table. The truth table for a NAND gate is shown in Table 6.1. For example, 74LS00 is a NAND gate and we will use it in the next example.
The most widely used subfamily for digital circuit simulation is the 74LSXX series where the digits after the LS indicate the gate’s function. The biggest difference among the different subfamilies is the response time of each one. We can see all of them if we open the parts menu and select TTL circuits. TTL (Transistor-Transistor Logic) is a family of digital gates with several subfamilies. VEE is a negative power supply that provides 5 VDC and VSS is the same as digital ground at 0 VDC. Them is the value of the provided voltage. The difference amongįigure 6.1: Digital power supplies in Multisim. There are 4 different power supplies for digital circuits (see Fig. When using component models from real world circuits a specific gate in a multi-gate IC can be specified and the supply voltage can be changed accordingly.
Ideal parts don’t need a dedicated power supply and can be used as stand-alone gates. Mathematical models for gates can be idealized so that race conditions will not affect the final result or they can be based on real world components in the 74XX and 40XX families for TTL and CMOS, respectively. They range from digital gates (both families, TTL and CMOS are supported), indicators (such as LED’s, LCD displays, Logic Analyzers and Word Generators), wires, data buses and power supplies. In Multisim we can find several useful tools and parts for digital circuits. Multisim has a wide set of features specifically designed for the analysis and simulation of digital circuits in several handy ways.
Their applications go from cellular telephones to PC applications, communications systems, process control, to name a few. Digital Circuits INTRODUCTION Digital circuits nowadays are part of our daily lives from several decades ago.