7401 Quad Nand Gate - With Open Collector Outputs

Essentially a variation of the 7400, the 7401 IC package contains four independent positive logic NAND GATES with open-collector outputs. Pins 14 and 7 provide power for all four logic gates. The outputs require pull-up resistors to function properly. The open-collector outputs can be connected together to produce higher output voltages or to implement an active-LOW wired-OR function or active-high wired-AND function with out the need for additional logic gates. Since the outputs are open collector, they can efficiently be attached to other outputs and only the active device will be able to have the final say on what the main output line will be. This allows the designer to build selector circuits where multiple data can be selected from a wide variety of sources and only one of them will be allowed to output at a specific time. Of course this requires some addition circuitry and planning, but this greatly extends the complexity with which can be obtained versus the 7400. The 7401 datasheet can be immediately downloaded HERE.

7400 Quad Nand Gate

The 7400 is the first of a series of logical TTL devices and is the perfect example of a basic, yet commonly applied, integrated circuit. Within this discrete component is contained 4 independently functional 2-input NAND gates. Pins 14 and 7 provide power to all of the four gates and the internal connections are omitted for added clarity, as is often the case in digital electronics. Once power is applied to the main inputs of the chip, each gate begins to perform it's primary function, which is to calculate the NAND function on two input lines. For example, consider the "first" gate inside the chip (wired to pins one, two, and three). Pins one and two are the gate's input lines. Pin three is the output. The output will only "activate" (or go low) when both inputs, one and two, are logical high. All other combinations of highs and lows (ones and zeros) on the input lines will result in a consistenly high output (logical one) or a voltage essentially the same as Vcc (+). Take a look at the second diagram to better understand the 7400's (74LS00, 74HCT00) role in a simple circuit. The gates, having limited power capabilities, may not be able to directly drive an output. Typically, most low current (20mA) LED's are no problem and can be directly connected to a gate's output through a current limiting resistor. However, if one were to attempt to power a halogen light, a motor, or other heavy duty, high current, device, the output would need to be amplified by either an opamp (operational amplifier), a transistor circuit, or possibly a combination of such approaches. Click here to download a datasheet for more information.