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Hello. And welcome to The Logic Minute. In this video, we will look at how to use logic to enable or disable a digital signal. A digital signal can be enabled or disabled with a wide variety of logic devices. This diagram shows a generic logic device with an input control and output. The control input is either an Enable or a Disable.

An Enable will allow an input signal, shown in green, to pass the output, shown in red, when the control signal is high. It will prevent a signal from passing when the control signal is switched to low. A Disable will prevent the signal from passing when the control signal is high and will allow a signal to pass when the control signal is low. A Disable is often referred to as an Active Low Enable. When a signal is enabled, the output will match the input. When a signal is disabled, there are four options for how the output should respond.

The first and most common option is that the output is forced low. This is typically achieved by using an AND gate, using one input as the Enable and the other input as the signal input. The second option is to force the output high. This is typically achieved by using an OR gate, using one input as the Disable and the other as the signal input. Devices that have an Output Enable, or OE pin, will typically place their outputs into a high impedance state.

One example of this is the 125 Buffer function, which is just a normal buffer that has an added OE pin. In this case, the output is not driven to a high or a low. And it is essentially disconnected. Most systems require a pull-up or a pull-down resistor at the output to maintain a valid logic level when an output is set to high impedance. It is also possible to hold the most recently known state at the output when the output is disabled. Note here that the output remains high after the input is disabled.

However, if the output is low when the input is disabled, the output is held low instead. This is best accomplished with a D-type latch, sometimes referred to as a transparent latch, since the output matches the input during normal operation. Thank you for watching. Please explore the other videos and training material on our site. And if you have any questions, come over to the E2E forums to ask us directly.

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