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Digital circuits can be analyzed with the same tools used for analog circuits. For example, a transistor-transistor logic (TTL) circuit will have inputs and outputs that are either 0 volts or +5 volts. A state of 0 volts is called a logical low and a state of +5 volts is called a logical high. To determine whether an input or output is a logical high or low all you need is a voltmeter. If the meter reads 0 volts, you have a low, if it reads +5 volts you have a high. You can use an oscilloscope in the same way. An oscilloscope will also show you when a state changes. If an input or output is changing rapidly, an oscilloscope is a good instrument to examine the state.
Logic probes are popular for examining digital circuits. A logic probe has one or more LEDs that light up to indicate the state of a circuit input or output.
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| A commercially-made logic probe[1] |
Most logic probes have two LEDs. A red LED indicates a logical high and a green LED indicates a logical low. If neither LED lights there is no usable signal where the probe is touching. Most logic probes have a circuit to indicate when a state changes for too short a time to see. This is usually called a pulse catcher. For example, a signal may go from low to high and back to low so fast that you can't see the red LED light up. The pulse catcher will cause a yellow LED to light and stay lit to indicate that this has happened. There is a reset switch to clear the pulse catcher. Many logic probes have an audible indicator also where a low pitch indicates a logical low and a high pitch indicates a logical high.
A primitive logic probe for TTL circuits can be made in a pinch with an LED and a clip lead. The cathode of the LED is connected to ground via the clip lead and the anode is used as a probe. The LED will light up when the anode is connected to a logical high.
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| An impromptu logic probe made from a clip lead and an LED[2]. |
Such a probe cannot tell the difference between a logical low and no connection at all. You may also need a series resistor to limit the current through the LED.
A logic clip is essentially a bunch of LEDs arranged in a spring clip that can be attached directly to a dual-inline package (DIP) logic circuit.
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| A logic clip[3]. |
Each LED connects to an input/output pin on the package. If an LED lights it is indicating a logical high on that pin.
A logic pulser looks much like a logic probe but is used to force input signals onto circuits instead of reading logic states. Typically, a logic pulser is used in hand with a logic probe. The pulser is used to change inputs and the probe is used to read the resulting outputs. (For certification test purposes, a logic pulser is a type of pulse generator.)
A logic analyzer is a sophisticated instrument that can display multiple inputs and outputs at one time, much like a multichannel oscilloscope. They can capture signals in real time and then store the information for display later.
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| Multiple signals on a logic analyzer.[4]. |
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