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Lesson 4: Digital Circuits

From "The Digital I/O Handbook"
by Jon Titus and Tom O’Hanlan
Sealevel System, Liberty, SC, 2004

Digital Inputs
Few computers can operate without connections to external devices such as sensors, switches, or other equipment that informs software about external conditions. Computers also receive data from keyboards, disk drives, touchscreens, and similar devices, all of which transfer their information to a computer through a device called an input port.

You can think of many practical uses for input ports. Imagine a controller that counts parts on a conveyor belt. An electronic counter in the controller would provide data that a computer could obtain from an input port. Similarly, an input port connected to a digital thermometer would let a computer read temperature values at any time. The computer simply retrieves the specified data as requested by a software command.
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Digital Outputs
Electronic devices receive information from computers through a circuit called an output port. That information can control a process, control individual devices, update a display, and so on. Each output port, as shown in Figure 2-1, receives information from a computer’s internal data bus and it also receives a unique strobe signal supplied by the internal circuitry of the computer. Because most people use output ports rather than design them, we won’t discuss port construction further.
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Sensor Interfacing
A thermal switch such as a Klixon Series 6786 from Texas Instruments ( provides a snap-action metallic disk that responds to temperature changes. At a specified temperature, the disk either makes or breaks an electrical contact, depending on the model. Buyers can specify an operating temperature and whether the thermal switch provides a normally-open (NO) or normally-closed (NC) contact. When the switch reaches the specified temperature, an NC switch will open and an NO switch will close.

Assume you have a NO switch that operates at 40°C (104°F). When the contacts close, you want a computer to start a process, say, turn on a fan. Because the switch supplies uncommitted contacts — no connections to power or ground — you can connect the switch to a computer in several ways, as shown in Figure 4-1a-d. In this type of application, it’s unlikely the circuit needs to debounce the switch.
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