And the world is one big "return to zero" full of positives and negatives. Someone already flogged this with the example of ECL, but I'll also say that digital electronics must interface with the outside world. Radio? AC powerlines? Transformers for a variety of purposes, not just audio, power, and RF, either.įor us digitals, GND is the most negative we deal with. Where did you get that idea? There are loads of things out there that require more than just a positive above common that are not audio. Negatives are only used for audio amplifiers What is the diff between positive voltages and negative voltages? Another reason for using a negative power supply is protection of the output transistors from an accidental short circuit developing between output and ground (but the outputs are not protected from a short circuit with the negative rail)." The positive power supply has another disadvantage - the output voltages will vary slightly (☐.4 V) against the background of high constant voltage (+3.9 V). In this case, the voltage divider R1-R2 compensates the voltage variations to some extent. As the constant voltage drops across the collector resistors change slightly (or not at all), the output voltages follow the supply voltage variations and the two circuit parts act as constant current level shifters. If the negative end of the power supply was grounded, the collector resistors would be attached to the positive rail. As the collector resistors are firmly "tied up" to ground, the output voltages "move" slightly (or not at all). In this connection, when the supply voltage varies, the voltage drops across the collector resistors change slightly (in the case of emitter constant current source, they do not change at all). Because ground should be the most stable voltage in a system, ECL is specified with a positive ground. This is done mainly to minimize the influence of the power supply variations on the logic levels as ECL is more sensitive to noise on the VCC and relatively immune to noise on VEE. "The ECL circuits usually operate with negative power supplies (positive end of the supply is connected to ground) in contrast to other logic families in which negative end of the supply is grounded. Potentiometer – measures angular positions continuously, using resistance.The Vcc/Vee question is answered in the wiki:.Switches – mechanical mechanisms will open or close electrical contacts for a logical signal.Proximity Switches – use inductance, capacitance or light to detect an object logically.Typical examples of sensors are listed below in relative order of popularity. Inputs come from sensors that translate physical phenomena into electrical signals. Hence, we will discuss logical I/O and leave continuous I/O for later. Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control.Īs a result, most control applications (and PLCs) use logical inputs and outputs for most applications. If the light can be dimmed to different levels, it is continuous. If it can only be turned on or off, it is logical control. Both inputs and outputs can be categorized into two basic types: logical and continuous.Ĭonsider the example of a light bulb. Inputs to, and outputs from, a PLC are necessary to monitor and control a process.
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