What is a pull-up resistor and what is a pull-down resistor? What do they do?
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A: The two Bias Resaitor resistors shown in the figure below are pull-up resistors and pull-down resistors. In the figure, the upper Bias Resaitor resistor is grounded, so it is called a pull-down resistor, which means pulling the level of circuit node A to the low direction (ground); similarly, in the figure, the lower Bias Resaitor resistor is a power supply (positive), so it is called a pull-up resistor, which means pulling the level of circuit node A to the high direction (power supply positive). Of course, in many circuits, the 12k resistor in the middle of the pull-up and pull-down resistors is not available or visible. I found this figure on the RS-485/RS-422 bus, and you can immediately understand the meaning of pull-up and pull-down. But many circuits have only one pull-up or pull-down resistor, and in practice, there are more pull-up resistors. The main function of the pull-up and pull-down resistors is to give the line (node) a fixed level when the circuit driver is turned off. 1 In the RS-485 bus, their main function is to make the voltage of the A-B terminal of all nodes 200mV or above (regardless of polarity) when all drivers of the line release the bus. Otherwise, if the voltage between the input terminals A and B of the receiver is lower than ±200mV (the absolute value is less than 200mV), the logic level output by the receiver will be received as the last bit of the transmitted data, which is obviously very easy to cause communication errors. 2 The most common pull-up resistor is when the 7th pin of the NE555 circuit is used as an output. In fact, it is the same as a resistor connected to the power supply + of the C pole of a triode or the D pole of a MOS tube. Its function is: when the tube (transistor or MOS tube) inputs the off level, the C pole or D pole has a high level (approximately equal to the power supply voltage when unloaded); when the tube (transistor or MOS tube) inputs the on level, the C pole or D pole will be connected to the power supply ground (-), so there is a low level. The ideal should be 0V, but because the tube has an on resistance, there is a certain voltage. Different tubes may be different, and the same tube may also have slight differences due to parameter differences. Even a real metal contact power switch has contact resistance/on voltage drop (although the voltage drop is different under different currents); just in terms of conduction, for different series of integrated circuits, because of different application objects, the output voltage after conduction has different regulations, typically the difference between TTL level and CMOS level. This aspect exceeds the content of this question and will be dealt with separately in the log. 3 Suggestion: Experiment yourself or use simulation software.
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