What is a pull-up resistor and a drop-down resistor? What are their functions?

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What is a pull-up resistor and a drop-down resistor? What are their functions?

The pull-up resistor is generally the resistor in the circuit where one end is connected to the power supply and the other end is connected to the chip pin. The pull-down resistor is generally the resistor that is connected to one end of the chip pin.

For example, the two bias resaitor resistors are the pull-up and drop-down resistors. In the figure, a bias resaitor resistor in the upper part is called a drop-down resistor because it is grounded, which means that the level of circuit node A is pulled to the lower direction (ground). Similarly, in the figure, A bias resaitor resistor in the lower part is called a pull-up resistor because it is a power supply (positive), which means to pull the level of circuit node A to a high direction (positive power supply. Of course, the 12 K resistor in the middle of the pulling resistance in many circuits does not exist or cannot be seen. I find this figure is RS-485/RS-422 bus, you can suddenly understand the pull up and down the meaning. However, many circuits only have one pull-up or drop-down resistor. In reality, the pull-up resistor is many.

The main function of the pull-down resistor is to assign a fixed level to the line (node) when the circuit drive is closed.

1 In the RS-485 bus, their main role is to make all nodes of the A-B end voltage above 200mV or 200mV when all drivers of the Line release the bus (regardless of polarity ). Otherwise, if the level between the receiver input A and B is less 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, this is obviously very prone to communication errors.

2 The most visible pull-up resistance should be the ne555 circuit with 7 feet as the output. In fact, it is the same as the C pole of a transistor or the D of a MOS tube. Its function is: when the input of the tube (transistor or MOs tube) is switched off, the C pole or D has a very high level (no-load is equal to the power supply voltage); when the tube (transistor or MOs tube) when the input power is turned on, the C pole or D pole is connected to the power source location (-), so there is a low level. The ideal value is 0 V, but the pipe has a conduction resistance, so there is a certain voltage. Different pipes may be different, and the same pipe may be slightly different due to parameter differences, even a real metal contact power switch has a contact resistance/conduction pressure drop (although the pressure drop varies under different currents). Just for the conduction, for different series of integrated circuits, due to different application objects, the output voltage after conduction varies. The typical difference is the TTL level and the CMOS level. This is beyond the content of this problem, and the log is processed separately.

3. Suggestion: Experiment on your own or use simulation software.